Create a Set implementation that use == instead of equals() in Java

  //from   w  ww.  ja v a2s  . c om


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 * Hibernate, Relational Persistence for Idiomatic Java
 *
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 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
 * for more details.
 *
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 */

import java.util.Collection;
import java.util.IdentityHashMap;
import java.util.Iterator;
import java.util.Set;

/**
 * Set implementation that use == instead of equals() as its comparison
 * mechanism.  This is achieved by internally using an IdentityHashMap.
 *
 * @author Emmanuel Bernard
 */
public class IdentitySet implements Set {
  private static final Object DUMP_VALUE = new Object();

  private final IdentityHashMap map;

  /**
   * Create an IdentitySet with default sizing.
   */
  public IdentitySet() {
    this.map = new IdentityHashMap();
  }

  /**
   * Create an IdentitySet with the given sizing.
   *
   * @param sizing The sizing of the set to create.
   */
  public IdentitySet(int sizing) {
    this.map = new IdentityHashMap( sizing );
  }

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

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

  public boolean contains(Object o) {
    return map.get( o ) == DUMP_VALUE;
  }

  public Iterator iterator() {
    return map.entrySet().iterator();
  }

  public Object[] toArray() {
    return map.entrySet().toArray();
  }

  public Object[] toArray(Object[] a) {
    return map.entrySet().toArray( a );
  }

  public boolean add(Object o) {
    return map.put( o, DUMP_VALUE ) == null;
  }

  public boolean remove(Object o) {
    return map.remove( o ) == DUMP_VALUE;
  }

  public boolean containsAll(Collection c) {
    Iterator it = c.iterator();
    while ( it.hasNext() ) {
      if ( !map.containsKey( it.next() ) ) {
        return false;
      }
    }
    return true;
  }

  public boolean addAll(Collection c) {
    Iterator it = c.iterator();
    boolean changed = false;
    while ( it.hasNext() ) {
      if ( this.add( it.next() ) ) {
        changed = true;
      }
    }
    return changed;
  }

  public boolean retainAll(Collection c) {
    //doable if needed
    throw new UnsupportedOperationException();
  }

  public boolean removeAll(Collection c) {
    Iterator it = c.iterator();
    boolean changed = false;
    while ( it.hasNext() ) {
      if ( this.remove( it.next() ) ) {
        changed = true;
      }
    }
    return changed;
  }

  public void clear() {
    map.clear();
  }
}