identityhashtable.java

java1.6众多例子参考

/*
 * @(#)IdentityHashtable.java	1.15 05/11/17
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

/*
 * Licensed Materials - Property of IBM
 * RMI-IIOP v1.0
 * Copyright IBM Corp. 1998 1999  All Rights Reserved
 *
 * US Government Users Restricted Rights - Use, duplication or
 * disclosure restricted by GSA ADP Schedule Contract with IBM Corp.
 */

package com.sun.corba.se.impl.util;

import java.util.Dictionary;
import java.util.Enumeration;
import java.util.NoSuchElementException;

/**
 * IdentityHashtable is a modified copy of the 1.1.6 Hashtable class which
 * does not rely on the hashCode() and equals() methods of the key or value; 
 * instead, it uses the System.identityHashcode() method and pointer comparison.
 * In addition, all synchronization has been removed.
 */
/**
 * IdentityHashtable collision list.
 */
class IdentityHashtableEntry {
	int hash;
	Object key;
	Object value;
	IdentityHashtableEntry next;
}

public final class IdentityHashtable extends Dictionary {
    /**
     * The hash table data.
     */
    private transient IdentityHashtableEntry table[];

    /**
     * The total number of entries in the hash table.
     */
    private transient int count;

    /**
     * Rehashes the table when count exceeds this threshold.
     */
    private int threshold;

    /**
     * The load factor for the hashtable.
     */
    private float loadFactor;

    /**
     * Constructs a new, empty hashtable with the specified initial 
     * capacity and the specified load factor. 
     *
     * @param      initialCapacity   the initial capacity of the hashtable.
     * @param      loadFactor        a number between 0.0 and 1.0.
     * @exception  IllegalArgumentException  if the initial capacity is less
     *               than or equal to zero, or if the load factor is less than
     *               or equal to zero.
     * @since      JDK1.0
     */
    public IdentityHashtable(int initialCapacity, float loadFactor) {
	if ((initialCapacity <= 0) || (loadFactor <= 0.0)) {
	    throw new IllegalArgumentException();
	}
	this.loadFactor = loadFactor;
	table = new IdentityHashtableEntry[initialCapacity];
	threshold = (int)(initialCapacity * loadFactor);
    }

    /**
     * Constructs a new, empty hashtable with the specified initial capacity
     * and default load factor.
     *
     * @param   initialCapacity   the initial capacity of the hashtable.
     * @since   JDK1.0
     */
    public IdentityHashtable(int initialCapacity) {
	this(initialCapacity, 0.75f);
    }

    /**
     * Constructs a new, empty hashtable with a default capacity and load
     * factor. 
     *
     * @since   JDK1.0
     */
    public IdentityHashtable() {
	this(101, 0.75f);
    }

    /**
     * Returns the number of keys in this hashtable.
     *
     * @return  the number of keys in this hashtable.
     * @since   JDK1.0
     */
    public int size() {
	return count;
    }

    /**
     * Tests if this hashtable maps no keys to values.
     *
     * @return  <code>true</code> if this hashtable maps no keys to values;
     *          <code>false</code> otherwise.
     * @since   JDK1.0
     */
    public boolean isEmpty() {
	return count == 0;
    }

    /**
     * Returns an enumeration of the keys in this hashtable.
     *
     * @return  an enumeration of the keys in this hashtable.
     * @see     java.util.Enumeration
     * @see     java.util.Hashtable#elements()
     * @since   JDK1.0
     */
    public Enumeration keys() {
	return new IdentityHashtableEnumerator(table, true);
    }

    /**
     * Returns an enumeration of the values in this hashtable.
     * Use the Enumeration methods on the returned object to fetch the elements
     * sequentially.
     *
     * @return  an enumeration of the values in this hashtable.
     * @see     java.util.Enumeration
     * @see     java.util.Hashtable#keys()
     * @since   JDK1.0
     */
    public Enumeration elements() {
	return new IdentityHashtableEnumerator(table, false);
    }

    /**
     * Tests if some key maps into the specified value in this hashtable.
     * This operation is more expensive than the <code>containsKey</code>
     * method.
     *
     * @param      value   a value to search for.
     * @return     <code>true</code> if some key maps to the
     *             <code>value</code> argument in this hashtable;
     *             <code>false</code> otherwise.
     * @exception  NullPointerException  if the value is <code>null</code>.
     * @see        java.util.Hashtable#containsKey(java.lang.Object)
     * @since      JDK1.0
     */
    public boolean contains(Object value) {
	if (value == null) {
	    throw new NullPointerException();
	}

	IdentityHashtableEntry tab[] = table;
	for (int i = tab.length ; i-- > 0 ;) {
	    for (IdentityHashtableEntry e = tab[i] ; e != null ; e = e.next) {
		if (e.value == value) {
		    return true;
		}
	    }
	}
	return false;
    }

    /**
     * Tests if the specified object is a key in this hashtable.
     * 
     * @param   key   possible key.
     * @return  <code>true</code> if the specified object is a key in this
     *          hashtable; <code>false</code> otherwise.
     * @see     java.util.Hashtable#contains(java.lang.Object)
     * @since   JDK1.0
     */
    public boolean containsKey(Object key) {
	IdentityHashtableEntry tab[] = table;
	int hash = System.identityHashCode(key);
	int index = (hash & 0x7FFFFFFF) % tab.length;
	for (IdentityHashtableEntry e = tab[index] ; e != null ; e = e.next) {
	    if ((e.hash == hash) && e.key == key) {
		return true;
	    }
	}
	return false;
    }

    /**
     * Returns the value to which the specified key is mapped in this hashtable.
     *
     * @param   key   a key in the hashtable.
     * @return  the value to which the key is mapped in this hashtable;
     *          <code>null</code> if the key is not mapped to any value in
     *          this hashtable.
     * @see     java.util.Hashtable#put(java.lang.Object, java.lang.Object)
     * @since   JDK1.0
     */
    public Object get(Object key) {
	IdentityHashtableEntry tab[] = table;
	int hash = System.identityHashCode(key);
	int index = (hash & 0x7FFFFFFF) % tab.length;
	for (IdentityHashtableEntry e = tab[index] ; e != null ; e = e.next) {
	    if ((e.hash == hash) && e.key == key) {
		return e.value;
	    }
	}
	return null;
    }

    /**
     * Rehashes the contents of the hashtable into a hashtable with a 
     * larger capacity. This method is called automatically when the 
     * number of keys in the hashtable exceeds this hashtable's capacity 
     * and load factor. 
     *
     * @since   JDK1.0
     */
    protected void rehash() {
	int oldCapacity = table.length;
	IdentityHashtableEntry oldTable[] = table;

	int newCapacity = oldCapacity * 2 + 1;
	IdentityHashtableEntry newTable[] = new IdentityHashtableEntry[newCapacity];

	threshold = (int)(newCapacity * loadFactor);
	table = newTable;

	//System.out.println("rehash old=" + oldCapacity + ", new=" + newCapacity + ", thresh=" + threshold + ", count=" + count);

	for (int i = oldCapacity ; i-- > 0 ;) {
	    for (IdentityHashtableEntry old = oldTable[i] ; old != null ; ) {
		IdentityHashtableEntry e = old;
		old = old.next;

		int index = (e.hash & 0x7FFFFFFF) % newCapacity;
		e.next = newTable[index];
		newTable[index] = e;
	    }
	}
    }

    /**
     * Maps the specified <code>key</code> to the specified 
     * <code>value</code> in this hashtable. Neither the key nor the 
     * value can be <code>null</code>. 
     * <p>
     * The value can be retrieved by calling the <code>get</code> method 
     * with a key that is equal to the original key. 
     *
     * @param      key     the hashtable key.
     * @param      value   the value.
     * @return     the previous value of the specified key in this hashtable,
     *             or <code>null</code> if it did not have one.
     * @exception  NullPointerException  if the key or value is
     *               <code>null</code>.
     * @see     java.util.Hashtable#get(java.lang.Object)
     * @since   JDK1.0
     */
    public Object put(Object key, Object value) {
	// Make sure the value is not null
	if (value == null) {
	    throw new NullPointerException();
	}

	// Makes sure the key is not already in the hashtable.
	IdentityHashtableEntry tab[] = table;
	int hash = System.identityHashCode(key);
	int index = (hash & 0x7FFFFFFF) % tab.length;
	for (IdentityHashtableEntry e = tab[index] ; e != null ; e = e.next) {
	    if ((e.hash == hash) && e.key == key) {
		Object old = e.value;
		e.value = value;
		return old;
	    }
	}

	if (count >= threshold) {
	    // Rehash the table if the threshold is exceeded
	    rehash();
	    return put(key, value);
	} 

	// Creates the new entry.
	IdentityHashtableEntry e = new IdentityHashtableEntry();
	e.hash = hash;
	e.key = key;
	e.value = value;
	e.next = tab[index];
	tab[index] = e;
	count++;
	return null;
    }

	/**
     * Removes the key (and its corresponding value) from this 
     * hashtable. This method does nothing if the key is not in the hashtable.
     *
     * @param   key   the key that needs to be removed.
     * @return  the value to which the key had been mapped in this hashtable,
     *          or <code>null</code> if the key did not have a mapping.
     * @since   JDK1.0
     */
    public Object remove(Object key) {
	IdentityHashtableEntry tab[] = table;
	int hash = System.identityHashCode(key);
	int index = (hash & 0x7FFFFFFF) % tab.length;
	for (IdentityHashtableEntry e = tab[index], prev = null ; e != null ; prev = e, e = e.next) {
	    if ((e.hash == hash) && e.key == key) {
		if (prev != null) {
		    prev.next = e.next;
		} else {
		    tab[index] = e.next;
		}
		count--;
		return e.value;
	    }
	}
	return null;
    }

    /**
     * Clears this hashtable so that it contains no keys. 
     *
     * @since   JDK1.0
     */
    public void clear() {
	IdentityHashtableEntry tab[] = table;
	for (int index = tab.length; --index >= 0; )
	    tab[index] = null;
	count = 0;
    }

    /**
     * Returns a rather long string representation of this hashtable.
     *
     * @return  a string representation of this hashtable.
     * @since   JDK1.0
     */
    public String toString() {
	int max = size() - 1;
	StringBuffer buf = new StringBuffer();
	Enumeration k = keys();
	Enumeration e = elements();
	buf.append("{");

	for (int i = 0; i <= max; i++) {
	    String s1 = k.nextElement().toString();
	    String s2 = e.nextElement().toString();
	    buf.append(s1 + "=" + s2);
	    if (i < max) {
		buf.append(", ");
	    }
	}
	buf.append("}");
	return buf.toString();
    }
}