hashtable.java

java源代码 请看看啊 提点宝贵的意见

/*
 * @(#)Hashtable.java	1.95 03/01/23
 *
 * Copyright 2003 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util;
import java.io.*;

/**
 * This class implements a hashtable, which maps keys to values. Any 
 * non-<code>null</code> object can be used as a key or as a value. <p>
 *
 * To successfully store and retrieve objects from a hashtable, the 
 * objects used as keys must implement the <code>hashCode</code> 
 * method and the <code>equals</code> method. <p>
 *
 * An instance of <code>Hashtable</code> has two parameters that affect its
 * performance: <i>initial capacity</i> and <i>load factor</i>.  The
 * <i>capacity</i> is the number of <i>buckets</i> in the hash table, and the
 * <i>initial capacity</i> is simply the capacity at the time the hash table
 * is created.  Note that the hash table is <i>open</i>: in the case of a "hash
 * collision", a single bucket stores multiple entries, which must be searched
 * sequentially.  The <i>load factor</i> is a measure of how full the hash
 * table is allowed to get before its capacity is automatically increased.
 * When the number of entries in the hashtable exceeds the product of the load
 * factor and the current capacity, the capacity is increased by calling the
 * <code>rehash</code> method.<p>
 *
 * Generally, the default load factor (.75) offers a good tradeoff between
 * time and space costs.  Higher values decrease the space overhead but
 * increase the time cost to look up an entry (which is reflected in most
 * <tt>Hashtable</tt> operations, including <tt>get</tt> and <tt>put</tt>).<p>
 *
 * The initial capacity controls a tradeoff between wasted space and the
 * need for <code>rehash</code> operations, which are time-consuming.
 * No <code>rehash</code> operations will <i>ever</i> occur if the initial
 * capacity is greater than the maximum number of entries the
 * <tt>Hashtable</tt> will contain divided by its load factor.  However,
 * setting the initial capacity too high can waste space.<p>
 *
 * If many entries are to be made into a <code>Hashtable</code>, 
 * creating it with a sufficiently large capacity may allow the 
 * entries to be inserted more efficiently than letting it perform 
 * automatic rehashing as needed to grow the table. <p>
 *
 * This example creates a hashtable of numbers. It uses the names of 
 * the numbers as keys:
 * <p><blockquote><pre>
 *     Hashtable numbers = new Hashtable();
 *     numbers.put("one", new Integer(1));
 *     numbers.put("two", new Integer(2));
 *     numbers.put("three", new Integer(3));
 * </pre></blockquote>
 * <p>
 * To retrieve a number, use the following code: 
 * <p><blockquote><pre>
 *     Integer n = (Integer)numbers.get("two");
 *     if (n != null) {
 *         System.out.println("two = " + n);
 *     }
 * </pre></blockquote>
 * <p>
 * As of the Java 2 platform v1.2, this class has been retrofitted to
 * implement Map, so that it becomes a part of Java's collection framework.
 * Unlike the new collection implementations, Hashtable is synchronized.<p>
 *
 * The Iterators returned by the iterator and listIterator methods
 * of the Collections returned by all of Hashtable's "collection view methods"
 * are <em>fail-fast</em>: if the Hashtable is structurally modified
 * at any time after the Iterator is created, in any way except through the
 * Iterator's own remove or add methods, the Iterator will throw a
 * ConcurrentModificationException.  Thus, in the face of concurrent
 * modification, the Iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the future.
 * The Enumerations returned by Hashtable's keys and values methods are
 * <em>not</em> fail-fast.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. 
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i><p>
 *
 * This class is a member of the 
 * <a href="{@docRoot}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Arthur van Hoff
 * @author  Josh Bloch
 * @version 1.95, 01/23/03
 * @see     Object#equals(java.lang.Object)
 * @see     Object#hashCode()
 * @see     Hashtable#rehash()
 * @see     Collection
 * @see	    Map
 * @see	    HashMap
 * @see	    TreeMap
 * @since JDK1.0
 */
public class Hashtable extends Dictionary implements Map, Cloneable,
                                                   java.io.Serializable {
    /**
     * The hash table data.
     */
    private transient Entry table[];

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

    /**
     * The table is rehashed when its size exceeds this threshold.  (The
     * value of this field is (int)(capacity * loadFactor).)
     *
     * @serial
     */
    private int threshold;
							 
    /**
     * The load factor for the hashtable.
     *
     * @serial
     */
    private float loadFactor;

    /**
     * The number of times this Hashtable has been structurally modified
     * Structural modifications are those that change the number of entries in
     * the Hashtable or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the Hashtable fail-fast.  (See ConcurrentModificationException).
     */
    private transient int modCount = 0;

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = 1421746759512286392L;

    /**
     * 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        the load factor of the hashtable.
     * @exception  IllegalArgumentException  if the initial capacity is less
     *             than zero, or if the load factor is nonpositive.
     */
    public Hashtable(int initialCapacity, float loadFactor) {
	if (initialCapacity < 0)
	    throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal Load: "+loadFactor);

        if (initialCapacity==0)
            initialCapacity = 1;
	this.loadFactor = loadFactor;
	table = new Entry[initialCapacity];
	threshold = (int)(initialCapacity * loadFactor);
    }

    /**
     * Constructs a new, empty hashtable with the specified initial capacity
     * and default load factor, which is <tt>0.75</tt>.
     *
     * @param     initialCapacity   the initial capacity of the hashtable.
     * @exception IllegalArgumentException if the initial capacity is less
     *              than zero.
     */
    public Hashtable(int initialCapacity) {
	this(initialCapacity, 0.75f);
    }

    /**
     * Constructs a new, empty hashtable with a default initial capacity (11)
     * and load factor, which is <tt>0.75</tt>. 
     */
    public Hashtable() {
	this(11, 0.75f);
    }

    /**
     * Constructs a new hashtable with the same mappings as the given 
     * Map.  The hashtable is created with an initial capacity sufficient to
     * hold the mappings in the given Map and a default load factor, which is
     * <tt>0.75</tt>.
     *
     * @param t the map whose mappings are to be placed in this map.
     * @throws NullPointerException if the specified map is null.
     * @since   1.2
     */
    public Hashtable(Map t) {
	this(Math.max(2*t.size(), 11), 0.75f);
	putAll(t);
    }

    /**
     * Returns the number of keys in this hashtable.
     *
     * @return  the number of keys in this hashtable.
     */
    public synchronized 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.
     */
    public synchronized boolean isEmpty() {
	return count == 0;
    }

    /**
     * Returns an enumeration of the keys in this hashtable.
     *
     * @return  an enumeration of the keys in this hashtable.
     * @see     Enumeration
     * @see     #elements()
     * @see	#keySet()
     * @see	Map
     */
    public synchronized Enumeration keys() {
	return getEnumeration(KEYS);
    }

    /**
     * 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     #keys()
     * @see	#values()
     * @see	Map
     */
    public synchronized Enumeration elements() {
	return getEnumeration(VALUES);
    }

    /**
     * Tests if some key maps into the specified value in this hashtable.
     * This operation is more expensive than the <code>containsKey</code>
     * method.<p>
     *
     * Note that this method is identical in functionality to containsValue,
     * (which is part of the Map interface in the collections framework).
     * 
     * @param      value   a value to search for.
     * @return     <code>true</code> if and only if some key maps to the
     *             <code>value</code> argument in this hashtable as 
     *             determined by the <tt>equals</tt> method;
     *             <code>false</code> otherwise.
     * @exception  NullPointerException  if the value is <code>null</code>.
     * @see        #containsKey(Object)
     * @see        #containsValue(Object)
     * @see	   Map
     */
    public synchronized boolean contains(Object value) {
	if (value == null) {
	    throw new NullPointerException();
	}

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

    /**
     * Returns true if this Hashtable maps one or more keys to this value.<p>
     *
     * Note that this method is identical in functionality to contains
     * (which predates the Map interface).
     *
     * @param value value whose presence in this Hashtable is to be tested.
     * @return <tt>true</tt> if this map maps one or more keys to the
     *         specified value.
     * @throws NullPointerException  if the value is <code>null</code>.
     * @see	   Map
     * @since 1.2
     */
    public boolean containsValue(Object value) {
	return contains(value);
    }

    /**
     * Tests if the specified object is a key in this hashtable.
     * 
     * @param   key   possible key.
     * @return  <code>true</code> if and only if the specified object 
     *          is a key in this hashtable, as determined by the 
     *          <tt>equals</tt> method; <code>false</code> otherwise.
     * @throws  NullPointerException  if the key is <code>null</code>.
     * @see     #contains(Object)
     */
    public synchronized boolean containsKey(Object key) {
	Entry tab[] = table;
	int hash = key.hashCode();
	int index = (hash & 0x7FFFFFFF) % tab.length;
	for (Entry e = tab[index] ; e != null ; e = e.next) {
	    if ((e.hash == hash) && e.key.equals(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.
     * @throws  NullPointerException  if the key is <code>null</code>.
     * @see     #put(Object, Object)
     */
    public synchronized Object get(Object key) {
	Entry tab[] = table;
	int hash = key.hashCode();
	int index = (hash & 0x7FFFFFFF) % tab.length;
	for (Entry e = tab[index] ; e != null ; e = e.next) {
	    if ((e.hash == hash) && e.key.equals(key)) {
		return e.value;
	    }
	}
	return null;
    }

    /**
     * Increases the capacity of and internally reorganizes this 
     * hashtable, in order to accommodate and access its entries more 
     * efficiently.  This method is called automatically when the 
     * number of keys in the hashtable exceeds this hashtable's capacity 
     * and load factor. 
     */
    protected void rehash() {
	int oldCapacity = table.length;
	Entry oldMap[] = table;

	int newCapacity = oldCapacity * 2 + 1;