An LRU cache class based on java.util.LinkedHashMap

An LRU (least recently used) cache is used to buffer a limited number of the MRU (most recently used) objects of a class in memory.

API documentation:LRUCache.htmlDownload full package: LRUCache.zip

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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

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// Test routine for the LRUCache class.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()); }

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Author: Christian d'Heureuse (www.source-code.biz, www.inventec.ch/chdh)
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