IdentityHashSet.java

/*
 * Copyright 2007  T-Rank AS
 * 
 * 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.
 */
package no.trank.openpipe.util;

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

/**
 * This class implements the <tt>Set</tt> interface with a hash table, using
 * reference-equality in place of object-equality when comparing objects.
 * <p/>
 * In other words, in an <tt>IdentityHashSet</tt>, two objects
 * <tt>k1</tt> and <tt>k2</tt> are considered equal if and only if
 * <tt>(k1==k2)</tt>.  (In normal <tt>Set</tt> implementations (like
 * <tt>HashSet</tt>) two objects <tt>k1</tt> and <tt>k2</tt> are considered equal
 * if and only if <tt>(k1==null ? k2==null : k1.equals(k2))</tt>.)
 * <p/>
 * <p><b>This class is <i>not</i> a general-purpose <tt>Set</tt>
 * implementation!  While this class implements the <tt>Set</tt> interface, it
 * intentionally violates <tt>Set's</tt> general contract, which mandates the
 * use of the <tt>equals</tt> method when comparing objects.  This class is
 * designed for use only in the rare cases wherein reference-equality
 * semantics are required.</b>
 *
 * @version $Revision$
 */
public class IdentityHashSet<E>
      extends AbstractSet<E>
      implements Set<E>, Cloneable, java.io.Serializable {
   private transient IdentityHashMap<E, Object> map;

   // Dummy value to associate with an Object in the backing Map
   private static final Object PRESENT = new Object();

   /**
    * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
    * default initial capacity (16) and load factor (0.75).
    */
   public IdentityHashSet() {
      map = new IdentityHashMap<E, Object>();
   }

   /**
    * Constructs a new set containing the elements in the specified
    * collection.  The <tt>HashMap</tt> is created with default load factor
    * (0.75) and an initial capacity sufficient to contain the elements in
    * the specified collection.
    *
    * @param c the collection whose elements are to be placed into this set
    * @throws NullPointerException if the specified collection is null
    */
   public IdentityHashSet(Collection<? extends E> c) {
      map = new IdentityHashMap<E, Object>(Math.max((int) (c.size() / .75f) + 1, 16));
      addAll(c);
   }

   /**
    * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
    * the specified initial capacity and default load factor (0.75).
    *
    * @param initialCapacity the initial capacity of the hash table
    * @throws IllegalArgumentException if the initial capacity is less
    *                                  than zero
    */
   public IdentityHashSet(int initialCapacity) {
      map = new IdentityHashMap<E, Object>(initialCapacity);
   }

   /**
    * Returns an iterator over the elements in this set.  The elements
    * are returned in no particular order.
    *
    * @return an Iterator over the elements in this set
    * @see java.util.ConcurrentModificationException
    */
   @Override
   public Iterator<E> iterator() {
      return map.keySet().iterator();
   }

   /**
    * Returns the number of elements in this set (its cardinality).
    *
    * @return the number of elements in this set (its cardinality)
    */
   @Override
   public int size() {
      return map.size();
   }

   /**
    * Returns <tt>true</tt> if this set contains no elements.
    *
    * @return <tt>true</tt> if this set contains no elements
    */
   @Override
   public boolean isEmpty() {
      return map.isEmpty();
   }

   /**
    * Returns <tt>true</tt> if this set contains the specified element.
    * More formally, returns <tt>true</tt> if and only if this set
    * contains an element <tt>e</tt> such that
    * <tt>(o==null ? e==null : o.equals(e))</tt>.
    *
    * @param o element whose presence in this set is to be tested
    * @return <tt>true</tt> if this set contains the specified element
    */
   @Override
   public boolean contains(Object o) {
      return map.containsKey(o);
   }

   /**
    * Adds the specified element to this set if it is not already present.
    * More formally, adds the specified element <tt>e</tt> to this set if
    * this set contains no element <tt>e2</tt> such that
    * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
    * If this set already contains the element, the call leaves the set
    * unchanged and returns <tt>false</tt>.
    *
    * @param e element to be added to this set
    * @return <tt>true</tt> if this set did not already contain the specified
    *         element
    */
   @Override
   public boolean add(E e) {
      return map.put(e, PRESENT) == null;
   }

   /**
    * Removes the specified element from this set if it is present.
    * More formally, removes an element <tt>e</tt> such that
    * <tt>(o==null ? e==null : o.equals(e))</tt>,
    * if this set contains such an element.  Returns <tt>true</tt> if
    * this set contained the element (or equivalently, if this set
    * changed as a result of the call).  (This set will not contain the
    * element once the call returns.)
    *
    * @param o object to be removed from this set, if present
    * @return <tt>true</tt> if the set contained the specified element
    */
   @Override
   public boolean remove(Object o) {
      return map.remove(o) == PRESENT;
   }

   /**
    * Removes all of the elements from this set.
    * The set will be empty after this call returns.
    */
   @Override
   public void clear() {
      map.clear();
   }

}