Java Programmer's Toolkit/Java Programmer's Toolkit.iso/solaris2/jdk/src/java/util/hashtabl.jav

Text File | 1995-10-30 | 11.0 KB | 411 lines

/*
 * @(#)Hashtable.java    1.29 95/09/28
 *
 * Copyright (c) 1994 Sun Microsystems, Inc. All Rights Reserved.
 *
 * Permission to use, copy, modify, and distribute this software
 * and its documentation for NON-COMMERCIAL purposes and without
 * fee is hereby granted provided that this copyright notice
 * appears in all copies. Please refer to the file "copyright.html"
 * for further important copyright and licensing information.
 *
 * SUN MAKES NO REPRESENTATIONS OR WARRANTIES ABOUT THE SUITABILITY OF
 * THE SOFTWARE, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
 * TO THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
 * PARTICULAR PURPOSE, OR NON-INFRINGEMENT. SUN SHALL NOT BE LIABLE FOR
 * ANY DAMAGES SUFFERED BY LICENSEE AS A RESULT OF USING, MODIFYING OR
 * DISTRIBUTING THIS SOFTWARE OR ITS DERIVATIVES.
 */

package java.util;

/**
 * Hashtable collision list.
 */
class HashtableEntry {
    int hash;
    Object key;
    Object value;
    HashtableEntry next;

    protected Object clone() {
    HashtableEntry entry = (HashtableEntry)super.clone();
    entry.next = (next != null) ? (HashtableEntry)next.clone() : null;
    return entry;
    }


}

/**
 * Hashtable class. Maps keys to values. Any object can be used as
 * a key and/or value.<p>
 *
 * To sucessfully store and retrieve objects from a hash table the
 * object used as the key must implement the hashCode() and equals()
 * methods.<p>
 *
 * This example creates a hashtable of numbers. It uses the names of
 * the numbers as keys:
 * <pre>
 *    Hashtable numbers = new Hashtable();
 *    numbers.put("one", new Integer(1));
 *    numbers.put("two", new Integer(2));
 *    numbers.put("three", new Integer(3));
 * </pre>
 * To retrieve a number use:
 * <pre>
 *    Integer n = (Integer)numbers.get("two");
 *    if (n != null) {
 *        System.out.println("two = " + n);
 *    }
 * </pre>
 *
 * @see java.lang.Object#hashCode
 * @see java.lang.Object#equals
 * @version     1.29, 09/28/95
 * @author    Arthur van Hoff
 */
public
class Hashtable extends Dictionary {
    /**
     * The hash table data.
     */
    private HashtableEntry table[];

    /**
     * The total number of entries in the hash table.
     */
    private 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 number of buckets
     * @param loadFactor a number between 0.0 and 1.0, it defines
     *        the threshold for rehashing the hashtable into
     *        a bigger one.
     * @exception IllegalArgumentException If the initial capacity
     * is less than or equal to zero.
     * @exception IllegalArgumentException If the load factor is
     * less than or equal to zero.
     */
    public Hashtable(int initialCapacity, float loadFactor) {
    if ((initialCapacity <= 0) || (loadFactor <= 0.0)) {
        throw new IllegalArgumentException();
    }
    this.loadFactor = loadFactor;
    table = new HashtableEntry[initialCapacity];
    threshold = (int)(initialCapacity * loadFactor);
    }

    /**
     * Constructs a new, empty hashtable with the specified initial
     * capacity.
     * @param initialCapacity the initial number of buckets
     */
    public Hashtable(int initialCapacity) {
    this(initialCapacity, 0.75f);
    }

    /**
     * Constructs a new, empty hashtable. A default capacity and load factor
     * is used. Note that the hashtable will automatically grow when it gets
     * full.
     */
    public Hashtable() {
    this(101, 0.75f);
    }

    /**
     * Returns the number of elements contained in the hashtable.
     */
    public int size() {
    return count;
    }

    /**
     * Returns true if the hashtable contains no elements.
     */
    public boolean isEmpty() {
    return count == 0;
    }

    /**
     * Returns an enumeration of the hashtable's keys.
     * @see Hashtable#elements
     * @see Enumeration
     */
    public synchronized Enumeration keys() {
    return new HashtableEnumerator(table, true);
    }

    /**
     * Returns an enumeration of the elements. Use the Enumeration methods
     * on the returned object to fetch the elements sequentially.
     * @see Hashtable#keys
     * @see Enumeration
     */
    public synchronized Enumeration elements() {
    return new HashtableEnumerator(table, false);
    }

    /**
     * Returns true if the specified object is an element of the hashtable.
     * This operation is more expensive than the containsKey() method.
     * @param value the value that we are looking for
     * @exception NullPointerException If the value being searched
     * for is equal to null.
     * @see Hashtable#containsKey
     */
    public synchronized boolean contains(Object value) {
    if (value == null) {
        throw new NullPointerException();
    }

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

    /**
     * Returns true if the collection contains an element for the key.
     * @param key the key that we are looking for
     * @see Hashtable#contains
     */
    public synchronized boolean containsKey(Object key) {
    HashtableEntry tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    for (HashtableEntry e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
        return true;
        }
    }
    return false;
    }

    /**
     * Gets the object associated with the specified key in the
     * hashtable.
     * @param key the specified key
     * @returns the element for the key or null if the key
     *         is not defined in the hash table.
     * @see Hashtable#put
     */
    public synchronized Object get(Object key) {
    HashtableEntry tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    for (HashtableEntry e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
        return e.value;
        }
    }
    return null;
    }

    /**
     * Rehashes the content of the table into a bigger table.
     * This method is called automatically when the hashtable's
     * size exceeds the threshold.
     */
    protected void rehash() {
    int oldCapacity = table.length;
    HashtableEntry oldTable[] = table;

    int newCapacity = oldCapacity * 2 + 1;
    HashtableEntry newTable[] = new HashtableEntry[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 (HashtableEntry old = oldTable[i] ; old != null ; ) {
        HashtableEntry e = old;
        old = old.next;

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

    /**
     * Puts the specified element into the hashtable, using the specified
     * key.  The element may be retrieved by doing a get() with the same key.
     * The key and the element cannot be null.
     * @param key the specified key in the hashtable
     * @param value the specified element
     * @exception NullPointerException If the value of the element
     * is equal to null.
     * @see Hashtable#get
     * @return the old value of the key, or null if it did not have one.
     */
    public synchronized 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.
    HashtableEntry tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    for (HashtableEntry e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(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.
    HashtableEntry e = new HashtableEntry();
    e.hash = hash;
    e.key = key;
    e.value = value;
    e.next = tab[index];
    tab[index] = e;
    count++;
    return null;
    }

    /**
     * Removes the element corresponding to the key. Does nothing if the
     * key is not present.
     * @param key the key that needs to be removed
     * @return the value of key, or null if the key was not found.
     */
    public synchronized Object remove(Object key) {
    HashtableEntry tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    for (HashtableEntry e = tab[index], prev = null ; e != null ; prev = e, e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
        if (prev != null) {
            prev.next = e.next;
        } else {
            tab[index] = e.next;
        }
        count--;
        return e.value;
        }
    }
    return null;
    }

    /**
     * Clears the hash table so that it has no more elements in it.
     */
    public synchronized void clear() {
    HashtableEntry tab[] = table;
    for (int index = tab.length; --index >= 0; )
        tab[index] = null;
    count = 0;
    }

    /**
     * Creates a clone of the hashtable. A shallow copy is made,
     * the keys and elements themselves are NOT cloned. This is a
     * relatively expensive operation.
     */
    public synchronized Object clone() {
    Hashtable t = (Hashtable)super.clone();
    t.table = new HashtableEntry[table.length];
    for (int i = table.length ; i-- > 0 ; ) {
        t.table[i] = (table[i] != null) ? (HashtableEntry)table[i].clone() : null;
    }
    return t;
    }

    /**
     * Converts to a rather lengthy String.
     */
    public synchronized 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();
    }
}

/**
 * A hashtable enumerator class.  This class should remain opaque
 * to the client. It will use the Enumeration interface.
 */
class HashtableEnumerator implements Enumeration {
    boolean keys;
    int index;
    HashtableEntry table[];
    HashtableEntry entry;

    HashtableEnumerator(HashtableEntry table[], boolean keys) {
    this.table = table;
    this.keys = keys;
    this.index = table.length;
    }

    public boolean hasMoreElements() {
    if (entry != null) {
        return true;
    }
    while (index-- > 0) {
        if ((entry = table[index]) != null) {
        return true;
        }
    }
    return false;
    }

    public Object nextElement() {
    if (entry == null) {
        while ((index-- > 0) && ((entry = table[index]) == null));
    }
    if (entry != null) {
        HashtableEntry e = entry;
        entry = e.next;
        return keys ? e.key : e.value;
    }
    throw new NoSuchElementException("HashtableEnumerator");
    }

}