Java/Collections Data Structure/ArrayList — различия между версиями

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Текущая версия на 07:23, 1 июня 2010

Содержание

A boolean is being stored and then retrieved from an ArrayList

   
import java.util.ArrayList;
public class Main {
  public static void main(String... args) {
    ArrayList<Boolean> list = new ArrayList<Boolean>();
    list.add(true);
    boolean flag = list.get(0);
  }
}





Add an element to specified index of Java ArrayList

   
import java.util.ArrayList;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    arrayList.add(1, "INSERTED");
    for (String str: arrayList)
      System.out.println(str);
  }
}
/*
1
INSERTED
2
3
*/





Append all elements of other Collection to Java ArrayList

   
import java.util.ArrayList;
import java.util.Vector;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    Vector<String> v = new Vector<String>();
    v.add("4");
    v.add("5");
    // append all elements of Vector to ArrayList
    arrayList.addAll(v);
    for (String str : arrayList)
      System.out.println(str);
  }
}
/*
1
2
3
4
5
*/





A variant of java.util.ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array

   
/*
 * File: CopyOnWriteArrayList.java
 * 
 * Written by Doug Lea. Adapted and released, under explicit permission, from
 * JDK1.2 ArrayList.java which carries the following copyright:
 * 
 * Copyright 1997 by Sun Microsystems, Inc., 901 San Antonio Road, Palo Alto,
 * California, 94303, U.S.A. All rights reserved.
 * 
 * This software is the confidential and proprietary information of Sun
 * Microsystems, Inc. ("Confidential Information"). You shall not disclose such
 * Confidential Information and shall use it only in accordance with the terms
 * of the license agreement you entered into with Sun.
 * 
 * History: Date Who What 21Jun1998 dl Create public version 9Oct1999 dl faster
 * equals 29jun2001 dl Serialization methods now private
 */

import java.util.*;
/**
 * This class implements a variant of java.util.ArrayList in which all mutative
 * operations (add, set, and so on) are implemented by making a fresh copy of
 * the underlying array.
 * <p>
 * This is ordinarily too costly, but it becomes attractive when traversal
 * operations vastly overwhelm mutations, and, especially, when you cannot or
 * don"t want to synchronize traversals, yet need to preclude interference among
 * concurrent threads. The iterator method uses a reference to the state of the
 * array at the point that the iterator was created. This array never changes
 * during the lifetime of the iterator, so interference is impossible. (The
 * iterator will not traverse elements added or changed since the iterator was
 * created, but usually this is a desirable feature.)
 * <p>
 * As much code and documentation as possible was shamelessly copied from
 * java.util.ArrayList (Thanks, Josh!), with the intent of preserving all
 * semantics of ArrayList except for the copy-on-write property. (The java.util
 * collection code could not be subclassed here since all of the existing
 * collection classes assume elementwise mutability.)
 * <p>
 * Because of the copy-on-write policy, some one-by-one mutative operations in
 * the java.util.Arrays and java.util.Collections classes are so time/space
 * intensive as to never be worth calling (except perhaps as benchmarks for
 * garbage collectors :-).
 * <p>
 * Three methods are supported in addition to those described in List and
 * ArrayList. The addIfAbsent and addAllAbsent methods provide Set semantics for
 * add, and are used in CopyOnWriteArraySet. However, they can also be used
 * directly from this List version. The copyIn method (and a constructor that
 * invokes it) allow you to copy in an initial array to use. This method can be
 * useful when you first want to perform many operations on a plain array, and
 * then make a copy available for use through the collection API.
 * <p>
 * Due to their strict read-only nature, element-changing operations on
 * iterators (remove, set, and add) are not supported. These are the only
 * methods throwing UnsupportedOperationException.
 * <p>
 * <p>[]
 * 
 * @see CopyOnWriteArraySet
 */

public class CopyOnWriteArrayList implements List, Cloneable, java.io.Serializable
{
  private static final long serialVersionUID = 1L;
  /**
   * The held array. Directly access only within synchronized methods
   */
  protected transient Object[] array_;
  /**
   * Accessor to the array intended to be called from within unsynchronized
   * read-only methods
   * 
   * @return The internal array
   */
  protected synchronized Object[] array()
  {
    return array_;
  }
  /**
   * Constructs an empty list
   * 
   */
  public CopyOnWriteArrayList()
  {
    array_ = new Object[0];
  }
  /**
   * Constructs an list containing the elements of the specified Collection,
   * in the order they are returned by the Collection"s iterator.
   * 
   * @param c The collection to get the objects from. 
   */
  public CopyOnWriteArrayList(Collection c)
  {
    array_ = new Object[c.size()];
    Iterator i = c.iterator();
    int size = 0;
    while (i.hasNext())
      array_[size++] = i.next();
  }
  /**
   * Create a new CopyOnWriteArrayList holding a copy of given array
   * 
   * @param toCopyIn
   *            the array. A copy of this array is used as the internal array.
   */
  public CopyOnWriteArrayList(Object[] toCopyIn)
  {
    copyIn(toCopyIn, 0, toCopyIn.length);
  }
  /**
   * Replace the held array with a copy of the <code>n</code> elements of
   * the provided array, starting at position <code>first</code>. To copy
   * an entire array, call with arguments (array, 0, array.length).
   * 
   * @param toCopyIn
   *            the array. A copy of the indicated elements of this array is
   *            used as the internal array.
   * @param first
   *            The index of first position of the array to start copying
   *            from.
   * @param n
   *            the number of elements to copy. This will be the new size of
   *            the list.
   */
  public synchronized void copyIn(Object[] toCopyIn, int first, int n)
  {
    array_ = new Object[n];
    System.arraycopy(toCopyIn, first, array_, 0, n);
  }
  /**
   * Returns the number of components in this list.
   * 
   * @return the number of components in this list.
   */
  public int size()
  {
    return array().length;
  }
  /**
   * Tests if this list has no components.
   * 
   * @return <code>true</code> if this list has no components;
   *         <code>false</code> otherwise.
   */
  public boolean isEmpty()
  {
    return size() == 0;
  }
  /**
   * Returns true if this list contains the specified element.
   * 
   * @param o
   *            element whose presence in this List is to be tested.
   */
  public boolean contains(Object elem)
  {
    Object[] elementData = array();
    int len = elementData.length;
    return indexOf(elem, elementData, len) >= 0;
  }
  /**
   * Searches for the first occurence of the given argument, testing for
   * equality using the <code>equals</code> method.
   * 
   * @param elem
   *            an object.
   * @return the index of the first occurrence of the argument in this list;
   *         returns <code>-1</code> if the object is not found.
   * @see Object#equals(Object)
   */
  public int indexOf(Object elem)
  {
    Object[] elementData = array();
    int len = elementData.length;
    return indexOf(elem, elementData, len);
  }

  /**
   * static version allows repeated call without needed to grab lock for array
   * each time
   * @param elem 
   * @param elementData 
   * @param len 
   * @return The index that is found. -1 if not found
   */
  protected static int indexOf(Object elem, Object[] elementData, int len)
  {
    if (elem == null)
    {
      for (int i = 0; i < len; i++)
        if (elementData[i] == null)
          return i;
    }
    else
    {
      for (int i = 0; i < len; i++)
        if (elem.equals(elementData[i]))
          return i;
    }
    return -1;
  }
  /**
   * Searches for the first occurence of the given argument, beginning the
   * search at <code>index</code>, and testing for equality using the
   * <code>equals</code> method.
   * 
   * @param elem
   *            an object.
   * @param index
   *            the index to start searching from.
   * @return the index of the first occurrence of the object argument in this
   *         List at position <code>index</code> or later in the List;
   *         returns <code>-1</code> if the object is not found.
   * @see Object#equals(Object)
   */
  // needed in order to compile on 1.2b3
  public int indexOf(Object elem, int index)
  {
    Object[] elementData = array();
    int elementCount = elementData.length;
    if (elem == null)
    {
      for (int i = index; i < elementCount; i++)
        if (elementData[i] == null)
          return i;
    }
    else
    {
      for (int i = index; i < elementCount; i++)
        if (elem.equals(elementData[i]))
          return i;
    }
    return -1;
  }
  /**
   * Returns the index of the last occurrence of the specified object in this
   * list.
   * 
   * @param elem
   *            the desired component.
   * @return the index of the last occurrence of the specified object in this
   *         list; returns -1 if the object is not found.
   */
  public int lastIndexOf(Object elem)
  {
    Object[] elementData = array();
    int len = elementData.length;
    return lastIndexOf(elem, elementData, len);
  }
  protected static int lastIndexOf(Object elem, Object[] elementData, int len)
  {
    if (elem == null)
    {
      for (int i = len - 1; i >= 0; i--)
        if (elementData[i] == null)
          return i;
    }
    else
    {
      for (int i = len - 1; i >= 0; i--)
        if (elem.equals(elementData[i]))
          return i;
    }
    return -1;
  }
  /**
   * Searches backwards for the specified object, starting from the specified
   * index, and returns an index to it.
   * 
   * @param elem
   *            the desired component.
   * @param index
   *            the index to start searching from.
   * @return the index of the last occurrence of the specified object in this
   *         List at position less than index in the List; -1 if the object is
   *         not found.
   */
  public int lastIndexOf(Object elem, int index)
  {
    // needed in order to compile on 1.2b3
    Object[] elementData = array();
    if (elem == null)
    {
      for (int i = index; i >= 0; i--)
        if (elementData[i] == null)
          return i;
    }
    else
    {
      for (int i = index; i >= 0; i--)
        if (elem.equals(elementData[i]))
          return i;
    }
    return -1;
  }
  /**
   * Returns a shallow copy of this list. (The elements themselves are not
   * copied.)
   * 
   * @return a clone of this list.
   */
  public Object clone()
  {
    try
    {
      Object[] elementData = array();
      CopyOnWriteArrayList v = (CopyOnWriteArrayList)super.clone();
      v.array_ = new Object[elementData.length];
      System.arraycopy(elementData, 0, v.array_, 0, elementData.length);
      return v;
    }
    catch (CloneNotSupportedException e)
    {
      // this shouldn"t happen, since we are Cloneable
      throw new InternalError();
    }
  }
  /**
   * Returns an array containing all of the elements in this list in the
   * correct order.
   */
  public Object[] toArray()
  {
    Object[] elementData = array();
    Object[] result = new Object[elementData.length];
    System.arraycopy(elementData, 0, result, 0, elementData.length);
    return result;
  }
  /**
   * Returns an array containing all of the elements in this list in the
   * correct order. The runtime type of the returned array is that of the
   * specified array. If the list fits in the specified array, it is returned
   * therein. Otherwise, a new array is allocated with the runtime type of the
   * specified array and the size of this list.
   * <p>
   * If the list fits in the specified array with room to spare (i.e., the
   * array has more elements than the list), the element in the array
   * immediately following the end of the collection is set to null. This is
   * useful in determining the length of the list <em>only</em> if the
   * caller knows that the list does not contain any null elements.
   * 
   * @param a
   *            the array into which the elements of the list are to be
   *            stored, if it is big enough; otherwise, a new array of the
   *            same runtime type is allocated for this purpose.
   * @return an array containing the elements of the list.
   * @exception ArrayStoreException
   *                the runtime type of a is not a supertype of the runtime
   *                type of every element in this list.
   */
  public Object[] toArray(Object a[])
  {
    Object[] elementData = array();
    if (a.length < elementData.length)
      a = (Object[])java.lang.reflect.Array.newInstance(a.getClass().getComponentType(),
          elementData.length);
    System.arraycopy(elementData, 0, a, 0, elementData.length);
    if (a.length > elementData.length)
      a[elementData.length] = null;
    return a;
  }
  // Positional Access Operations
  /**
   * Returns the element at the specified position in this list.
   * 
   * @param index
   *            index of element to return.
   * @exception IndexOutOfBoundsException
   *                index is out of range (index &lt; 0 || index &gt;=
   *                size()).
   */
  public Object get(int index)
  {
    Object[] elementData = array();
    rangeCheck(index, elementData.length);
    return elementData[index];
  }
  /**
   * Replaces the element at the specified position in this list with the
   * specified element.
   * 
   * @param index
   *            index of element to replace.
   * @param element
   *            element to be stored at the specified position.
   * @return the element previously at the specified position.
   * @exception IndexOutOfBoundsException
   *                index out of range (index &lt; 0 || index &gt;= size()).
   */
  public synchronized Object set(int index, Object element)
  {
    int len = array_.length;
    rangeCheck(index, len);
    Object oldValue = array_[index];
    boolean same = (oldValue == element || (element != null && element.equals(oldValue)));
    if (!same)
    {
      Object[] newArray = new Object[len];
      System.arraycopy(array_, 0, newArray, 0, len);
      newArray[index] = element;
      array_ = newArray;
    }
    return oldValue;
  }
  /**
   * Appends the specified element to the end of this list.
   * 
   * @param element
   *            element to be appended to this list.
   * @return true (as per the general contract of Collection.add).
   */
  public synchronized boolean add(Object element)
  {
    int len = array_.length;
    Object[] newArray = new Object[len + 1];
    System.arraycopy(array_, 0, newArray, 0, len);
    newArray[len] = element;
    array_ = newArray;
    return true;
  }
  /**
   * Inserts the specified element at the specified position in this list.
   * Shifts the element currently at that position (if any) and any subsequent
   * elements to the right (adds one to their indices).
   * 
   * @param index
   *            index at which the specified element is to be inserted.
   * @param element
   *            element to be inserted.
   * @exception IndexOutOfBoundsException
   *                index is out of range (index &lt; 0 || index &gt; size()).
   */
  public synchronized void add(int index, Object element)
  {
    int len = array_.length;
    if (index > len || index < 0)
      throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + len);
    Object[] newArray = new Object[len + 1];
    System.arraycopy(array_, 0, newArray, 0, index);
    newArray[index] = element;
    System.arraycopy(array_, index, newArray, index + 1, len - index);
    array_ = newArray;
  }
  /**
   * Removes the element at the specified position in this list. Shifts any
   * subsequent elements to the left (subtracts one from their indices).
   * Returns the element that was removed from the list.
   * 
   * @param index
   *            the index of the element to removed.
   * @exception IndexOutOfBoundsException
   *                index out of range (index &lt; 0 || index &gt;= size()).
   */
  public synchronized Object remove(int index)
  {
    int len = array_.length;
    rangeCheck(index, len);
    Object oldValue = array_[index];
    Object[] newArray = new Object[len - 1];
    System.arraycopy(array_, 0, newArray, 0, index);
    int numMoved = len - index - 1;
    if (numMoved > 0)
      System.arraycopy(array_, index + 1, newArray, index, numMoved);
    array_ = newArray;
    return oldValue;
  }
  /**
   * Removes a single instance of the specified element from this Collection,
   * if it is present (optional operation). More formally, removes an element
   * <code>e</code> such that <code>(o==null ? e==null :
   * o.equals(e))</code>,
   * if the Collection contains one or more such elements. Returns true if the
   * Collection contained the specified element (or equivalently, if the
   * Collection changed as a result of the call).
   * 
   * @param element
   *            element to be removed from this Collection, if present.
   * @return true if the Collection changed as a result of the call.
   */
  public synchronized boolean remove(Object element)
  {
    int len = array_.length;
    if (len == 0)
      return false;
    // Copy while searching for element to remove
    // This wins in the normal case of element being present
    int newlen = len - 1;
    Object[] newArray = new Object[newlen];
    for (int i = 0; i < newlen; ++i)
    {
      if (element == array_[i] || (element != null && element.equals(array_[i])))
      {
        // found one; copy remaining and exit
        for (int k = i + 1; k < len; ++k)
          newArray[k - 1] = array_[k];
        array_ = newArray;
        return true;
      }
      else
        newArray[i] = array_[i];
    }
    // special handling for last cell
    if (element == array_[newlen] || (element != null && element.equals(array_[newlen])))
    {
      array_ = newArray;
      return true;
    }
    else
      return false; // throw away copy
  }

  /**
   * Removes from this List all of the elements whose index is between
   * fromIndex, inclusive and toIndex, exclusive. Shifts any succeeding
   * elements to the left (reduces their index). This call shortens the List
   * by (toIndex - fromIndex) elements. (If toIndex==fromIndex, this operation
   * has no effect.)
   * 
   * @param fromIndex
   *            index of first element to be removed.
   * @param toIndex
   *            index after last element to be removed.
   * @exception IndexOutOfBoundsException
   *                fromIndex or toIndex out of range (fromIndex &lt; 0 ||
   *                fromIndex &gt;= size() || toIndex &gt; size() || toIndex
   *                &lt; fromIndex).
   */
  public synchronized void removeRange(int fromIndex, int toIndex)
  {
    int len = array_.length;
    if (fromIndex < 0 || fromIndex >= len || toIndex > len || toIndex < fromIndex)
      throw new IndexOutOfBoundsException();
    int numMoved = len - toIndex;
    int newlen = len - (toIndex - fromIndex);
    Object[] newArray = new Object[newlen];
    System.arraycopy(array_, 0, newArray, 0, fromIndex);
    System.arraycopy(array_, toIndex, newArray, fromIndex, numMoved);
    array_ = newArray;
  }

  /**
   * Append the element if not present. This operation can be used to obtain
   * Set semantics for lists.
   * 
   * @param element
   *            element to be added to this Collection, if absent.
   * @return true if added
   */
  public synchronized boolean addIfAbsent(Object element)
  {
    // Copy while checking if already present.
    // This wins in the most common case where it is not present
    int len = array_.length;
    Object[] newArray = new Object[len + 1];
    for (int i = 0; i < len; ++i)
    {
      if (element == array_[i] || (element != null && element.equals(array_[i])))
        return false; // exit, throwing away copy
      else
        newArray[i] = array_[i];
    }
    newArray[len] = element;
    array_ = newArray;
    return true;
  }
  /**
   * Returns true if this Collection contains all of the elements in the
   * specified Collection.
   * <p>
   * This implementation iterates over the specified Collection, checking each
   * element returned by the Iterator in turn to see if it"s contained in this
   * Collection. If all elements are so contained true is returned, otherwise
   * false.
   * 
   */
  public boolean containsAll(Collection c)
  {
    Object[] elementData = array();
    int len = elementData.length;
    Iterator e = c.iterator();
    while (e.hasNext())
      if (indexOf(e.next(), elementData, len) < 0)
        return false;
    return true;
  }

  /**
   * Removes from this Collection all of its elements that are contained in
   * the specified Collection. This is a particularly expensive operation in
   * this class because of the need for an internal temporary array.
   * <p>
   * 
   * @return true if this Collection changed as a result of the call.
   */
  public synchronized boolean removeAll(Collection c)
  {
    Object[] elementData = array_;
    int len = elementData.length;
    if (len == 0)
      return false;
    // temp array holds those elements we know we want to keep
    Object[] temp = new Object[len];
    int newlen = 0;
    for (int i = 0; i < len; ++i)
    {
      Object element = elementData[i];
      if (!c.contains(element))
      {
        temp[newlen++] = element;
      }
    }
    if (newlen == len)
      return false;
    // copy temp as new array
    Object[] newArray = new Object[newlen];
    System.arraycopy(temp, 0, newArray, 0, newlen);
    array_ = newArray;
    return true;
  }
  /**
   * Retains only the elements in this Collection that are contained in the
   * specified Collection (optional operation). In other words, removes from
   * this Collection all of its elements that are not contained in the
   * specified Collection.
   * 
   * @return true if this Collection changed as a result of the call.
   */
  public synchronized boolean retainAll(Collection c)
  {
    Object[] elementData = array_;
    int len = elementData.length;
    if (len == 0)
      return false;
    Object[] temp = new Object[len];
    int newlen = 0;
    for (int i = 0; i < len; ++i)
    {
      Object element = elementData[i];
      if (c.contains(element))
      {
        temp[newlen++] = element;
      }
    }
    if (newlen == len)
      return false;
    Object[] newArray = new Object[newlen];
    System.arraycopy(temp, 0, newArray, 0, newlen);
    array_ = newArray;
    return true;
  }
  /**
   * Appends all of the elements in the specified Collection that are not
   * already contained in this list, to the end of this list, in the order
   * that they are returned by the specified Collection"s Iterator.
   * 
   * @param c
   *            elements to be added into this list.
   * @return the number of elements added
   */
  public synchronized int addAllAbsent(Collection c)
  {
    int numNew = c.size();
    if (numNew == 0)
      return 0;
    Object[] elementData = array_;
    int len = elementData.length;
    Object[] temp = new Object[numNew];
    int added = 0;
    Iterator e = c.iterator();
    while (e.hasNext())
    {
      Object element = e.next();
      if (indexOf(element, elementData, len) < 0)
      {
        if (indexOf(element, temp, added) < 0)
        {
          temp[added++] = element;
        }
      }
    }
    if (added == 0)
      return 0;
    Object[] newArray = new Object[len + added];
    System.arraycopy(elementData, 0, newArray, 0, len);
    System.arraycopy(temp, 0, newArray, len, added);
    array_ = newArray;
    return added;
  }
  /**
   * Removes all of the elements from this list.
   * 
   */
  public synchronized void clear()
  {
    array_ = new Object[0];
  }
  /**
   * Appends all of the elements in the specified Collection to the end of
   * this list, in the order that they are returned by the specified
   * Collection"s Iterator.
   * 
   * @param c
   *            elements to be inserted into this list.
   */
  public synchronized boolean addAll(Collection c)
  {
    int numNew = c.size();
    if (numNew == 0)
      return false;
    int len = array_.length;
    Object[] newArray = new Object[len + numNew];
    System.arraycopy(array_, 0, newArray, 0, len);
    Iterator e = c.iterator();
    for (int i = 0; i < numNew; i++)
      newArray[len++] = e.next();
    array_ = newArray;
    return true;
  }
  /**
   * Inserts all of the elements in the specified Collection into this list,
   * starting at the specified position. Shifts the element currently at that
   * position (if any) and any subsequent elements to the right (increases
   * their indices). The new elements will appear in the list in the order
   * that they are returned by the specified Collection"s iterator.
   * 
   * @param index
   *            index at which to insert first element from the specified
   *            collection.
   * @param c
   *            elements to be inserted into this list.
   * @exception IndexOutOfBoundsException
   *                index out of range (index &lt; 0 || index &gt; size()).
   */
  public synchronized boolean addAll(int index, Collection c)
  {
    int len = array_.length;
    if (index > len || index < 0)
      throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + len);
    int numNew = c.size();
    if (numNew == 0)
      return false;
    Object[] newArray = new Object[len + numNew];
    System.arraycopy(array_, 0, newArray, 0, len);
    int numMoved = len - index;
    if (numMoved > 0)
      System.arraycopy(array_, index, newArray, index + numNew, numMoved);
    Iterator e = c.iterator();
    for (int i = 0; i < numNew; i++)
      newArray[index++] = e.next();
    array_ = newArray;
    return true;
  }
  /**
   * Check if the given index is in range. If not, throw an appropriate
   * runtime exception.
   * @param index 
   * @param length 
   */
  protected void rangeCheck(int index, int length)
  {
    if (index >= length || index < 0)
      throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + length);
  }
  /**
   * Save the state of the list to a stream (i.e., serialize it).
   * @param s 
   * @throws java.io.IOException 
   * 
   * @serialData The length of the array backing the list is emitted (int),
   *             followed by all of its elements (each an Object) in the
   *             proper order.
   */
  private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException
  {
    // Write out element count, and any hidden stuff
    s.defaultWriteObject();
    Object[] elementData = array();
    // Write out array length
    s.writeInt(elementData.length);
    // Write out all elements in the proper order.
    for (int i = 0; i < elementData.length; i++)
      s.writeObject(elementData[i]);
  }
  /**
   * Reconstitute the list from a stream (i.e., deserialize it).
   * @param s 
   * @throws java.io.IOException 
   * @throws ClassNotFoundException 
   */
  private synchronized void readObject(java.io.ObjectInputStream s) throws java.io.IOException,
      ClassNotFoundException
  {
    // Read in size, and any hidden stuff
    s.defaultReadObject();
    // Read in array length and allocate array
    int arrayLength = s.readInt();
    Object[] elementData = new Object[arrayLength];
    // Read in all elements in the proper order.
    for (int i = 0; i < elementData.length; i++)
      elementData[i] = s.readObject();
    array_ = elementData;
  }
  /**
   * Returns a string representation of this Collection, containing the String
   * representation of each element.
   */
  public String toString()
  {
    StringBuffer buf = new StringBuffer();
    Iterator e = iterator();
    buf.append("[");
    int maxIndex = size() - 1;
    for (int i = 0; i <= maxIndex; i++)
    {
      buf.append(String.valueOf(e.next()));
      if (i < maxIndex)
        buf.append(", ");
    }
    buf.append("]");
    return buf.toString();
  }

  /**
   * Compares the specified Object with this List for equality. Returns true
   * if and only if the specified Object is also a List, both Lists have the
   * same size, and all corresponding pairs of elements in the two Lists are
   * <em>equal</em>. (Two elements <code>e1</code> and <code>e2</code>
   * are <em>equal</em> if
   * <code>(e1==null ? e2==null : e1.equals(e2))</code>.) In other words,
   * two Lists are defined to be equal if they contain the same elements in
   * the same order.
   * <p>
   * This implementation first checks if the specified object is this List. If
   * so, it returns true; if not, it checks if the specified object is a List.
   * If not, it returns false; if so, it iterates over both lists, comparing
   * corresponding pairs of elements. If any comparison returns false, this
   * method returns false. If either Iterator runs out of elements before
   * before the other it returns false (as the Lists are of unequal length);
   * otherwise it returns true when the iterations complete.
   * 
   * @param o
   *            the Object to be compared for equality with this List.
   * @return true if the specified Object is equal to this List.
   */
  public boolean equals(Object o)
  {
    if (o == this)
      return true;
    if (!(o instanceof List))
      return false;
    List l2 = (List)(o);
    if (size() != l2.size())
      return false;
    ListIterator e1 = listIterator();
    ListIterator e2 = l2.listIterator();
    while (e1.hasNext())
    {
      Object o1 = e1.next();
      Object o2 = e2.next();
      if (!(o1 == null ? o2 == null : o1.equals(o2)))
        return false;
    }
    return true;
  }
  /**
   * Returns the hash code value for this List.
   * <p>
   * This implementation uses exactly the code that is used to define the List
   * hash function in the documentation for List.hashCode.
   */
  public int hashCode()
  {
    int hashCode = 1;
    Iterator i = iterator();
    while (i.hasNext())
    {
      Object obj = i.next();
      hashCode = 31 * hashCode + (obj == null ? 0 : obj.hashCode());
    }
    return hashCode;
  }
  /**
   * Returns an Iterator over the elements contained in this collection. The
   * iterator provides a snapshot of the state of the list when the iterator
   * was constructed. No synchronization is needed while traversing the
   * iterator. The iterator does <em>NOT</em> support the
   * <code>remove</code> method.
   */
  public Iterator iterator()
  {
    return new COWIterator(array(), 0);
  }
  /**
   * Returns an Iterator of the elements in this List (in proper sequence).
   * The iterator provides a snapshot of the state of the list when the
   * iterator was constructed. No synchronization is needed while traversing
   * the iterator. The iterator does <em>NOT</em> support the
   * <code>remove</code>, <code>set</code>, or <code>add</code>
   * methods.
   * 
   */
  public ListIterator listIterator()
  {
    return new COWIterator(array(), 0);
  }
  /**
   * Returns a ListIterator of the elements in this List (in proper sequence),
   * starting at the specified position in the List. The specified index
   * indicates the first element that would be returned by an initial call to
   * nextElement. An initial call to previousElement would return the element
   * with the specified index minus one. The ListIterator returned by this
   * implementation will throw an UnsupportedOperationException in its remove,
   * set and add methods.
   * 
   * @param index
   *            index of first element to be returned from the ListIterator
   *            (by a call to getNext).
   * @exception IndexOutOfBoundsException
   *                index is out of range (index &lt; 0 || index &gt; size()).
   */
  public ListIterator listIterator(final int index)
  {
    Object[] elementData = array();
    int len = elementData.length;
    if (index < 0 || index > len)
      throw new IndexOutOfBoundsException("Index: " + index);
    return new COWIterator(array(), index);
  }
  protected static class COWIterator implements ListIterator
  {
    /** Snapshot of the array * */
    protected final Object[] array;
    /**
     * Index of element to be returned by subsequent call to next.
     */
    protected int cursor;
    protected COWIterator(Object[] elementArray, int initialCursor)
    {
      array = elementArray;
      cursor = initialCursor;
    }
    public boolean hasNext()
    {
      return cursor < array.length;
    }
    public boolean hasPrevious()
    {
      return cursor > 0;
    }
    public Object next()
    {
      try
      {
        return array[cursor++];
      }
      catch (IndexOutOfBoundsException ex)
      {
        throw new NoSuchElementException();
      }
    }
    public Object previous()
    {
      try
      {
        return array[--cursor];
      }
      catch (IndexOutOfBoundsException e)
      {
        throw new NoSuchElementException();
      }
    }
    public int nextIndex()
    {
      return cursor;
    }
    public int previousIndex()
    {
      return cursor - 1;
    }
    /**
     * Not supported. Always throws UnsupportedOperationException.
     * 
     * @exception UnsupportedOperationException
     *                remove is not supported by this Iterator.
     */
    public void remove()
    {
      throw new UnsupportedOperationException();
    }
    /**
     * Not supported. Always throws UnsupportedOperationException.
     * 
     * @exception UnsupportedOperationException
     *                set is not supported by this Iterator.
     */
    public void set(Object o)
    {
      throw new UnsupportedOperationException();
    }
    /**
     * Not supported. Always throws UnsupportedOperationException.
     * 
     * @exception UnsupportedOperationException
     *                add is not supported by this Iterator.
     */
    public void add(Object o)
    {
      throw new UnsupportedOperationException();
    }
  }

  /**
   * Returns a view of the portion of this List between fromIndex, inclusive,
   * and toIndex, exclusive. The returned List is backed by this List, so
   * changes in the returned List are reflected in this List, and vice-versa.
   * While mutative operations are supported, they are probably not very
   * useful for CopyOnWriteArrays.
   * </p>
   * The semantics of the List returned by this method become undefined if the
   * backing list (i.e., this List) is <i>structurally modified</i> in any
   * way other than via the returned List. (Structural modifications are those
   * that change the size of the List, or otherwise perturb it in such a
   * fashion that iterations in progress may yield incorrect results.)
   * 
   * @param fromIndex
   *            low endpoint (inclusive) of the subList.
   * @param toKey
   *            high endpoint (exclusive) of the subList.
   * @return a view of the specified range within this List.
   * @exception IndexOutOfBoundsException
   *                Illegal endpoint index value (fromIndex &lt; 0 || toIndex
   *                &gt; size || fromIndex &gt; toIndex).
   */
  public synchronized List subList(int fromIndex, int toIndex)
  {
    // synchronized since sublist ctor depends on it.
    int len = array_.length;
    if (fromIndex < 0 || toIndex > len || fromIndex > toIndex)
      throw new IndexOutOfBoundsException();
    return new COWSubList(this, fromIndex, toIndex);
  }
  protected static class COWSubList extends AbstractList
  {
    /*
     * This is currently a bit sleazy. The class extends AbstractList merely
     * for convenience, to avoid having to define addAll, etc. This doesn"t
     * hurt, but is stupid and wasteful. This class does not need or use
     * modCount mechanics in AbstractList, but does need to check for
     * concurrent modification using similar mechanics. On each operation,
     * the array that we expect the backing list to use is checked and
     * updated. Since we do this for all of the base operations invoked by
     * those defined in AbstractList, all is well.
     * 
     * It"s not clear whether this is worth cleaning up. The kinds of list
     * operations inherited from AbstractList are are already so slow on COW
     * sublists that adding a bit more space/time doesn"t seem even
     * noticeable.
     */
    protected final CopyOnWriteArrayList l;
    protected final int offset;
    protected int size;
    protected Object[] expectedArray;
    protected COWSubList(CopyOnWriteArrayList list, int fromIndex, int toIndex)
    {
      l = list;
      expectedArray = l.array();
      offset = fromIndex;
      size = toIndex - fromIndex;
    }
    // only call this holding l"s lock
    protected void checkForComodification()
    {
      if (l.array_ != expectedArray)
        throw new ConcurrentModificationException();
    }
    // only call this holding l"s lock
    protected void rangeCheck(int index)
    {
      if (index < 0 || index >= size)
        throw new IndexOutOfBoundsException("Index: " + index + ",Size: " + size);
    }

    public Object set(int index, Object element)
    {
      synchronized (l)
      {
        rangeCheck(index);
        checkForComodification();
        Object x = l.set(index + offset, element);
        expectedArray = l.array_;
        return x;
      }
    }
    public Object get(int index)
    {
      synchronized (l)
      {
        rangeCheck(index);
        checkForComodification();
        return l.get(index + offset);
      }
    }
    public int size()
    {
      synchronized (l)
      {
        checkForComodification();
        return size;
      }
    }
    public void add(int index, Object element)
    {
      synchronized (l)
      {
        checkForComodification();
        if (index < 0 || index > size)
          throw new IndexOutOfBoundsException();
        l.add(index + offset, element);
        expectedArray = l.array_;
        size++;
      }
    }
    public Object remove(int index)
    {
      synchronized (l)
      {
        rangeCheck(index);
        checkForComodification();
        Object result = l.remove(index + offset);
        expectedArray = l.array_;
        size--;
        return result;
      }
    }
    public Iterator iterator()
    {
      synchronized (l)
      {
        checkForComodification();
        return new COWSubListIterator(0);
      }
    }
    public ListIterator listIterator(final int index)
    {
      synchronized (l)
      {
        checkForComodification();
        if (index < 0 || index > size)
          throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
        return new COWSubListIterator(index);
      }
    }
    protected class COWSubListIterator implements ListIterator
    {
      protected final ListIterator i;
      protected final int index;
      protected COWSubListIterator(int index)
      {
        this.index = index;
        i = l.listIterator(index + offset);
      }
      public boolean hasNext()
      {
        return nextIndex() < size;
      }
      public Object next()
      {
        if (hasNext())
          return i.next();
        else
          throw new NoSuchElementException();
      }
      public boolean hasPrevious()
      {
        return previousIndex() >= 0;
      }
      public Object previous()
      {
        if (hasPrevious())
          return i.previous();
        else
          throw new NoSuchElementException();
      }
      public int nextIndex()
      {
        return i.nextIndex() - offset;
      }
      public int previousIndex()
      {
        return i.previousIndex() - offset;
      }
      public void remove()
      {
        throw new UnsupportedOperationException();
      }
      public void set(Object o)
      {
        throw new UnsupportedOperationException();
      }
      public void add(Object o)
      {
        throw new UnsupportedOperationException();
      }
    }

    public List subList(int fromIndex, int toIndex)
    {
      synchronized (l)
      {
        checkForComodification();
        if (fromIndex < 0 || toIndex > size)
          throw new IndexOutOfBoundsException();
        return new COWSubList(l, fromIndex + offset, toIndex + offset);
      }
    }

  }
}





Convert a List (ArrayList) to an Array with full length array

   
import java.util.ArrayList;
import java.util.List;
public class Main {
  public static void main(String[] args) {
    List<String> carList = new ArrayList<String>();
    carList.add("A");
    carList.add("B");
    carList.add("C");
    carList.add("D");
    String[] carArray = carList.toArray(new String[0]);
    for (String car : carArray) {
      System.out.println(car);
    }
  }
}
/*
A
B
C
D
*/





Convert a List (ArrayList) to an Array with zero length array

   
import java.util.ArrayList;
import java.util.List;
public class Main {
  public static void main(String[] args) {
    List<String> carList = new ArrayList<String>();
    carList.add("A");
    carList.add("B");
    carList.add("C");
    carList.add("D");
    String[] carArray = carList.toArray(new String[0]);
    for (String car : carArray) {
      System.out.println(car);
    }
  }
}
/*
A
B
C
D
*/





Convert an ArrayList into an array.

    
import java.util.ArrayList;
class ArrayListToArray {
  public static void main(String args[]) {
    ArrayList<Integer> al = new ArrayList<Integer>();
    al.add(1);
    al.add(2);
    al.add(3);
    al.add(4);
    System.out.println("Contents of al: " + al);
    Integer ia[] = new Integer[al.size()];
    ia = al.toArray(ia);
    int sum = 0;
    for (int i : ia)
      sum += i;
    System.out.println("Sum is: " + sum);
  }
}





Copy all elements of Java ArrayList to an Object Array

   
import java.util.ArrayList;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    arrayList.add("4");
    arrayList.add("5");
    Object[] objArray = arrayList.toArray();
    for (Object obj : objArray)
      System.out.println(obj);
  }
}





Copy Elements of ArrayList to Java Vector

   
import java.util.ArrayList;
import java.util.Collections;
import java.util.Vector;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    arrayList.add("4");
    arrayList.add("5");
    Vector<String> v = new Vector<String>();
    v.add("A");
    v.add("B");
    v.add("D");
    v.add("E");
    v.add("F");
    v.add("G");
    v.add("H");
    System.out.println(v);
    Collections.copy(v, arrayList);
    System.out.println(v);
  }
}
/*
[A, B, D, E, F, G, H]
[1, 2, 3, 4, 5, G, H]
*/





Copy Elements of One Java ArrayList to Another Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList1 = new ArrayList<String>();
    arrayList1.add("1");
    arrayList1.add("2");
    arrayList1.add("3");
    ArrayList<String> arrayList2 = new ArrayList<String>();
    arrayList2.add("One");
    arrayList2.add("Two");
    arrayList2.add("Three");
    arrayList2.add("Four");
    arrayList2.add("Five");
    System.out.println(arrayList2);
    Collections.copy(arrayList2, arrayList1);
    System.out.println(arrayList2);
  }
}
/*
[One, Two, Three, Four, Five]
[1, 2, 3, Four, Five]
*/





Copy On Write ArrayList

   
/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group.  Adapted and released, under explicit permission,
 * from JDK ArrayList.java which carries the following copyright:
 *
 * Copyright 1997 by Sun Microsystems, Inc.,
 * 901 San Antonio Road, Palo Alto, California, 94303, U.S.A.
 * All rights reserved.
 *
 * This software is the confidential and proprietary information
 * of Sun Microsystems, Inc. ("Confidential Information").  You
 * shall not disclose such Confidential Information and shall use
 * it only in accordance with the terms of the license agreement
 * you entered into with Sun.
 */
import java.util.AbstractList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.RandomAccess;

/**
 * A thread-safe variant of {@link java.util.ArrayList} in which all mutative
 * operations (<tt>add</tt>, <tt>set</tt>, and so on) are implemented by
 * making a fresh copy of the underlying array.
 *
 * <p> This is ordinarily too costly, but may be <em>more</em> efficient
 * than alternatives when traversal operations vastly outnumber
 * mutations, and is useful when you cannot or don"t want to
 * synchronize traversals, yet need to preclude interference among
 * concurrent threads.  The "snapshot" style iterator method uses a
 * reference to the state of the array at the point that the iterator
 * was created. This array never changes during the lifetime of the
 * iterator, so interference is impossible and the iterator is
 * guaranteed not to throw <tt>ConcurrentModificationException</tt>.
 * The iterator will not reflect additions, removals, or changes to
 * the list since the iterator was created.  Element-changing
 * operations on iterators themselves (<tt>remove</tt>, <tt>set</tt>, and
 * <tt>add</tt>) are not supported. These methods throw
 * <tt>UnsupportedOperationException</tt>.
 *
 * <p>All elements are permitted, including <tt>null</tt>.
 *
 * <p>This class is a member of the
 * .
 *
 * @since 1.5
 * @author Doug Lea
 */
public class CopyOnWriteArrayList
    implements List, RandomAccess, Cloneable, java.io.Serializable {
    private static final long serialVersionUID = 8673264195747942595L;
    /** The array, accessed only via getArray/setArray. */
    private volatile transient Object[] array;
    /**
     * This has been made public to support more efficient iteration.
     * <strong>DO NOT MODIFY this array upon getting it</strong>.
     * Otherwise you risk wreaking havoc on your list. In fact, if you are
     * not the author of this comment, you probably shouldn"t use it at all.
     * @return this lists internal array
     */
    public Object[]  getArray()    { return array; }
    
    void      setArray(Object[] a) { array = a; }
    /**
     * Creates an empty list.
     */
    public CopyOnWriteArrayList() {
        setArray(new Object[0]);
    }
    /**
     * Creates a list containing the elements of the specified
     * collection, in the order they are returned by the collection"s
     * iterator.
     *
     * @param c the collection of initially held elements
     * @throws NullPointerException if the specified collection is null
     */
    public CopyOnWriteArrayList(Collection c) {
        Object[] elements = new Object[c.size()];
        int size = 0;
        for (Iterator itr = c.iterator(); itr.hasNext(); ) {
            Object e = itr.next();
            elements[size++] = e;
        }
        setArray(elements);
    }
    /**
     * Creates a list holding a copy of the given array.
     *
     * @param toCopyIn the array (a copy of this array is used as the
     *        internal array)
     * @throws NullPointerException if the specified array is null
     */
    public CopyOnWriteArrayList(Object[] toCopyIn) {
        copyIn(toCopyIn, 0, toCopyIn.length);
    }
    /**
     * Replaces the held array with a copy of the <tt>n</tt> elements
     * of the provided array, starting at position <tt>first</tt>.  To
     * copy an entire array, call with arguments (array, 0,
     * array.length).
     * @param toCopyIn the array. A copy of the indicated elements of
     * this array is used as the internal array.
     * @param first The index of first position of the array to
     * start copying from.
     * @param n the number of elements to copy. This will be the new size of
     * the list.
     */
    private void copyIn(Object[] toCopyIn, int first, int n) {
        int limit = first + n;
        if (limit > toCopyIn.length)
            throw new IndexOutOfBoundsException();
        Object[] newElements = copyOfRange(toCopyIn, first, limit,
                                          Object[].class);
        synchronized (this) { setArray(newElements); }
    }
    /**
     * Returns the number of elements in this list.
     *
     * @return the number of elements in this list
     */
    public int size() {
        return getArray().length;
    }
    /**
     * Returns <tt>true</tt> if this list contains no elements.
     *
     * @return <tt>true</tt> if this list contains no elements
     */
    public boolean isEmpty() {
        return size() == 0;
    }
    /**
     * Test for equality, coping with nulls.
     */
    private static boolean eq(Object o1, Object o2) {
        return (o1 == null ? o2 == null : o1.equals(o2));
    }
    /**
     * static version of indexOf, to allow repeated calls without
     * needing to re-acquire array each time.
     * @param o element to search for
     * @param elements the array
     * @param index first index to search
     * @param fence one past last index to search
     * @return index of element, or -1 if absent
     */
    private static int indexOf(Object o, Object[] elements,
                               int index, int fence) {
        if (o == null) {
            for (int i = index; i < fence; i++)
                if (elements[i] == null)
                    return i;
        } else {
            for (int i = index; i < fence; i++)
                if (o.equals(elements[i]))
                    return i;
        }
        return -1;
    }
    /**
     * static version of lastIndexOf.
     * @param o element to search for
     * @param elements the array
     * @param index first index to search
     * @return index of element, or -1 if absent
     */
    private static int lastIndexOf(Object o, Object[] elements, int index) {
        if (o == null) {
            for (int i = index; i >= 0; i--)
                if (elements[i] == null)
                    return i;
        } else {
            for (int i = index; i >= 0; i--)
                if (o.equals(elements[i]))
                    return i;
        }
        return -1;
    }
    /**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested
     * @return <tt>true</tt> if this list contains the specified element
     */
    public boolean contains(Object o) {
        Object[] elements = getArray();
        return indexOf(o, elements, 0, elements.length) >= 0;
    }
    /**
     * {@inheritDoc}
     */
    public int indexOf(Object o) {
        Object[] elements = getArray();
        return indexOf(o, elements, 0, elements.length);
    }

    /**
     * Returns the index of the first occurrence of the specified element in
     * this list, searching forwards from <tt>index</tt>, or returns -1 if
     * the element is not found.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(i&nbsp;&gt;=&nbsp;index&nbsp;&amp;&amp;&nbsp;(e==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;e.equals(get(i))))</tt>,
     * or -1 if there is no such index.
     *
     * @param e element to search for
     * @param index index to start searching from
     * @return the index of the first occurrence of the element in
     *         this list at position <tt>index</tt> or later in the list;
     *         <tt>-1</tt> if the element is not found.
     * @throws IndexOutOfBoundsException if the specified index is negative
     */
    public int indexOf(Object e, int index) {
        Object[] elements = getArray();
        return indexOf(e, elements, index, elements.length);
    }
    /**
     * {@inheritDoc}
     */
    public int lastIndexOf(Object o) {
        Object[] elements = getArray();
        return lastIndexOf(o, elements, elements.length - 1);
    }
    /**
     * Returns the index of the last occurrence of the specified element in
     * this list, searching backwards from <tt>index</tt>, or returns -1 if
     * the element is not found.
     * More formally, returns the highest index <tt>i</tt> such that
     * <tt>(i&nbsp;&lt;=&nbsp;index&nbsp;&amp;&amp;&nbsp;(e==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;e.equals(get(i))))</tt>,
     * or -1 if there is no such index.
     *
     * @param e element to search for
     * @param index index to start searching backwards from
     * @return the index of the last occurrence of the element at position
     *         less than or equal to <tt>index</tt> in this list;
     *         -1 if the element is not found.
     * @throws IndexOutOfBoundsException if the specified index is greater
     *         than or equal to the current size of this list
     */
    public int lastIndexOf(Object e, int index) {
        Object[] elements = getArray();
        return lastIndexOf(e, elements, index);
    }
    /**
     * Returns a shallow copy of this list.  (The elements themselves
     * are not copied.)
     *
     * @return a clone of this list
     */
    public Object clone() {
        try {
            return super.clone();
        } catch (CloneNotSupportedException e) {
            // this shouldn"t happen, since we are Cloneable
            throw new InternalError();
        }
    }
    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all the elements in this list
     */
    public Object[] toArray() {
        Object[] elements = getArray();
        return copyOf(elements, elements.length);
    }
    /**
     * Returns an array containing all of the elements in this list in
     * proper sequence (from first to last element); the runtime type of
     * the returned array is that of the specified array.  If the list fits
     * in the specified array, it is returned therein.  Otherwise, a new
     * array is allocated with the runtime type of the specified array and
     * the size of this list.
     *
     * <p>If this list fits in the specified array with room to spare
     * (i.e., the array has more elements than this list), the element in
     * the array immediately following the end of the list is set to
     * <tt>null</tt>.  (This is useful in determining the length of this
     * list <i>only</i> if the caller knows that this list does not contain
     * any null elements.)
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose <tt>x</tt> is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of the list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing all the elements in this list
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this list
     * @throws NullPointerException if the specified array is null
     */
    public Object[] toArray(Object a[]) {
        Object[] elements = getArray();
        int len = elements.length;
        if (a.length < len)
            return copyOf(elements, len, a.getClass());
        else {
            System.arraycopy(elements, 0, a, 0, len);
            if (a.length > len)
                a[len] = null;
            return a;
        }
    }
    // Positional Access Operations
    /**
     * {@inheritDoc}
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public Object get(int index) {
        return (getArray()[index]);
    }
    /**
     * Replaces the element at the specified position in this list with the
     * specified element.
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public synchronized Object set(int index, Object element) {
        Object[] elements = getArray();
        int len = elements.length;
        Object oldValue = elements[index];
        if (oldValue != element) {
            Object[] newElements = copyOf(elements, len);
            newElements[index] = element;
            setArray(newElements);
        }
        return oldValue;
    }
    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as per the spec for {@link Collection#add})
     */
    public boolean add(Object e) {
        synchronized (this) {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = copyOf(elements, len + 1);
            newElements[len] = e;
            setArray(newElements);
        }
        return true;
    }
    /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public synchronized void add(int index, Object element) {
        Object[] elements = getArray();
        int len = elements.length;
        if (index > len || index < 0)
            throw new IndexOutOfBoundsException("Index: " + index+
                                                ", Size: " + len);
        Object[] newElements;
        int numMoved = len - index;
        if (numMoved == 0)
            newElements = copyOf(elements, len + 1);
        else {
            newElements = new Object[len + 1];
            System.arraycopy(elements, 0, newElements, 0, index);
            System.arraycopy(elements, index, newElements, index + 1,
                             numMoved);
        }
        newElements[index] = element;
        setArray(newElements);
    }
    /**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).  Returns the element that was removed from the list.
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public synchronized Object remove(int index) {
        Object[] elements = getArray();
        int len = elements.length;
        Object oldValue = elements[index];
        int numMoved = len - index - 1;
        if (numMoved == 0)
            setArray(copyOf(elements, len - 1));
        else {
            Object[] newElements = new Object[len - 1];
            System.arraycopy(elements, 0, newElements, 0, index);
            System.arraycopy(elements, index + 1, newElements, index,
                             numMoved);
            setArray(newElements);
        }
        return oldValue;
    }
    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If this list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    public synchronized boolean remove(Object o) {
        Object[] elements = getArray();
        int len = elements.length;
        if (len != 0) {
            // Copy while searching for element to remove
            // This wins in the normal case of element being present
            int newlen = len - 1;
            Object[] newElements = new Object[newlen];
            for (int i = 0; i < newlen; ++i) {
                if (eq(o, elements[i])) {
                    // found one;  copy remaining and exit
                    for (int k = i + 1; k < len; ++k)
                        newElements[k-1] = elements[k];
                    setArray(newElements);
                    return true;
                } else
                    newElements[i] = elements[i];
            }
            // special handling for last cell
            if (eq(o, elements[newlen])) {
                setArray(newElements);
                return true;
            }
        }
        return false;
    }
    /**
     * Removes from this list all of the elements whose index is between
     * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.
     * Shifts any succeeding elements to the left (reduces their index).
     * This call shortens the list by <tt>(toIndex - fromIndex)</tt> elements.
     * (If <tt>toIndex==fromIndex</tt>, this operation has no effect.)
     *
     * @param fromIndex index of first element to be removed
     * @param toIndex index after last element to be removed
     * @throws IndexOutOfBoundsException if fromIndex or toIndex out of
     *              range (fromIndex &lt; 0 || fromIndex &gt;= size() || toIndex
     *              &gt; size() || toIndex &lt; fromIndex)
     */
    private synchronized void removeRange(int fromIndex, int toIndex) {
        Object[] elements = getArray();
        int len = elements.length;
        if (fromIndex < 0 || fromIndex >= len ||
            toIndex > len || toIndex < fromIndex)
            throw new IndexOutOfBoundsException();
        int newlen = len - (toIndex - fromIndex);
        int numMoved = len - toIndex;
        if (numMoved == 0)
            setArray(copyOf(elements, newlen));
        else {
            Object[] newElements = new Object[newlen];
            System.arraycopy(elements, 0, newElements, 0, fromIndex);
            System.arraycopy(elements, toIndex, newElements,
                             fromIndex, numMoved);
            setArray(newElements);
        }
    }
    /**
     * Append the element if not present.
     *
     * @param e element to be added to this list, if absent
     * @return <tt>true</tt> if the element was added
     */
    public synchronized boolean addIfAbsent(Object e) {
        // Copy while checking if already present.
        // This wins in the most common case where it is not present
        Object[] elements = getArray();
        int len = elements.length;
        Object[] newElements = new Object[len + 1];
        for (int i = 0; i < len; ++i) {
            if (eq(e, elements[i]))
                return false; // exit, throwing away copy
            else
                newElements[i] = elements[i];
        }
        newElements[len] = e;
        setArray(newElements);
        return true;
    }
    /**
     * Returns <tt>true</tt> if this list contains all of the elements of the
     * specified collection.
     *
     * @param c collection to be checked for containment in this list
     * @return <tt>true</tt> if this list contains all of the elements of the
     *         specified collection
     * @throws NullPointerException if the specified collection is null
     * @see #contains(Object)
     */
    public boolean containsAll(Collection c) {
        Object[] elements = getArray();
        int len = elements.length;
        for (Iterator itr = c.iterator(); itr.hasNext(); ) {
            Object e = itr.next();
            if (indexOf(e, elements, 0, len) < 0)
                return false;
        }
        return true;
    }
    /**
     * Removes from this list all of its elements that are contained in
     * the specified collection. This is a particularly expensive operation
     * in this class because of the need for an internal temporary array.
     *
     * @param c collection containing elements to be removed from this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection (optional)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements (optional),
     *         or if the specified collection is null
     * @see #remove(Object)
     */
    public synchronized boolean removeAll(Collection c) {
        Object[] elements = getArray();
        int len = elements.length;
        if (len != 0) {
            // temp array holds those elements we know we want to keep
            int newlen = 0;
            Object[] temp = new Object[len];
            for (int i = 0; i < len; ++i) {
                Object element = elements[i];
                if (!c.contains(element))
                    temp[newlen++] = element;
            }
            if (newlen != len) {
                setArray(copyOfRange(temp, 0, newlen, Object[].class));
                return true;
            }
        }
        return false;
    }
    /**
     * Retains only the elements in this list that are contained in the
     * specified collection.  In other words, removes from this list all of
     * its elements that are not contained in the specified collection.
     *
     * @param c collection containing elements to be retained in this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection (optional)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements (optional),
     *         or if the specified collection is null
     * @see #remove(Object)
     */
    public synchronized boolean retainAll(Collection c) {
        Object[] elements = getArray();
        int len = elements.length;
        if (len != 0) {
            // temp array holds those elements we know we want to keep
            int newlen = 0;
            Object[] temp = new Object[len];
            for (int i = 0; i < len; ++i) {
                Object element = elements[i];
                if (c.contains(element))
                    temp[newlen++] = element;
            }
            if (newlen != len) {
                setArray(copyOfRange(temp, 0, newlen, Object[].class));
                return true;
            }
        }
        return false;
    }
    /**
     * Appends all of the elements in the specified collection that
     * are not already contained in this list, to the end of
     * this list, in the order that they are returned by the
     * specified collection"s iterator.
     *
     * @param c collection containing elements to be added to this list
     * @return the number of elements added
     * @throws NullPointerException if the specified collection is null
     * @see #addIfAbsent(Object)
     */
    public int addAllAbsent(Collection c) {
        int numNew = c.size();
        if (numNew == 0)
            return 0;
        synchronized (this) {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] temp = new Object[numNew];
            int added = 0;
            for (Iterator itr = c.iterator(); itr.hasNext(); ) {
                Object e = itr.next();
                if (indexOf(e, elements, 0, len) < 0 &&
                    indexOf(e, temp, 0, added) < 0)
                    temp[added++] = e;
            }
            if (added != 0) {
                Object[] newElements = new Object[len + added];
                System.arraycopy(elements, 0, newElements, 0, len);
                System.arraycopy(temp, 0, newElements, len, added);
                setArray(newElements);
            }
            return added;
        }
    }
    /**
     * Removes all of the elements from this list.
     * The list will be empty after this call returns.
     */
    public synchronized void clear() {
        setArray(new Object[0]);
    }
    /**
     * Appends all of the elements in the specified collection to the end
     * of this list, in the order that they are returned by the specified
     * collection"s iterator.
     *
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     * @see #add(Object)
     */
    public boolean addAll(Collection c) {
        int numNew = c.size();
        if (numNew == 0)
            return false;
        synchronized (this) {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = new Object[len + numNew];
            System.arraycopy(elements, 0, newElements, 0, len);
            for (Iterator itr = c.iterator(); itr.hasNext(); ) {
                Object e = itr.next();
                newElements[len++] = e;
            }
            setArray(newElements);
            return true;
        }
    }
    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in this list in the order that they are returned by the
     * specified collection"s iterator.
     *
     * @param index index at which to insert the first element
     *        from the specified collection
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     * @see #add(int,Object)
     */
    public boolean addAll(int index, Collection c) {
        int numNew = c.size();
        synchronized (this) {
            Object[] elements = getArray();
            int len = elements.length;
            if (index > len || index < 0)
                throw new IndexOutOfBoundsException("Index: " + index +
                                                    ", Size: "+ len);
            if (numNew == 0)
                return false;
            int numMoved = len - index;
            Object[] newElements;
            if (numMoved == 0)
                newElements = copyOf(elements, len + numNew);
            else {
                newElements = new Object[len + numNew];
                System.arraycopy(elements, 0, newElements, 0, index);
                System.arraycopy(elements, index,
                                 newElements, index + numNew,
                                 numMoved);
            }
            for (Iterator itr = c.iterator(); itr.hasNext(); ) {
                Object e = itr.next();
                newElements[index++] = e;
            }
            setArray(newElements);
            return true;
        }
    }
    /**
     * Save the state of the list to a stream (i.e., serialize it).
     *
     * @serialData The length of the array backing the list is emitted
     *               (int), followed by all of its elements (each an Object)
     *               in the proper order.
     * @param s the stream
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        s.defaultWriteObject();
        Object[] elements = getArray();
        int len = elements.length;
        // Write out array length
        s.writeInt(len);
        // Write out all elements in the proper order.
        for (int i = 0; i < len; i++)
            s.writeObject(elements[i]);
    }
    /**
     * Reconstitute the list from a stream (i.e., deserialize it).
     * @param s the stream
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in size, and any hidden stuff
        s.defaultReadObject();
        // Read in array length and allocate array
        int len = s.readInt();
        Object[] elements = new Object[len];
        // Read in all elements in the proper order.
        for (int i = 0; i < len; i++)
            elements[i] = s.readObject();
        setArray(elements);
    }
    /**
     * Returns a string representation of this list, containing
     * the String representation of each element.
     */
    public String toString() {
        Object[] elements = getArray();
        int maxIndex = elements.length - 1;
        StringBuffer buf = new StringBuffer();
        buf.append("[");
        for (int i = 0; i <= maxIndex; i++) {
            buf.append(String.valueOf(elements[i]));
            if (i < maxIndex)
                buf.append(", ");
        }
        buf.append("]");
        return buf.toString();
    }
    /**
     * Compares the specified object with this list for equality.
     * Returns true if and only if the specified object is also a {@link
     * List}, both lists have the same size, and all corresponding pairs
     * of elements in the two lists are <em>equal</em>.  (Two elements
     * <tt>e1</tt> and <tt>e2</tt> are <em>equal</em> if <tt>(e1==null ?
     * e2==null : e1.equals(e2))</tt>.)  In other words, two lists are
     * defined to be equal if they contain the same elements in the same
     * order.
     *
     * @param o the object to be compared for equality with this list
     * @return <tt>true</tt> if the specified object is equal to this list
     */
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof List))
            return false;
        List l2 = (List)(o);
        if (size() != l2.size())
            return false;
        ListIterator e1 = listIterator();
        ListIterator e2 = l2.listIterator();
        while (e1.hasNext()) {
            if (!eq(e1.next(), e2.next()))
                return false;
        }
        return true;
    }
    /**
     * Returns the hash code value for this list.
     *
     * <p>This implementation uses the definition in {@link List#hashCode}.
     *
     * @return the hash code value for this list
     */
    public int hashCode() {
        int hashCode = 1;
        Object[] elements = getArray();
        int len = elements.length;
        for (int i = 0; i < len; ++i) {
            Object obj = elements[i];
            hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
        }
        return hashCode;
    }
    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * <p>The returned iterator provides a snapshot of the state of the list
     * when the iterator was constructed. No synchronization is needed while
     * traversing the iterator. The iterator does <em>NOT</em> support the
     * <tt>remove</tt> method.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    public Iterator iterator() {
        return new COWIterator(getArray(), 0);
    }
    /**
     * {@inheritDoc}
     *
     * <p>The returned iterator provides a snapshot of the state of the list
     * when the iterator was constructed. No synchronization is needed while
     * traversing the iterator. The iterator does <em>NOT</em> support the
     * <tt>remove</tt>, <tt>set</tt> or <tt>add</tt> methods.
     */
    public ListIterator listIterator() {
        return new COWIterator(getArray(), 0);
    }
    /**
     * {@inheritDoc}
     *
     * <p>The list iterator returned by this implementation will throw an
     * <tt>UnsupportedOperationException</tt> in its <tt>remove</tt>,
     * <tt>set</tt> and <tt>add</tt> methods.
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public ListIterator listIterator(final int index) {
        Object[] elements = getArray();
        int len = elements.length;
        if (index < 0 || index > len)
            throw new IndexOutOfBoundsException("Index: " + index);
        return new COWIterator(getArray(), index);
    }
    private static class COWIterator implements ListIterator {
        /** Snapshot of the array **/
        private final Object[] snapshot;
        /** Index of element to be returned by subsequent call to next.  */
        private int cursor;
        private COWIterator(Object[] elements, int initialCursor) {
            cursor = initialCursor;
            snapshot = elements;
        }
        public boolean hasNext() {
            return cursor < snapshot.length;
        }
        public boolean hasPrevious() {
            return cursor > 0;
        }
        public Object next() {
            try {
                return (snapshot[cursor++]);
            } catch (IndexOutOfBoundsException ex) {
                throw new NoSuchElementException();
            }
        }
        public Object previous() {
            try {
                return (snapshot[--cursor]);
            } catch (IndexOutOfBoundsException e) {
                throw new NoSuchElementException();
            }
        }
        public int nextIndex() {
            return cursor;
        }
        public int previousIndex() {
            return cursor - 1;
        }
        /**
         * Not supported. Always throws UnsupportedOperationException.
         * @throws UnsupportedOperationException always; <tt>remove</tt>
         *         is not supported by this iterator.
         */
        public void remove() {
            throw new UnsupportedOperationException();
        }
        /**
         * Not supported. Always throws UnsupportedOperationException.
         * @throws UnsupportedOperationException always; <tt>set</tt>
         *         is not supported by this iterator.
         */
        public void set(Object e) {
            throw new UnsupportedOperationException();
        }
        /**
         * Not supported. Always throws UnsupportedOperationException.
         * @throws UnsupportedOperationException always; <tt>add</tt>
         *         is not supported by this iterator.
         */
        public void add(Object e) {
            throw new UnsupportedOperationException();
        }
    }
    /**
     * Returns a view of the portion of this list between
     * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.
     * The returned list is backed by this list, so changes in the
     * returned list are reflected in this list, and vice-versa.
     * While mutative operations are supported, they are probably not
     * very useful for CopyOnWriteArrayLists.
     *
     * <p>The semantics of the list returned by this method become
     * undefined if the backing list (i.e., this list) is
     * <i>structurally modified</i> in any way other than via the
     * returned list.  (Structural modifications are those that change
     * the size of the list, or otherwise perturb it in such a fashion
     * that iterations in progress may yield incorrect results.)
     *
     * @param fromIndex low endpoint (inclusive) of the subList
     * @param toIndex high endpoint (exclusive) of the subList
     * @return a view of the specified range within this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public synchronized List subList(int fromIndex, int toIndex) {
        Object[] elements = getArray();
        int len = elements.length;
        if (fromIndex < 0 || toIndex > len  || fromIndex > toIndex)
            throw new IndexOutOfBoundsException();
        return new COWSubList(this, fromIndex, toIndex);
    }

    /**
     * Sublist for CopyOnWriteArrayList.
     * This class extends AbstractList merely for convenience, to
     * avoid having to define addAll, etc. This doesn"t hurt, but
     * is wasteful.  This class does not need or use modCount
     * mechanics in AbstractList, but does need to check for
     * concurrent modification using similar mechanics.  On each
     * operation, the array that we expect the backing list to use
     * is checked and updated.  Since we do this for all of the
     * base operations invoked by those defined in AbstractList,
     * all is well.  While inefficient, this is not worth
     * improving.  The kinds of list operations inherited from
     * AbstractList are already so slow on COW sublists that
     * adding a bit more space/time doesn"t seem even noticeable.
     */
    private static class COWSubList extends AbstractList {
        private final CopyOnWriteArrayList l;
        private final int offset;
        private int size;
        private Object[] expectedArray;
        // only call this holding l"s lock
        private COWSubList(CopyOnWriteArrayList list,
                           int fromIndex, int toIndex) {
            l = list;
            expectedArray = l.getArray();
            offset = fromIndex;
            size = toIndex - fromIndex;
        }
        // only call this holding l"s lock
        private void checkForComodification() {
            if (l.getArray() != expectedArray)
                throw new ConcurrentModificationException();
        }
        // only call this holding l"s lock
        private void rangeCheck(int index) {
            if (index < 0 || index >= size)
                throw new IndexOutOfBoundsException("Index: " + index +
                                                    ",Size: " + size);
        }
        public Object set(int index, Object element) {
            synchronized (l) {
                rangeCheck(index);
                checkForComodification();
                Object x = l.set(index + offset, element);
                expectedArray = l.getArray();
                return x;
            }
        }
        public Object get(int index) {
            synchronized (l) {
                rangeCheck(index);
                checkForComodification();
                return l.get(index + offset);
            }
        }
        public int size() {
            synchronized (l) {
                checkForComodification();
                return size;
            }
        }
        public void add(int index, Object element) {
            synchronized (l) {
                checkForComodification();
                if (index<0 || index>size)
                    throw new IndexOutOfBoundsException();
                l.add(index + offset, element);
                expectedArray = l.getArray();
                size++;
            }
        }
        public void clear() {
            synchronized (l) {
                checkForComodification();
                l.removeRange(offset, offset+size);
                expectedArray = l.getArray();
                size = 0;
            }
        }
        public Object remove(int index) {
            synchronized (l) {
                rangeCheck(index);
                checkForComodification();
                Object result = l.remove(index + offset);
                expectedArray = l.getArray();
                size--;
                return result;
            }
        }
        public Iterator iterator() {
            synchronized (l) {
                checkForComodification();
                return new COWSubListIterator(l, 0, offset, size);
            }
        }
        public ListIterator listIterator(final int index) {
            synchronized (l) {
                checkForComodification();
                if (index<0 || index>size)
                    throw new IndexOutOfBoundsException("Index: "+index+
                                                        ", Size: "+size);
                return new COWSubListIterator(l, index, offset, size);
            }
        }
        public List subList(int fromIndex, int toIndex) {
            synchronized (l) {
                checkForComodification();
                if (fromIndex<0 || toIndex>size)
                    throw new IndexOutOfBoundsException();
                return new COWSubList(l, fromIndex + offset,
                                         toIndex + offset);
            }
        }
    }

    private static class COWSubListIterator implements ListIterator {
        private final ListIterator i;
        private final int offset;
        private final int size;
        private COWSubListIterator(List l, int index, int offset,
                                   int size) {
            this.offset = offset;
            this.size = size;
            i = l.listIterator(index + offset);
        }
        public boolean hasNext() {
            return nextIndex() < size;
        }
        public Object next() {
            if (hasNext())
                return i.next();
            else
                throw new NoSuchElementException();
        }
        public boolean hasPrevious() {
            return previousIndex() >= 0;
        }
        public Object previous() {
            if (hasPrevious())
                return i.previous();
            else
                throw new NoSuchElementException();
        }
        public int nextIndex() {
            return i.nextIndex() - offset;
        }
        public int previousIndex() {
            return i.previousIndex() - offset;
        }
        public void remove() {
            throw new UnsupportedOperationException();
        }
        public void set(Object e) {
            throw new UnsupportedOperationException();
        }
        public void add(Object e) {
            throw new UnsupportedOperationException();
        }
    }
//    // Support for resetting lock while deserializing
//    private static final Unsafe unsafe =  Unsafe.getUnsafe();
//    private static final long lockOffset;
//    static {
//        try {
//            lockOffset = unsafe.objectFieldOffset
//                (CopyOnWriteArrayList.class.getDeclaredField("lock"));
//            } catch (Exception ex) { throw new Error(ex); }
//    }
//    private void resetLock() {
//        unsafe.putObjectVolatile(this, lockOffset, new ReentrantLock());
//    }
//
    // Temporary emulations of anticipated new j.u.Arrays functions
    private static Object[] copyOfRange(Object[] original, int from, int to,
                                        Class newType) {
        int newLength = to - from;
        if (newLength < 0)
            throw new IllegalArgumentException(from + " > " + to);
        Object[] copy = (Object[]) java.lang.reflect.Array.newInstance
            (newType.getComponentType(), newLength);
        System.arraycopy(original, from, copy, 0,
                         Math.min(original.length - from, newLength));
        return copy;
    }
    private static Object[] copyOf(Object[] original, int newLength,
                                   Class newType) {
        Object[] copy = (Object[]) java.lang.reflect.Array.newInstance
            (newType.getComponentType(), newLength);
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }
    private static Object[] copyOf(Object[] original, int newLength) {
        return copyOf(original, newLength, original.getClass());
    }
}





Find maximum element of Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> arrayList = new ArrayList<Integer>();
    arrayList.add(new Integer("3"));
    arrayList.add(new Integer("1"));
    arrayList.add(new Integer("8"));
    arrayList.add(new Integer("3"));
    arrayList.add(new Integer("5"));
    Object obj = Collections.max(arrayList);
    System.out.println(obj);
  }
}
//8





Find Minimum element of Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> arrayList = new ArrayList<Integer>();
    arrayList.add(new Integer("1"));
    arrayList.add(new Integer("2"));
    arrayList.add(new Integer("3"));
    arrayList.add(new Integer("4"));
    arrayList.add(new Integer("5"));
    Object obj = Collections.min(arrayList);
    System.out.println(obj);
  }
}





Get element in an ArrayList by index

   

import java.util.ArrayList;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    System.out.println(arrayList.get(0));
    System.out.println(arrayList.get(1));
    System.out.println(arrayList.get(2));
  }
}





Get Enumeration over Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
import java.util.Enumeration;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("A");
    arrayList.add("B");
    arrayList.add("D");
    arrayList.add("E");
    arrayList.add("F");
    Enumeration e = Collections.enumeration(arrayList);
    while (e.hasMoreElements())
      System.out.println(e.nextElement());
  }
}





Get generic Iterator from generic ArrayList

   
/*
 * Copyright (c) 1995 - 2008 Sun Microsystems, Inc.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 *   - Neither the name of Sun Microsystems nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
import java.text.DateFormatSymbols;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
public class OysterMonths {
  Collection<String> safeMonths;
  public Collection<String> filter(Collection<String> c) {
    Collection<String> filteredCollection = new ArrayList<String>();
    for (Iterator<String> i = c.iterator(); i.hasNext();) {
      String s = i.next();
      if (condition(s)) {
        filteredCollection.add(s);
      }
    }
    return filteredCollection;
  }
  public boolean condition(String s) {
    return s.contains("r");
  }
  public static void main(String[] args) {
    OysterMonths om = new OysterMonths();
    DateFormatSymbols dfs = new DateFormatSymbols();
    String[] monthArray = dfs.getMonths();
    Collection<String> months = Arrays.asList(monthArray);
    om.safeMonths = om.filter(months);
    System.out.println("The following months are safe for oysters:");
    System.out.println(om.safeMonths);
  }
}





Get Previous and next index using Java ListIterator

   
import java.util.ArrayList;
import java.util.ListIterator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> aList = new ArrayList<String>();
    aList.add("1");
    aList.add("2");
    aList.add("3");
    aList.add("4");
    aList.add("5");
    ListIterator<String> listIterator = aList.listIterator();
    System.out.println("Previous: " + listIterator.previousIndex());
    System.out.println("Next: " + listIterator.nextIndex());
    // advance current position by one using next method
    listIterator.next();
    System.out.println("Previous: " + listIterator.previousIndex());
    System.out.println("Next: " + listIterator.nextIndex());
  }
}





Get Size of Java ArrayList and loop through elements

   
import java.util.ArrayList;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    int totalElements = arrayList.size();
    for (int index = 0; index < totalElements; index++)
      System.out.println(arrayList.get(index));
  }
}





Get Sub List of Java ArrayList

   
import java.util.ArrayList;
import java.util.List;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    arrayList.add("4");
    arrayList.add("5");
    List lst = arrayList.subList(1, 3);
    for (int i = 0; i < lst.size(); i++)
      System.out.println(lst.get(i));
    // remove one element from sub list
    Object obj = lst.remove(0);
    System.out.println(obj + " is removed");
    for (String str: arrayList)
      System.out.println(str);
  }
}





Get Synchronized List from Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class Main {
  public static void main(String[] args) {
    ArrayList arrayList = new ArrayList();
    List list = Collections.synchronizedList(arrayList);
  }
}





Get the size of an arraylist after and before add and remove methods

    
import java.util.ArrayList;
class ArrayListDemo {
  public static void main(String args[]) {
    ArrayList<String> al = new ArrayList<String>();
    System.out.println("Initial size of al: " + al.size());
    al.add("C");
    al.add("A");
    al.add("E");
    al.add("B");
    al.add("D");
    al.add("F");
    al.add(1, "A2");
    System.out.println("Size of al after additions: " + al.size());
    System.out.println("Contents of al: " + al);
    al.remove("F");
    al.remove(2);
    System.out.println("Size of al after deletions: " + al.size());
    System.out.println("Contents of al: " + al);
  }
}





How to Convert an ArrayList into an array

   
import java.util.ArrayList;
public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> al = new ArrayList<Integer>();
    al.add(new Integer(1));
    al.add(new Integer(2));
    al.add(new Integer(3));
    al.add(new Integer(4));
    al.add(new Integer(5));
    System.out.println("contents of al : " + al);
    Object ia[] = al.toArray(); // get array
    int sum = 0;
    for (int i = 0; i < ia.length; i++){
      sum += ((Integer) ia[i]).intValue();
    }
    System.out.println("Sum is :" + sum);
  }
}





If an ArrayList contains a given item

   
 
import java.util.ArrayList;
import java.util.List;
public class Main {
  public static void main(String[] args) {
    List list = new ArrayList();
    list.add("Item 1");
    list.add("Item 2");
    if (list.contains("Item 1")) {
      System.out.println("True");
    } else {
      System.out.println("False");
    }
  }
}





Insert all elements of other Collection to Specified Index of Java ArrayList

   
import java.util.ArrayList;
import java.util.Vector;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    Vector<String> v = new Vector<String>();
    v.add("4");
    v.add("5");
    // insert all elements of Vector to ArrayList at index 1
    arrayList.addAll(1, v);
    
    for (String str: arrayList)
      System.out.println(str);
  }
}
/*
1
4
5
2
3
*/





Iterate through a Collection using Java Iterator

   
import java.util.ArrayList;
import java.util.Iterator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> aList = new ArrayList<String>();
    aList.add("1");
    aList.add("2");
    aList.add("3");
    aList.add("4");
    aList.add("5");
    Iterator itr = aList.iterator();
    // iterate through the ArrayList values using Iterator"s hasNext and next methods
    while (itr.hasNext()){
      System.out.println(itr.next());
    }
  }
}





Iterate through elements Java ArrayList using Iterator

   
import java.util.ArrayList;
import java.util.Iterator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    arrayList.add("4");
    arrayList.add("5");
    Iterator itr = arrayList.iterator();
    while (itr.hasNext()){
      System.out.println(itr.next());
    }
  }
}





Iterate through elements Java ArrayList using ListIterator

   
import java.util.ArrayList;
import java.util.ListIterator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    arrayList.add("4");
    arrayList.add("5");
    ListIterator itr = arrayList.listIterator();
    System.out.println("in forward direction");
    while (itr.hasNext()) {
      System.out.println(itr.next());
    }
    System.out.println("in backward direction");
    while (itr.hasPrevious()) {
      System.out.println(itr.previous());
    }
  }
}
/*in forward direction
1
2
3
4
5
in backward direction
5
4
3
2
1
*/





Looping through a Collection object: while loop, iterator, and for each

   
import java.util.ArrayList;
import java.util.Iterator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> list = new ArrayList<String>();
    list.add("Monday");
    list.add("Tuesdag");
    list.add("Wednesday");
    list.add("Thursday");
    list.add("Friday");
    list.add("Saturday");
    list.add("Sunday");
    Iterator<String> iterator = null;
    iterator = list.iterator();
    while (iterator.hasNext()) {
      String element = iterator.next();
      System.out.println(element);
    }
    for (iterator = list.iterator(); iterator.hasNext();) {
      String element = iterator.next();
      System.out.println(element);
    }
    for (String element : list) {
      System.out.println(element);
    }
  }
}
/*
Monday
Tuesdag
Wednesday
Thursday
Friday
Saturday
Sunday
Monday
Tuesdag
Wednesday
Thursday
Friday
Saturday
Sunday
Monday
Tuesdag
Wednesday
Thursday
Friday
Saturday
Sunday
*/





pass the actual object you want removed.

    
import java.util.ArrayList;
public class MainClass {
  public static void main(String[] a) {
    ArrayList<Employee> emps = new ArrayList<Employee>();
    Employee emp1 = new Employee("A", "G");
    Employee emp2 = new Employee("T", "A");
    Employee emp3 = new Employee("K", "J");
    emps.add(emp1);
    emps.add(emp2);
    emps.add(emp3);
    System.out.println(emps);
    emps.remove(emp2);
    System.out.println(emps);
  }
}
class Address {
}
class Employee {
  private String lastName;
  private String firstName;
  private Double salary;
  public Address address;
  public Employee(String lastName, String firstName) {
    this.lastName = lastName;
    this.firstName = firstName;
    this.address = new Address();
  }
  public String getLastName() {
    return this.lastName;
  }
  public void setLastName(String lastName) {
    this.lastName = lastName;
  }
  public String getFirstName() {
    return this.firstName;
  }
  public void setFirstName(String firstName) {
    this.firstName = firstName;
  }
  public double getSalary() {
    return this.salary;
  }
  public void setSalary(double salary) {
    this.salary = salary;
  }
}





Perform Binary Search on Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("4");
    arrayList.add("2");
    arrayList.add("5");
    arrayList.add("3");
    Collections.sort(arrayList);
    System.out.println("Sorted ArrayList contains : " + arrayList);
    int index = Collections.binarySearch(arrayList, "4");
    System.out.println("Element found at : " + index);
  }
}





Pre-generics example that uses a collection.

    
import java.util.ArrayList;
import java.util.Iterator;
class OldStyle {
  public static void main(String args[]) {
    ArrayList list = new ArrayList();
    list.add("one");
    list.add("two");
    list.add("three");
    list.add("four");
    Iterator itr = list.iterator();
    while (itr.hasNext()) {
      String str = (String) itr.next(); // explicit cast needed here.
      System.out.println(str + " is " + str.length() + " chars long.");
    }
  }
}





Remove all elements from Java ArrayList

   
import java.util.ArrayList;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    System.out.println(arrayList.size());
    arrayList.clear();
    System.out.println(arrayList.size());
  }
}





Remove an element from ArrayList using Java ListIterator

   
import java.util.ArrayList;
import java.util.ListIterator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> aList = new ArrayList<String>();
    aList.add("1");
    aList.add("2");
    aList.add("3");
    aList.add("4");
    aList.add("5");
    // Get an object of ListIterator using listIterator() method
    ListIterator listIterator = aList.listIterator();
    listIterator.next();
    listIterator.next();
    // remove element returned by last next method
    listIterator.remove();
    for (String str: aList){
      System.out.println(str);
    }
  }
}





Remove an element from Collection using Java Iterator

   
import java.util.ArrayList;
import java.util.Iterator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> aList = new ArrayList<String>();
    aList.add("1");
    aList.add("2");
    aList.add("3");
    aList.add("4");
    aList.add("5");
    for (String str: aList) {
      System.out.println(str);
    }
    Iterator itr = aList.iterator();
    // remove 2 from ArrayList using Iterator"s remove method.
    String strElement = "";
    while (itr.hasNext()) {
      strElement = (String) itr.next();
      if (strElement.equals("2")) {
        itr.remove();
        break;
      }
    }
    for (String str: aList) {
      System.out.println(str);
    }
  }
}





Remove an element from specified index of Java ArrayList

   

import java.util.ArrayList;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    Object obj = arrayList.remove(1);
    System.out.println(obj + " is removed from ArrayList");
    for (String str: arrayList)
      System.out.println(str);
  }
}





Remove duplicate items from an ArrayList

   
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
public class Main {
  public static void main(String[] argv) {
    List<String> arrayList1 = new ArrayList<String>();
    arrayList1.add("A");
    arrayList1.add("A");
    arrayList1.add("B");
    arrayList1.add("B");
    arrayList1.add("B");
    arrayList1.add("C");
    HashSet<String> hashSet = new HashSet<String>(arrayList1);
    List<String> arrayList2 = new ArrayList<String>(hashSet);
    for (Object item : arrayList2)
      System.out.println(item);
  }
}
/*
A
B
C
*/





Replace All Elements Of Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("A");
    arrayList.add("B");
    arrayList.add("D");
    System.out.println(arrayList);
    Collections.fill(arrayList, "REPLACED");
    System.out.println(arrayList);
  }
}
/*
[A, B, D]
[REPLACED, REPLACED, REPLACED]
*/





Replace all occurrences of specified element of Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("A");
    arrayList.add("B");
    arrayList.add("A");
    arrayList.add("C");
    arrayList.add("D");
    System.out.println(arrayList);
    Collections.replaceAll(arrayList, "A", "Replace All");
    System.out.println(arrayList);
  }
}
/*
[A, B, A, C, D]
[Replace All, B, Replace All, C, D]
*/





Replace an element at specified index of Java ArrayList

   

import java.util.ArrayList;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");
    arrayList.set(1, "REPLACED ELEMENT");
    for (String str: arrayList){
      System.out.println(str);
    }
  }
}
/*
1
REPLACED ELEMENT
3
*/





Replace an element from ArrayList using Java ListIterator

   
import java.util.ArrayList;
import java.util.ListIterator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> aList = new ArrayList<String>();
    aList.add("1");
    aList.add("2");
    aList.add("3");
    aList.add("4");
    aList.add("5");
    ListIterator<String> listIterator = aList.listIterator();
    listIterator.next();
    listIterator.set("100");
    for (String str : aList) {
      System.out.println(str);
    }
  }
}





Reverse order of all elements of Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("A");
    arrayList.add("B");
    arrayList.add("C");
    arrayList.add("D");
    arrayList.add("E");
    System.out.println(arrayList);
    Collections.reverse(arrayList);
    System.out.println(arrayList);
  }
}





Rotate elements of a collection

   
 
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
public class Main {
  public static void main(String[] args) {
    List numbers = new ArrayList();
    for (int i = 0; i < 25; i++) {
      numbers.add(i);
    }
    System.out.println(Arrays.toString(numbers.toArray()));
    Collections.rotate(numbers, 10);
    System.out.println(Arrays.toString(numbers.toArray()));
  }
}





Search an element of Java ArrayList

   

import java.util.ArrayList;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("2");
    arrayList.add("2");
    arrayList.add("3");
    arrayList.add("4");
    arrayList.add("5");
    arrayList.add("1");
    arrayList.add("2");
    System.out.println(arrayList.contains("2"));
    int index = arrayList.indexOf("4");
    if (index == -1)
      System.out.println("not contain 4");
    else
      System.out.println("4 at index :" + index);
    int lastIndex = arrayList.lastIndexOf("1");
    if (lastIndex == -1)
      System.out.println("not contain 1");
    else
      System.out.println("Last index :"+ lastIndex);
  }
}
/*true
4 at index :3
Last index :5
*/





Search collection element

   
 
import java.text.DateFormatSymbols;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
public class Main {
  public static void main(String[] args) {
    List list = new LinkedList();
    DateFormatSymbols dfs = new DateFormatSymbols();
    String[] months = dfs.getMonths();
    for (int i = 0; i < months.length; i++) {
      String month = months[i];
      list.add(month);
    }
    Collections.sort(list);
    System.out.println("Month Names = " + list);
    int index = Collections.binarySearch(list, "October");
    if (index > 0) {
      System.out.println("Found at index = " + index);
      String month = (String) list.get(index);
      System.out.println("Month = " + month);
    }
  }
}





shows the modern, generic form of collection classes

    
import java.util.ArrayList;
import java.util.Iterator;
class NewStyle {
  public static void main(String args[]) {
    ArrayList<String> list = new ArrayList<String>();
    list.add("one");
    list.add("two");
    list.add("three");
    list.add("four");
    Iterator<String> itr = list.iterator();
    while (itr.hasNext()) {
      String str = itr.next(); // no cast needed
      System.out.println(str + " is " + str.length() + " chars long.");
    }
  }
}





Shuffle elements of Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("A");
    arrayList.add("B");
    arrayList.add("C");
    arrayList.add("D");
    arrayList.add("E");
    System.out.println(arrayList);
    Collections.shuffle(arrayList);
    System.out.println(arrayList);
  }
}
/*
[A, B, C, D, E]
[E, D, A, C, B]
*/





Sort elements of Java ArrayList

   

import java.util.ArrayList;
import java.util.Collections;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("1");
    arrayList.add("3");
    arrayList.add("5");
    arrayList.add("2");
    arrayList.add("4");
    Collections.sort(arrayList);
    for (String str: arrayList)
      System.out.println(str);
  }
}
/*
1
2
3
4
5
*/





Sort items of an ArrayList

   
 
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
public class Main {
  public static void main(String[] args) {
    List<String> colours = new ArrayList<String>();
    colours.add("red");
    colours.add("green");
    colours.add("blue");
    colours.add("yellow");
    colours.add("cyan");
    colours.add("white");
    colours.add("black");
    Collections.sort(colours);
    System.out.println(Arrays.toString(colours.toArray()));
    Collections.sort(colours, Collections.reverseOrder());
    System.out.println(Arrays.toString(colours.toArray()));
  }
}





Sort Java ArrayList in descending order using comparator

   
import java.util.ArrayList;
import java.util.Collections;
import java.util.ruparator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("A");
    arrayList.add("B");
    arrayList.add("C");
    arrayList.add("D");
    arrayList.add("E");
    
    Comparator comparator = Collections.reverseOrder();
    System.out.println(arrayList);
    Collections.sort(arrayList, comparator);
    System.out.println(arrayList);
  }
}





Store user-defined objects in arraylist

    
import java.util.ArrayList;
public class MainClass {
  public static void main(String[] a) {
    ArrayList<Employee> emps = new ArrayList<Employee>();
    emps.add(new Employee("A", "G"));
    emps.add(new Employee("T", "A"));
    emps.add(new Employee("K", "J"));
    System.out.println(emps);
    Employee e = emps.get(1);
    e.setLastName("new");
    System.out.println(emps);
  }
}
class Address {
}
class Employee {
  private String lastName;
  private String firstName;
  private Double salary;
  public Address address;
  public Employee(String lastName, String firstName) {
    this.lastName = lastName;
    this.firstName = firstName;
    this.address = new Address();
  }
  public String getLastName() {
    return this.lastName;
  }
  public void setLastName(String lastName) {
    this.lastName = lastName;
  }
  public String getFirstName() {
    return this.firstName;
  }
  public void setFirstName(String firstName) {
    this.firstName = firstName;
  }
  public double getSalary() {
    return this.salary;
  }
  public void setSalary(double salary) {
    this.salary = salary;
  }
}





Swap elements of Java ArrayList

   
import java.util.ArrayList;
import java.util.Collections;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> arrayList = new ArrayList<String>();
    arrayList.add("A");
    arrayList.add("B");
    arrayList.add("C");
    arrayList.add("D");
    arrayList.add("E");
    System.out.println(arrayList);
    Collections.swap(arrayList, 0, 4);
    System.out.println(arrayList);
  }
}
/*
[A, B, C, D, E]
[E, B, C, D, A]
*/





Traverse through ArrayList in forward direction using Java ListIterator

   
       
import java.util.ArrayList;
import java.util.ListIterator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> aList = new ArrayList<String>();
    aList.add("1");
    aList.add("2");
    aList.add("3");
    aList.add("4");
    aList.add("5");
    ListIterator listIterator = aList.listIterator();
    while (listIterator.hasNext()){
      System.out.println(listIterator.next());
    }
  }
}





Traverse through ArrayList in reverse direction using Java ListIterator

   
import java.util.ArrayList;
import java.util.ListIterator;
public class Main {
  public static void main(String[] args) {
    ArrayList<String> aList = new ArrayList<String>();
    aList.add("1");
    aList.add("2");
    aList.add("3");
    aList.add("4");
    aList.add("5");
    ListIterator<String> listIterator = aList.listIterator();
    while (listIterator.hasNext()){
      System.out.println(listIterator.next());
    }
    while (listIterator.hasPrevious()){
      System.out.println(listIterator.previous());
    }
  }
}





Use set method to change the value in an array list

    
import java.util.ArrayList;
public class MainClass {
  public static void main(String[] a) {
    ArrayList<String> nums = new ArrayList<String>();
    nums.clear();
    nums.add("One");
    nums.add("Two");
    nums.add("Three");
    System.out.println(nums);
    nums.set(0, "Uno");
    nums.set(1, "Dos");
    nums.set(2, "Tres");
    System.out.println(nums);
  }
}





Use the Iterator returned from ArrayList to loop through an array list

    
import java.util.ArrayList;
import java.util.Iterator;
public class MainClass {
  public static void main(String[] a) {
    ArrayList<String> nums = new ArrayList<String>();
    nums.add("O");
    nums.add("Two");
    nums.add("Three");
    nums.add("Four");
    String s;
    Iterator e = nums.iterator();
    while (e.hasNext()) {
      s = (String) e.next();
      System.out.println(s);
    }
  }
}