/*
* @(#)Cache.java 1.11 95/09/06
*
* 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 sun.misc;
import java.util.Dictionary;
import java.util.Enumeration;
import java.util.NoSuchElementException;
/**
* 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.<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
* @version 1.11, 06 Sep 1995
*/
public
class Cache extends Dictionary {
/**
* The hash table data.
*/
private CacheEntry 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;
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.
* @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.
* @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.
*/
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.
*/
public int size() {
return count;
}
/**
* Returns true if the Cache contains no elements.
*/
public boolean isEmpty() {
return count == 0;
}
/**
* 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 Cache#keys
* @see Enumeration
*/
public synchronized Enumeration elements() {
return new CacheEnumerator(table, false);
}
/**
* 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.
*/
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.
* @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.
* @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.
*/
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");
}
}