LRUCache

// Copyright 2007 Christian d'Heureuse, Inventec Informatik AG, Zurich, Switzerland

// www.source-code.biz, www.inventec.ch/chdh

//

// This module is multi-licensed and may be used under the terms

// of any of the following licenses:

//

//  EPL, Eclipse Public License, V1.0 or later, http://www.eclipse.org/legal

//  LGPL, GNU Lesser General Public License, V2 or later, http://www.gnu.org/licenses/lgpl.html

//  GPL, GNU General Public License, V2 or later, http://www.gnu.org/licenses/gpl.html

//  AL, Apache License, V2.0 or later, http://www.apache.org/licenses

//  BSD, BSD License, http://www.opensource.org/licenses/bsd-license.php

//

// Please contact the author if you need another license.

// This module is provided "as is", without warranties of any kind.



import java.util.LinkedHashMap;

import java.util.Collection;

import java.util.Map;

import java.util.ArrayList;



/**

* An LRU cache, based on <code>LinkedHashMap</code>.

*

* <p>

* This cache has a fixed maximum number of elements (<code>cacheSize</code>).

* If the cache is full and another entry is added, the LRU (least recently used) entry is dropped.

*

* <p>

* This class is thread-safe. All methods of this class are synchronized.

*

* <p>

* Author: Christian d'Heureuse, Inventec Informatik AG, Zurich, Switzerland<br>

* Multi-licensed: EPL / LGPL / GPL / AL / BSD.

*/

public class LRUCache<K,V> {



private static final float   hashTableLoadFactor = 0.75f;



private LinkedHashMap<K,V>   map;

private int                  cacheSize;



/**

* Creates a new LRU cache.

* @param cacheSize the maximum number of entries that will be kept in this cache.

*/

public LRUCache (int cacheSize) {

   this.cacheSize = cacheSize;

   int hashTableCapacity = (int)Math.ceil(cacheSize / hashTableLoadFactor) + 1;

   map = new LinkedHashMap<K,V>(hashTableCapacity, hashTableLoadFactor, true) {

      // (an anonymous inner class)

      private static final long serialVersionUID = 1;

      @Override protected boolean removeEldestEntry (Map.Entry<K,V> eldest) {

         return size() > LRUCache.this.cacheSize; }}; }



/**

* Retrieves an entry from the cache.<br>

* The retrieved entry becomes the MRU (most recently used) entry.

* @param key the key whose associated value is to be returned.

* @return    the value associated to this key, or null if no value with this key exists in the cache.

*/

public synchronized V get (K key) {

   return map.get(key); }



/**

* Adds an entry to this cache.

* The new entry becomes the MRU (most recently used) entry.

* If an entry with the specified key already exists in the cache, it is replaced by the new entry.

* If the cache is full, the LRU (least recently used) entry is removed from the cache.

* @param key    the key with which the specified value is to be associated.

* @param value  a value to be associated with the specified key.

*/

public synchronized void put (K key, V value) {

   map.put (key, value); }



/**

* Clears the cache.

*/

public synchronized void clear() {

   map.clear(); }



/**

* Returns the number of used entries in the cache.

* @return the number of entries currently in the cache.

*/

public synchronized int usedEntries() {

   return map.size(); }



/**

* Returns a <code>Collection</code> that contains a copy of all cache entries.

* @return a <code>Collection</code> with a copy of the cache content.

*/

public synchronized Collection<Map.Entry<K,V>> getAll() {

   return new ArrayList<Map.Entry<K,V>>(map.entrySet()); }



} // end class LRUCache

import java.util.Map;

// Test program for the LRUCache class.
public class TestLRUCache {

public static void main (String[] args) {
   LRUCache<String,String> c = new LRUCache<String, String>(3);
   c.put ("1", "one");                           // 1
   c.put ("2", "two");                           // 2 1
   c.put ("3", "three");                         // 3 2 1
   c.put ("4", "four");                          // 4 3 2
   if (c.get("2") == null) throw new Error();    // 2 4 3
   c.put ("5", "five");                          // 5 2 4
   c.put ("4", "second four");                   // 4 5 2
   // Verify cache content.
   if (c.usedEntries() != 3)              throw new Error();
   if (!c.get("4").equals("second four")) throw new Error();
   if (!c.get("5").equals("five"))        throw new Error();
   if (!c.get("2").equals("two"))         throw new Error();
   // List cache content.
   for (Map.Entry<String, String> e : c.getAll())
      System.out.println (e.getKey() + " : " + e.getValue()); }

} // end class TestLRUCache