Set that compares object by identity rather than equality : Set « Collections Data Structure « Java

Set that compares object by identity rather than equality

// $Id: 16179 2009-03-18 11:08:10Z hardy.ferentschik $
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import java.util.Collection;
import java.util.IdentityHashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

 * Set that compares object by identity rather than equality. Wraps around a <code>IdentityHashMap</code>
 * @author Emmanuel Bernard
public class IdentitySet implements Set {
  private Map<Object, Object> map;
  private Object CONTAINS = new Object();

  public IdentitySet() {
    this( 10 );

  public IdentitySet(int size) { = new IdentityHashMap<Object, Object>( size );

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

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

  public boolean contains(Object o) {
    return map.containsKey( o );

  public Iterator iterator() {
    return map.keySet().iterator();

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

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

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

  public boolean addAll(Collection c) {
    boolean doThing = false;
    for ( Object o : c ) {
      doThing = doThing || add( o );
    return doThing;

  public void clear() {

  public boolean removeAll(Collection c) {
    boolean remove = false;
    for ( Object o : c ) {
      remove = remove || remove( o );
    return remove;

  public boolean retainAll(Collection c) {
    throw new UnsupportedOperationException();

  public boolean containsAll(Collection c) {
    for ( Object o : c ) {
      if ( !contains( o ) ) {
        return false;
    return true;

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

  public String toString() {
    return "IdentitySet{" +
        "map=" + map +


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