Java/Collections Data Structure/Weak List

A bag of weakly referenced objects

   
/*
 * Copyright (c) 1998 - 2005 Versant Corporation
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 * Versant Corporation - initial API and implementation
 */
import java.util.HashSet;
import java.util.List;
import java.util.ArrayList;
import java.util.Iterator;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.lang.ref.Reference;
/**
 * This maintains a bag of weakly referenced objects. The clean method
 * must be called from time to time to get rid of the objects that the
 * garbage collector wants to nuke. This class is not synchronized.
 */
public final class WeakBag {
    private HashSet set = new HashSet();
    private ReferenceQueue refQueue = new ReferenceQueue();
    public WeakBag() {
    }
    /**
     * Add o to the bag and return the WeakReference that can be used to
     * remove it.
     */
    public WeakReference add(Object o) {
        WeakReference ans = new WeakReference(o, refQueue);
        set.add(ans);
        return ans;
    }
    /**
     * Remove o from the bag.
     */
    public void remove(Reference o) {
        set.remove(o);
    }
    /**
     * Get the approximate number of objects in the bag.
     */
    public int size() {
        return set.size();
    }
    /**
     * Get all the objects still in the bag.
     */
    public List values() {
        ArrayList a = new ArrayList(set.size());
        for (Iterator i = set.iterator(); i.hasNext(); ) {
            WeakReference r = (WeakReference)i.next();
            Object o = r.get();
            if (o != null) a.add(o);
        }
        return a;
    }
    /**
     * Get rid of objects in the bag that the garbage collector wants to
     * nuke. This does not block.
     */
    public void clean() {
        for (;;) {
            Object r = refQueue.poll();
            if (r == null) return;
            set.remove(r);
        }
    }
}

Weak List from objectweb jac

  
//Revised from objectweb jac
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractList;
import java.util.ArrayList;
import java.util.List;
public class WeakList extends AbstractList {
    /**
     * Reference queue for cleared weak references
     */
    private final ReferenceQueue queue = new ReferenceQueue();
   
    private final List list=new ArrayList();
    public boolean add(Object o) {
        expungeStaleEntries();
        return list.add(new ListEntry(o,queue));
    }
    public Object get(int i) {
        expungeStaleEntries();
        return ((Reference)list.get(i)).get();
    }
    public int size() {
        //new Exception().printStackTrace();
        expungeStaleEntries();
        return list.size();
    }
    public Object remove(int index) {
        return ((ListEntry)list.remove(index)).get();
    }
    /**
     * Expunge stale entries from the list.
     */
    private void expungeStaleEntries() {
        Object r;
        while ( (r = queue.poll()) != null) {
            ListEntry e = (ListEntry)r;
            int i=list.indexOf(r);
            if(i!=-1) {
                list.remove(i);
            }
        }
    }
    private static class ListEntry extends WeakReference {
        String objectString;
        public ListEntry(Object o,ReferenceQueue queue) {
            super(o,queue);
            objectString=o.toString();
        }
        public boolean equals(Object o) {
            if(o==null) {
                return false;
            } else {
                if((o instanceof ListEntry)) {
                    return o.hashCode()==this.hashCode();
                } else {
                    if(this.get()==null) return false;
                    return this.get().equals(o);
                }
            }
        }
        public String toString() {
            if(this.get()==null) {
                return ""entry "+objectString+" <GARBAGED>"";
            } else {
                return ""entry "+this.get()+""";
            }
        }
      
    }
}

WeakReference list uses java.lang.ref.WeakReferences to store its contents.

   
/**
 * 
 * JFreeReport : a free Java reporting library
 * 
 *
 * Project Info:  http://reporting.pentaho.org/
 *
 * (C) Copyright 2001-2007, by Object Refinery Ltd, Pentaho Corporation and Contributors.
 *
 * 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., 59 Temple Place, Suite 330,
 * Boston, MA 02111-1307, USA.
 *
 * [Java is a trademark or registered trademark of Sun Microsystems, Inc.
 * in the United States and other countries.]
 *
 * ------------
 * WeakReferenceList.java
 * ------------
 * (C) Copyright 2001-2007, by Object Refinery Ltd, Pentaho Corporation and Contributors.
 */

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
/**
 * The WeakReference list uses <code>java.lang.ref.WeakReference</code>s to store its contents. In contrast to the
 * WeakHashtable, this list knows how to restore missing content, so that garbage collected elements can be restored
 * when they are accessed.
 * <p/>
 * By default this list can contain 25 elements, where the first element is stored using a strong reference, which is
 * not garbage collected.
 * <p/>
 * Restoring the elements is not implemented, concrete implementations will have to override the
 * <code>restoreChild(int)</code> method. The <code>getMaxChildCount</code> method defines the maxmimum number of
 * children in the list. When more than <code>maxChildCount</code> elements are contained in this list, add will always
 * return false to indicate that adding the element failed.
 * <p/>
 * To customize the list, override createReference to create a different kind of reference.
 * <p/>
 * This list is able to add or replace elements, but inserting or removing of elements is not possible.
 *
 * @author Thomas Morgner
 */
public abstract class WeakReferenceList implements Serializable, Cloneable
{
  /**
   * The master element.
   */
  private Object master;
  /**
   * Storage for the references.
   */
  private transient Reference[] childs;
  /**
   * The current number of elements.
   */
  private int size;
  /**
   * The maximum number of elements.
   */
  private final int maxChilds;
  /**
   * Creates a new weak reference list. The storage of the list is limited to getMaxChildCount() elements.
   *
   * @param maxChildCount the maximum number of elements.
   */
  protected WeakReferenceList(final int maxChildCount)
  {
    this.maxChilds = maxChildCount;
    this.childs = new Reference[maxChildCount - 1];
  }
  /**
   * Returns the maximum number of children in this list.
   *
   * @return the maximum number of elements in this list.
   */
  protected final int getMaxChildCount()
  {
    return maxChilds;
  }
  /**
   * Returns the master element of this list. The master element is the element stored by a strong reference and cannot
   * be garbage collected.
   *
   * @return the master element
   */
  protected Object getMaster()
  {
    return master;
  }
  /**
   * Attempts to restore the child stored on the given index.
   *
   * @param index the index.
   * @return null if the child could not be restored or the restored child.
   */
  protected abstract Object restoreChild(int index);
  /**
   * Returns the child stored at the given index. If the child has been garbage collected, it gets restored using the
   * restoreChild function.
   *
   * @param index the index.
   * @return the object.
   */
  public Object get(final int index)
  {
    if (isMaster(index))
    {
      return master;
    }
    else
    {
      final Reference ref = childs[getChildPos(index)];
      if (ref == null)
      {
        throw new IllegalStateException("State: " + index);
      }
      Object ob = ref.get();
      if (ob == null)
      {
        ob = restoreChild(index);
        childs[getChildPos(index)] = createReference(ob);
      }
      return ob;
    }
  }
  /**
   * Replaces the child stored at the given index with the new child which can be null.
   *
   * @param report the object.
   * @param index  the index.
   */
  public void set(final Object report, final int index)
  {
    if (isMaster(index))
    {
      master = report;
    }
    else
    {
      childs[getChildPos(index)] = createReference(report);
    }
  }
  /**
   * Creates a new reference for the given object.
   *
   * @param o the object.
   * @return a WeakReference for the object o without any ReferenceQueue attached.
   */
  private Reference createReference(final Object o)
  {
    return new WeakReference(o);
  }
  /**
   * Adds the element to the list. If the maximum size of the list is exceeded, this function returns false to indicate
   * that adding failed.
   *
   * @param rs the object.
   * @return true, if the object was successfully added to the list, false otherwise
   */
  public boolean add(final Object rs)
  {
    if (size == 0)
    {
      master = rs;
      size = 1;
      return true;
    }
    else
    {
      if (size < getMaxChildCount())
      {
        childs[size - 1] = createReference(rs);
        size++;
        return true;
      }
      else
      {
        // was not able to add this to this list, maximum number of entries reached.
        return false;
      }
    }
  }
  /**
   * Returns true, if the given index denotes a master index of this list.
   *
   * @param index the index.
   * @return true if the index is a master index.
   */
  protected boolean isMaster(final int index)
  {
    return index % getMaxChildCount() == 0;
  }
  /**
   * Returns the internal storage position for the child.
   *
   * @param index the index.
   * @return the internal storage index.
   */
  protected int getChildPos(final int index)
  {
    return index % getMaxChildCount() - 1;
  }
  /**
   * Returns the size of the list.
   *
   * @return the size.
   */
  public int getSize()
  {
    return size;
  }
  /**
   * Serialisation support. The transient child elements were not saved.
   *
   * @param in the input stream.
   * @throws IOException            if there is an I/O error.
   * @throws ClassNotFoundException if a serialized class is not defined on this system.
   */
  private void readObject(final ObjectInputStream in)
      throws IOException, ClassNotFoundException
  {
    in.defaultReadObject();
    childs = new Reference[getMaxChildCount() - 1];
    for (int i = 0; i < childs.length; i++)
    {
      childs[i] = createReference(null);
    }
  }
  /**
   * Creates and returns a copy of this object.  The precise meaning of "copy" may depend on the class of the object.
   * The general intent is that, for any object <tt>x</tt>, the expression: <blockquote>
   * <pre>
   * x.clone() != x</pre></blockquote>
   * will be true, and that the expression: <blockquote>
   * <pre>
   * x.clone().getClass() == x.getClass()</pre></blockquote>
   * will be <tt>true</tt>, but these are not absolute requirements. While it is typically the case that: <blockquote>
   * <pre>
   * x.clone().equals(x)</pre></blockquote>
   * will be <tt>true</tt>, this is not an absolute requirement.
   * <p/>
   * By convention, the returned object should be obtained by calling <tt>super.clone</tt>.  If a class and all of its
   * superclasses (except <tt>Object</tt>) obey this convention, it will be the case that <tt>x.clone().getClass() ==
   * x.getClass()</tt>.
   * <p/>
   * By convention, the object returned by this method should be independent of this object (which is being cloned).  To
   * achieve this independence, it may be necessary to modify one or more fields of the object returned by
   * <tt>super.clone</tt> before returning it.  Typically, this means copying any mutable objects that comprise the
   * internal "deep structure" of the object being cloned and replacing the references to these objects with references
   * to the copies.  If a class contains only primitive fields or references to immutable objects, then it is usually
   * the case that no fields in the object returned by <tt>super.clone</tt> need to be modified.
   * <p/>
   * The method <tt>clone</tt> for class <tt>Object</tt> performs a specific cloning operation. First, if the class of
   * this object does not implement the interface <tt>Cloneable</tt>, then a <tt>CloneNotSupportedException</tt> is
   * thrown. Note that all arrays are considered to implement the interface <tt>Cloneable</tt>. Otherwise, this method
   * creates a new instance of the class of this object and initializes all its fields with exactly the contents of the
   * corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned.
   * Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation.
   * <p/>
   * The class <tt>Object</tt> does not itself implement the interface <tt>Cloneable</tt>, so calling the <tt>clone</tt>
   * method on an object whose class is <tt>Object</tt> will result in throwing an exception at run time.
   *
   * @return a clone of this instance.
   * @throws CloneNotSupportedException if the object"s class does not support the <code>Cloneable</code> interface.
   *                                    Subclasses that override the <code>clone</code> method can also throw this
   *                                    exception to indicate that an instance cannot be cloned.
   * @see Cloneable
   */
  protected Object clone()
      throws CloneNotSupportedException
  {
    final WeakReferenceList list = (WeakReferenceList) super.clone();
    list.childs = (Reference[]) childs.clone();
    return list;
  }
}