An implementation of Set that manages a map of soft references to the set values. : Reference « Collections « Java Tutorial

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 * full listing of individual contributors.
 *
 * This 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
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 *
 * This software is distributed in the hope that it will be useful,
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 * 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 software; if not, write to the Free
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 */

import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.reflect.Array;
import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Set;

/**
 * An implementation of Set that manages a map of soft references to the set
 * values. The map is keyed by the value hashCode and so this is only useful for
 * value whose hashCode is a valid identity representation (String, primative
 * wrappers, etc).
 * 
 * @author Scott.Stark@jboss.org
 * @version $Revision: 2787 $
 */
@SuppressWarnings("unchecked")
public class SoftSet implements Set {
  private HashMap map = new HashMap();

  /** The queue of garbage collected soft references */
  private ReferenceQueue gcqueue = new ReferenceQueue();

  static class ComparableSoftReference extends SoftReference {
    private Integer key;

    ComparableSoftReference(Integer key, Object o, ReferenceQueue q) {
      super(o, q);
      this.key = key;
    }

    Integer getKey() {
      return key;
    }
  }

  static class ComparableSoftReferenceIterator implements Iterator {
    Iterator theIter;

    ComparableSoftReferenceIterator(Iterator theIter) {
      this.theIter = theIter;
    }

    public boolean hasNext() {
      return theIter.hasNext();
    }

    public Object next() {
      ComparableSoftReference csr = (ComparableSoftReference) theIter.next();
      return csr.get();
    }

    public void remove() {
      theIter.remove();
    }
  }

  /**
   * 
   */
  public SoftSet() {
  }

  public int size() {
    processQueue();
    return map.size();
  }

  public boolean isEmpty() {
    processQueue();
    return map.isEmpty();
  }

  public boolean contains(Object o) {
    processQueue();
    Integer key = new Integer(o.hashCode());
    boolean contains = map.containsKey(key);
    return contains;
  }

  public Iterator iterator() {
    processQueue();
    Iterator theIter = map.values().iterator();
    return new ComparableSoftReferenceIterator(theIter);
  }

  public Object[] toArray() {
    processQueue();
    return toArray(new Object[0]);
  }

  public Object[] toArray(Object[] a) {
    processQueue();
    int size = map.size();
    Object[] array = {};
    if (a.length >= size)
      array = a;
    Iterator iter = map.values().iterator();
    int index = 0;
    while (iter.hasNext()) {
      ComparableSoftReference csr = (ComparableSoftReference) iter.next();
      Object value = csr.get();
      // Create the correct array type
      if (array.length == 0) {
        if (value == null) {
          index++;
          continue;
        }
        Array.newInstance(value.getClass(), size);
      }
      array[index] = value;
      index++;
    }
    return array;
  }

  public boolean add(Object o) {
    processQueue();
    Integer key = new Integer(o.hashCode());
    ComparableSoftReference sr = new ComparableSoftReference(key, o, gcqueue);
    return map.put(key, sr) == null;
  }

  public boolean remove(Object o) {
    processQueue();
    Integer key = new Integer(o.hashCode());
    return map.remove(key) != null;
  }

  public boolean containsAll(Collection c) {
    processQueue();
    Iterator iter = c.iterator();
    boolean contains = true;
    while (iter.hasNext()) {
      Object value = iter.next();
      Integer key = new Integer(value.hashCode());
      contains &= map.containsKey(key);
    }
    return contains;
  }

  public boolean addAll(Collection c) {
    processQueue();
    Iterator iter = c.iterator();
    boolean added = false;
    while (iter.hasNext()) {
      Object value = iter.next();
      Integer key = new Integer(value.hashCode());
      ComparableSoftReference sr = new ComparableSoftReference(key, value, gcqueue);
      added |= map.put(key, sr) == null;
    }
    return added;
  }

  public boolean retainAll(Collection c) {
    Iterator iter = iterator();
    boolean removed = false;
    while (iter.hasNext()) {
      Object value = iter.next();
      if (c.contains(value) == false) {
        iter.remove();
        removed = true;
      }
    }
    return removed;
  }

  public boolean removeAll(Collection c) {
    processQueue();
    Iterator iter = c.iterator();
    boolean removed = false;
    while (iter.hasNext()) {
      Object value = iter.next();
      removed |= remove(value);
    }
    return removed;
  }

  public void clear() {
    while (gcqueue.poll() != null)
      ;
    map.clear();
  }

  public boolean equals(Object o) {
    return map.equals(o);
  }

  public int hashCode() {
    return map.hashCode();
  }

  /**
   * Iterate through the gcqueue for for any cleared reference, remove the
   * associated value from the underlying set.
   */
  private void processQueue() {
    ComparableSoftReference cr;
    while ((cr = (ComparableSoftReference) gcqueue.poll()) != null) {
      map.remove(cr.getKey());
    }
  }

}