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

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// $Id: IdentitySet.java 16179 2009-03-18 11:08:10Z hardy.ferentschik $
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* by the @authors tag. See the copyright.txt in the distribution for a
* full listing of individual contributors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* http://www.apache.org/licenses/LICENSE-2.0
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

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) {
    this.map = 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() {
    map.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 );
  }

  @Override
  public String toString() {
    return "IdentitySet{" +
        "map=" + map +
        '}';
  }
}