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/*
 * Copyright (c) 1995, 1996, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 * questions.
 */

package sun.misc;

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


Caches the collision list.

/**
 * Caches the collision list.
 */
class CacheEntry extends Ref {
    int hash;
    Object key;
    CacheEntry next;
    public Object reconstitute() {
        return null;
    }
}


The Cache class. Maps keys to values. Any object can be used as
a key and/or value.  This is very similar to the Hashtable
class, except that after putting an object into the Cache,
it is not guaranteed that a subsequent get will return it.
The Cache will automatically remove entries if memory is
getting tight and if the entry is not referenced from outside
the Cache.

To sucessfully store and retrieve objects from a hash table the
object used as the key must implement the hashCode() and equals()
methods.

This example creates a Cache of numbers. It uses the names of
the numbers as keys:

     Cache numbers = new Cache();
     numbers.put("one", new Integer(1));
     numbers.put("two", new Integer(1));
     numbers.put("three", new Integer(1));


To retrieve a number use:

     Integer n = (Integer)numbers.get("two");
     if (n != null) {
         System.out.println("two = " + n);
     }



See Also:
/**
 * The Cache class. Maps keys to values. Any object can be used as
 * a key and/or value.  This is very similar to the Hashtable
 * class, except that after putting an object into the Cache,
 * it is not guaranteed that a subsequent get will return it.
 * The Cache will automatically remove entries if memory is
 * getting tight and if the entry is not referenced from outside
 * the Cache.<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 Cache of numbers. It uses the names of
 * the numbers as keys:
 * <pre>
 *      Cache numbers = new Cache();
 *      numbers.put("one", new Integer(1));
 *      numbers.put("two", new Integer(1));
 *      numbers.put("three", new Integer(1));
 * </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
 * @see sun.misc.Ref
 */
public
class Cache extends Dictionary {
    
The hash table data.

/**
     * The hash table data.
     */
    private CacheEntry table[];

    
The total number of entries in the hash table.

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

    
Rehashes the table when count exceeds this threshold.

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

    
The load factor for the hashtable.

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

    private void init(int initialCapacity, float loadFactor) {
        if ((initialCapacity <= 0) || (loadFactor <= 0.0)) {
            throw new IllegalArgumentException();
        }
        this.loadFactor = loadFactor;
        table = new CacheEntry[initialCapacity];
        threshold = (int) (initialCapacity * loadFactor);
    }

    
Constructs a new, empty Cache with the specified initial
capacity and the specified load factor.

Params:
  • initialCapacity – the initial number of buckets
  • loadFactor – a number between 0.0 and 1.0, it defines
         the threshold for rehashing the Cache into
         a bigger one.
Throws:
/**
     * Constructs a new, empty Cache 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 Cache 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 Cache (int initialCapacity, float loadFactor) {
        init(initialCapacity, loadFactor);
    }

    
Constructs a new, empty Cache with the specified initial
capacity.

Params:
  • initialCapacity – the initial number of buckets
/**
     * Constructs a new, empty Cache with the specified initial
     * capacity.
     * @param initialCapacity the initial number of buckets
     */
    public Cache (int initialCapacity) {
        init(initialCapacity, 0.75f);
    }

    
Constructs a new, empty Cache. A default capacity and load factor
is used. Note that the Cache will automatically grow when it gets
full.

/**
     * Constructs a new, empty Cache. A default capacity and load factor
     * is used. Note that the Cache will automatically grow when it gets
     * full.
     */
    public Cache () {
        try {
            init(101, 0.75f);
        } catch (IllegalArgumentException ex) {
            // This should never happen
            throw new Error("panic");
        }
    }

    
Returns the number of elements contained within the Cache.

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

    
Returns true if the Cache contains no elements.

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

    
Returns an enumeration of the Cache's keys.

See Also:
/**
     * Returns an enumeration of the Cache's keys.
     * @see Cache#elements
     * @see Enumeration
     */
    public synchronized Enumeration keys() {
        return new CacheEnumerator(table, true);
    }

    
Returns an enumeration of the elements. Use the Enumeration methods
on the returned object to fetch the elements sequentially.

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

    
Gets the object associated with the specified key in the Cache.

Params:
  • key – the key in the hash table
See Also:
@returnsthe element for the key or null if the key
         is not defined in the hash table.
/**
     * Gets the object associated with the specified key in the Cache.
     * @param key the key in the hash table
     * @returns the element for the key or null if the key
     *          is not defined in the hash table.
     * @see Cache#put
     */
    public synchronized Object get(Object key) {
        CacheEntry tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (CacheEntry e = tab[index]; e != null; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                return e.check();
            }
        }
        return null;
    }

    
Rehashes the contents of the table into a bigger table.
This is method is called automatically when the Cache's
size exceeds the threshold.

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

        int newCapacity = oldCapacity * 2 + 1;
        CacheEntry newTable[] = new CacheEntry[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 (CacheEntry old = oldTable[i]; old != null;) {
                CacheEntry e = old;
                old = old.next;
                if (e.check() != null) {
                    int index = (e.hash & 0x7FFFFFFF) % newCapacity;
                    e.next = newTable[index];
                    newTable[index] = e;
                } else
                    count--;    /* remove entries that have disappeared */
            }
        }
    }

    
Puts the specified element into the Cache, 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.

Params:
  • key – the specified hashtable key
  • value – the specified element
Throws:
See Also:
Returns:the old value of the key, or null if it did not have one.
/**
     * Puts the specified element into the Cache, 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 hashtable key
     * @param value the specified element
     * @return the old value of the key, or null if it did not have one.
     * @exception NullPointerException If the value of the specified
     * element is null.
     * @see Cache#get
     */
    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 cache.
        CacheEntry tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
        CacheEntry ne = null;
        for (CacheEntry e = tab[index]; e != null; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                Object old = e.check();
                e.setThing(value);
                return old;
            } else if (e.check() == null)
                ne = e;         /* reuse old flushed value */
        }

        if (count >= threshold) {
            // Rehash the table if the threshold is exceeded
            rehash();
            return put(key, value);
        }
        // Creates the new entry.
        if (ne == null) {
            ne = new CacheEntry ();
            ne.next = tab[index];
            tab[index] = ne;
            count++;
        }
        ne.hash = hash;
        ne.key = key;
        ne.setThing(value);
        return null;
    }

    
Removes the element corresponding to the key. Does nothing if the
key is not present.

Params:
  • key – the key that needs to be removed
Returns:the value of key, or null if the key was not found.
/**
     * 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) {
        CacheEntry tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (CacheEntry 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.check();
            }
        }
        return null;
    }
}


A Cache enumerator class.  This class should remain opaque
to the client. It will use the Enumeration interface.

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

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

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

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

}