Java Tutorial/Collections/Collections Framework

Conversion of different Collection data types

Implementations of the Collection interface normally have a constructor that accepts a Collection object. This enables you to convert a Collection to a different type of Collection, such as a Queue to a List, or a List to a Set, etc. Here are the constructors of some implementations:

   <source lang="java">

public ArrayList (Collection c)

    public HashSet (Collection c)
    public LinkedList (Collection c)</source>
   
  
 
  

[World, Hello]

Deep clone collection: Returns a new collection containing clones of all the items in the specified collection.

   <source lang="java">

/*

* JCommon : a free general purpose class library for the Java(tm) platform
* 
*
* (C) Copyright 2000-2008, by Object Refinery Limited and Contributors.
*
* Project Info:  http://www.jfree.org/jcommon/index.html
*
* This library is free software; you can redistribute it and/or modify it
* under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation; either version 2.1 of the License, or
* (at your option) any later version.
*
* This library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
* License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
* USA.
*
* [Java is a trademark or registered trademark of Sun Microsystems, Inc.
* in the United States and other countries.]
*
* -------------------
* ShapeUtilities.java
* -------------------
* (C)opyright 2003-2008, by Object Refinery Limited and Contributors.
*
* Original Author:  David Gilbert (for Object Refinery Limited);
* Contributor(s):   -;
*
* $Id: ShapeUtilities.java,v 1.18 2008/06/02 06:58:28 mungady Exp $
*
* Changes
* -------
* 13-Aug-2003 : Version 1 (DG);
* 16-Mar-2004 : Moved rotateShape() from RefineryUtilities.java to here (DG);
* 13-May-2004 : Added new shape creation methods (DG);
* 30-Sep-2004 : Added createLineRegion() method (DG);
*               Moved drawRotatedShape() method from RefineryUtilities class
*               to this class (DG);
* 04-Oct-2004 : Renamed ShapeUtils --> ShapeUtilities (DG);
* 26-Oct-2004 : Added a method to test the equality of two Line2D
*               instances (DG);
* 10-Nov-2004 : Added new translateShape() and equal(Ellipse2D, Ellipse2D)
*               methods (DG);
* 11-Nov-2004 : Renamed translateShape() --> createTranslatedShape() (DG);
* 07-Jan-2005 : Minor Javadoc fix (DG);
* 11-Jan-2005 : Removed deprecated code in preparation for 1.0.0 release (DG);
* 21-Jan-2005 : Modified return type of RectangleAnchor.coordinates()
*               method (DG);
* 22-Feb-2005 : Added equality tests for Arc2D and GeneralPath (DG);
* 16-Mar-2005 : Fixed bug where equal(Shape, Shape) fails for two Polygon
*               instances (DG);
* 01-Jun-2008 : Fixed bug in equal(GeneralPath, GeneralPath) method (DG);
*
*/

import java.awt.Shape; import java.awt.geom.AffineTransform; import java.awt.geom.Point2D; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import java.lang.reflect.Modifier; import java.util.Collection; import java.util.Iterator; /**

* Utility methods for {@link Shape} objects.
*
* @author David Gilbert
*/

public class Main {

 /**
  * Returns a new collection containing clones of all the items in the
  * specified collection.
  * 
  * @param collection
  *          the collection (null not permitted).
  * @return A new collection containing clones of all the items in the
  *         specified collection.
  * @throws CloneNotSupportedException
  *           if any of the items in the collection cannot be cloned.
  */
 public static Collection deepClone(final Collection collection) throws CloneNotSupportedException {
   if (collection == null) {
     throw new IllegalArgumentException("Null "collection" argument.");
   }
   // all JDK-Collections are cloneable ...
   // and if the collection is not clonable, then we should throw
   // a CloneNotSupportedException anyway ...
   final Collection result = (Collection) clone(collection);
   result.clear();
   final Iterator iterator = collection.iterator();
   while (iterator.hasNext()) {
     final Object item = iterator.next();
     if (item != null) {
       result.add(clone(item));
     } else {
       result.add(null);
     }
   }
   return result;
 }
 /**
  * Returns a clone of the specified object, if it can be cloned, otherwise
  * throws a CloneNotSupportedException.
  * 
  * @param object
  *          the object to clone (null not permitted).
  * @return A clone of the specified object.
  * @throws CloneNotSupportedException
  *           if the object cannot be cloned.
  */
 public static Object clone(final Object object) throws CloneNotSupportedException {
   if (object == null) {
     throw new IllegalArgumentException("Null "object" argument.");
   }
   try {
     final Method method = object.getClass().getMethod("clone", (Class[]) null);
     if (Modifier.isPublic(method.getModifiers())) {
       return method.invoke(object, (Object[]) null);
     }
   } catch (NoSuchMethodException e) {
     System.out.println("Object without clone() method is impossible.");
   } catch (IllegalAccessException e) {
     System.out.println("Object.clone(): unable to call method.");
   } catch (InvocationTargetException e) {
     System.out.println("Object without clone() method is impossible.");
   }
   throw new CloneNotSupportedException("Failed to clone.");
 }

}</source>

Framework Interfaces

The framework consists of four core interfaces with two specializations for sorting

1. Collection
2. List
3. Set
4. Map
5. SortedSet
6. SortedMap

Interface type and its implementation

Interface TypeImplemented bySetHashSet, LinkedHashSet, EnumSetSortedSetTreeSetListVector, Stack, ArrayList, LinkedListQueuePriorityQueue, LinkedListMap<K,V>Hashtable<K,V>, HashMap<K,V>, LinkedHashMap<K,V>, WeakHashMap<K,V>, IdentityHashMap<K,V>SortedMapTreeMap

Making Your Objects Comparable and Sortable

You can make objects comparable by implementing the java.lang.ruparable and java.util.ruparator interfaces.

Classes such as java.lang.String, java.util.Date, and primitive wrapper classes all implement java.lang.ruparable.

   <source lang="java">

import java.util.Arrays; public class MainClass {

 public static void main(String[] args) {
   int[] intArray = new int[] { 5, 4, 3, 2, 1 };
   Arrays.sort(intArray);
   for (int i : intArray) {
     System.out.println(i);
   }
 }

}</source>

1
2
3
4
5

The Collections Framework consists of three parts

1. interfaces, the abstract data types that the framework supports.
2. implementations, the concrete versions of these interfaces.
3. algorithms, the predefined actions that can be defined on either the interfaces or their implementations.

1. All implementations are unsynchronized.
2. All implementations are serializable and cloneable
3. All implementations support having null elements.

The predefined algorithms for supporting the framework are found in the Collections and Arrays classes.

Using Comparable and Comparator

1. Implementing java.lang.ruparable enables you to define one way to compare instances of your class.
2. Objects sometimes might be comparable in more ways.
3. Comparator defines how two objects should be compared.
4. To make objects comparable in two ways, you need two comparators.

   <source lang="java">

import java.util.Arrays; import java.util.ruparator; class Person implements Comparable {

 private String firstName;
 private String lastName;
 private int age;
 public String getFirstName() {
   return firstName;
 }
 public void setFirstName(String firstName) {
   this.firstName = firstName;
 }
 public String getLastName() {
   return lastName;
 }
 public void setLastName(String lastName) {
   this.lastName = lastName;
 }
 public int getAge() {
   return age;
 }
 public void setAge(int age) {
   this.age = age;
 }
 public int compareTo(Object anotherPerson) throws ClassCastException {
   if (!(anotherPerson instanceof Person)) {
     throw new ClassCastException("A Person object expected.");
   }
   int anotherPersonAge = ((Person) anotherPerson).getAge();
   return this.age - anotherPersonAge;
 }

} class LastNameComparator implements Comparator {

 public int compare(Object person, Object anotherPerson) {
   String lastName1 = ((Person) person).getLastName().toUpperCase();
   String firstName1 = ((Person) person).getFirstName().toUpperCase();
   String lastName2 = ((Person) anotherPerson).getLastName().toUpperCase();
   String firstName2 = ((Person) anotherPerson).getFirstName().toUpperCase();
   
   if (lastName1.equals(lastName2)) {
     return firstName1.rupareTo(firstName2);
   } else {
     return lastName1.rupareTo(lastName2);
   }
 }

} class FirstNameComparator implements Comparator {

 public int compare(Object person, Object anotherPerson) {
   String lastName1 = ((Person) person).getLastName().toUpperCase();
   String firstName1 = ((Person) person).getFirstName().toUpperCase();
   String lastName2 = ((Person) anotherPerson).getLastName().toUpperCase();
   String firstName2 = ((Person) anotherPerson).getFirstName().toUpperCase();
   if (firstName1.equals(firstName2)) {
     return lastName1.rupareTo(lastName2);
   } else {
     return firstName1.rupareTo(firstName2);
   }
 }

} public class MainClass {

 public static void main(String[] args) {
   Person[] persons = new Person[4];
   persons[0] = new Person();
   persons[0].setFirstName("A");
   persons[0].setLastName("X");
   persons[0].setAge(56);
   persons[1] = new Person();
   persons[1].setFirstName("S");
   persons[1].setLastName("C");
   persons[1].setAge(8);
   persons[2] = new Person();
   persons[2].setFirstName("E");
   persons[2].setLastName("H");
   persons[2].setAge(16);
   persons[3] = new Person();
   persons[3].setFirstName("B");
   persons[3].setLastName("Q");
   persons[3].setAge(69);
   System.out.println("Natural Order");
   for (int i = 0; i < 4; i++) {
     Person person = persons[i];
     String lastName = person.getLastName();
     String firstName = person.getFirstName();
     int age = person.getAge();
     System.out.println(lastName + ", " + firstName + ". Age:" + age);
   }
   Arrays.sort(persons, new LastNameComparator());
   System.out.println();
   System.out.println("Sorted by last name");
   for (int i = 0; i < 4; i++) {
     Person person = persons[i];
     String lastName = person.getLastName();
     String firstName = person.getFirstName();
     int age = person.getAge();
     System.out.println(lastName + ", " + firstName + ". Age:" + age);
   }
   Arrays.sort(persons, new FirstNameComparator());
   System.out.println();
   System.out.println("Sorted by first name");
   for (int i = 0; i < 4; i++) {
     Person person = persons[i];
     String lastName = person.getLastName();
     String firstName = person.getFirstName();
     int age = person.getAge();
     System.out.println(lastName + ", " + firstName + ". Age:" + age);
   }
   Arrays.sort(persons);
   System.out.println();
   System.out.println("Sorted by age");
   for (int i = 0; i < 4; i++) {
     Person person = persons[i];
     String lastName = person.getLastName();
     String firstName = person.getFirstName();
     int age = person.getAge();
     System.out.println(lastName + ", " + firstName + ". Age:" + age);
   }
 }

}</source>

Natural Order
X, A. Age:56
C, S. Age:8
H, E. Age:16
Q, B. Age:69
Sorted by last name
C, S. Age:8
H, E. Age:16
Q, B. Age:69
X, A. Age:56
Sorted by first name
X, A. Age:56
Q, B. Age:69
H, E. Age:16
C, S. Age:8
Sorted by age
C, S. Age:8
H, E. Age:16
X, A. Age:56
Q, B. Age:69