Set implementation that use == instead of equals() : Set « Collections Data Structure « Java

Set implementation that use == instead of equals()


 * Hibernate, Relational Persistence for Idiomatic Java
 * Copyright (c) 2008, Red Hat Middleware LLC or third-party contributors as
 * indicated by the @author tags or express copyright attribution
 * statements applied by the authors.  All third-party contributions are
 * distributed under license by Red Hat Middleware LLC.
 * This copyrighted material is made available to anyone wishing to use, modify,
 * copy, or redistribute it subject to the terms and conditions of the GNU
 * Lesser General Public License, as published by the Free Software Foundation.
 * This program 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 distribution; if not, write to:
 * Free Software Foundation, Inc.
 * 51 Franklin Street, Fifth Floor
 * Boston, MA  02110-1301  USA

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() { = new IdentityHashMap();

   * Create an IdentitySet with the given sizing.
   * @param sizing The sizing of the set to create.
  public IdentitySet(int sizing) { = 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( ) ) {
        return false;
    return true;

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

  public void clear() {


Related examples in the same category

1.Set, HashSet and TreeSet
2.Things you can do with SetsThings you can do with Sets
3.Set operations: union, intersection, difference, symmetric difference, is subset, is superset
4.Set that compares object by identity rather than equality
5.Set union and intersection
6.Set with values iterated in insertion order.
7.Putting your own type in a SetPutting your own type in a Set
8.Use setUse set
9.Another Set demo
10.Set subtractionSet subtraction
11.Working with HashSet and TreeSetWorking with HashSet and TreeSet
12.TreeSet DemoTreeSet Demo
13.Show the union and intersection of two sets
14.Demonstrate the Set interface
15.Array Set extends AbstractSetArray Set extends AbstractSet
16.Sync Test
17.Set Copy
18.Set and TreeSet
20.What you can do with a TreeSetWhat you can do with a TreeSet
21.Remove all elements from a set
22.Copy all the elements from set2 to set1 (set1 += set2), set1 becomes the union of set1 and set2
23.Remove all the elements in set1 from set2 (set1 -= set2), set1 becomes the asymmetric difference of set1 and set2
24.Get the intersection of set1 and set2, set1 becomes the intersection of set1 and set2
25.Extend AbstractSet to Create Simple Set
26.Int Set
27.One Item Set
28.Small sets whose elements are known to be unique by construction
29.List Set implements Set
30.Converts a char array to a Set
31.Converts a string to a Set
32.Implements the Set interface, backed by a ConcurrentHashMap instance
33.An IdentitySet that uses reference-equality instead of object-equality
34.An implementation of the java.util.Stack based on an ArrayList instead of a Vector, so it is not synchronized to protect against multi-threaded access.
35.A thin wrapper around a List transforming it into a modifiable Set.
36.A thread-safe Set that manages canonical objects
37.This program uses a set to print all unique words in
38.Indexed Set
39.An ObjectToSet provides a java.util.Map from arbitrary objects to objects of class java.util.Set.
40.Sorted Multi Set
41.Fixed Size Sorted Set
42.Set operations
43.A NumberedSet is a generic container of Objects where each element is identified by an integer id.
44.Set which counts the number of times a values are added to it.
45.Set which counts the number of times a values are added to it and assigns them a unique positive index.
46.Indexed Set
47.A set acts like array.
48.Implements a Bloom filter. Which, as you may not know, is a space-efficient structure for storing a set.
49.Implementation of disjoint-set data structure
50.Call it an unordered list or a multiset, this collection is defined by oxymorons