Programming Languages Suite/ProgLangD.iso/VCAFE.3.0A/Main.bin/Hashtable.java

Java Source | 1998-09-22 | 16.4 KB | 545 lines

/*
 * @(#)Hashtable.java    1.42 98/07/01
 *
 * Copyright 1995-1998 by Sun Microsystems, Inc.,
 * 901 San Antonio Road, Palo Alto, California, 94303, U.S.A.
 * All rights reserved.
 *
 * This software is the confidential and proprietary information
 * of Sun Microsystems, Inc. ("Confidential Information").  You
 * shall not disclose such Confidential Information and shall use
 * it only in accordance with the terms of the license agreement
 * you entered into with Sun.
 */

package java.util;

import java.io.*;

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

    protected Object clone() {
    HashtableEntry entry = new HashtableEntry();
    entry.hash = hash;
    entry.key = key;
    entry.value = value;
    entry.next = (next != null) ? (HashtableEntry)next.clone() : null;
    return entry;
    }
}

/**
 * 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 efficiency: its <i>capacity</i> and its <i>load
 * factor</i>. The load factor should be between 0.0 and 1.0. 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. Larger load factors use
 * memory more efficiently, at the expense of larger expected time
 * per lookup.
 * <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>
 *
 * @author  Arthur van Hoff
 * @version 1.42, 07/01/98
 * @see     java.lang.Object#equals(java.lang.Object)
 * @see     java.lang.Object#hashCode()
 * @see     java.util.Hashtable#rehash()
 * @since   JDK1.0
 */
public
class Hashtable extends Dictionary implements Cloneable, java.io.Serializable {
    /**
     * The hash table data.
     */
    private transient HashtableEntry 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;

    /** 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        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 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
     * and default load factor.
     *
     * @param   initialCapacity   the initial capacity of the hashtable.
     * @since   JDK1.0
     */
    public Hashtable(int initialCapacity) {
    this(initialCapacity, 0.75f);
    }

    /**
     * Constructs a new, empty hashtable with a default capacity and load
     * factor.
     *
     * @since   JDK1.0
     */
    public Hashtable() {
    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 synchronized Enumeration keys() {
    return new HashtableEnumerator(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 synchronized Enumeration elements() {
    return new HashtableEnumerator(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 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;
    }

    /**
     * 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 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;
    }

    /**
     * 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 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 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;
    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;
        }
    }
    }

    /**
     * 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.lang.Object#equals(java.lang.Object)
     * @see     java.util.Hashtable#get(java.lang.Object)
     * @since   JDK1.0
     */
    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 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 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 this hashtable so that it contains no keys.
     *
     * @since   JDK1.0
     */
    public synchronized void clear() {
    HashtableEntry tab[] = table;
    for (int index = tab.length; --index >= 0; )
        tab[index] = null;
    count = 0;
    }

    /**
     * Creates a shallow copy of this hashtable. The keys and values
     * themselves are not cloned.
     * This is a relatively expensive operation.
     *
     * @return  a clone of the hashtable.
     * @since   JDK1.0
     */
    public synchronized Object clone() {
    try {
        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;
    } catch (CloneNotSupportedException e) {
        // this shouldn't happen, since we are Cloneable
        throw new InternalError();
    }
    }

    /**
     * Returns a rather long string representation of this hashtable.
     *
     * @return  a string representation of this hashtable.
     * @since   JDK1.0
     */
    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();
    }

    /**
     * WriteObject is called to save the state of the hashtable to a stream.
     * Only the keys and values are serialized since the hash values may be
     * different when the contents are restored.
     * iterate over the contents and write out the keys and values.
     */
    private synchronized void writeObject(java.io.ObjectOutputStream s)
        throws IOException
    {
    // Write out the length, threshold, loadfactor
    s.defaultWriteObject();

    // Write out length, count of elements and then the key/value objects
    s.writeInt(table.length);
    s.writeInt(count);
    for (int index = table.length-1; index >= 0; index--) {
        HashtableEntry entry = table[index];

        while (entry != null) {
        s.writeObject(entry.key);
        s.writeObject(entry.value);
        entry = entry.next;
        }
    }
    }

    /**
     * readObject is called to restore the state of the hashtable from
     * a stream.  Only the keys and values are serialized since the
     * hash values may be different when the contents are restored.
     * Read count elements and insert into the hashtable.
     */
    private synchronized void readObject(java.io.ObjectInputStream s)
         throws IOException, ClassNotFoundException
    {
    // Read in the length, threshold, and loadfactor
    s.defaultReadObject();

    // Read the original length of the array and number of elements
    int origlength = s.readInt();
    int elements = s.readInt();

    // Compute new size with a bit of room 5% to grow but
    // No larger than the original size.  Make the length
    // odd if it's large enough, this helps distribute the entries.
    // Guard against the length ending up zero, that's not valid.
    int length = (int)(elements * loadFactor) + (elements / 20) + 3;
    if (length > elements && (length & 1) == 0)
        length--;
    if (origlength > 0 && length > origlength)
        length = origlength;

    table = new HashtableEntry[length];
    count = 0;

    // Read the number of elements and then all the key/value objects
    for (; elements > 0; elements--) {
        Object key = s.readObject();
        Object value = s.readObject();
        put(key, value);
    }
    }
}

/**
 * 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");
    }
}