hashtable.java

来自「《移动Agent技术》一书的所有章节源代码。」· Java 代码 · 共 553 行 · 第 1/2 页

     * 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");
    }
}