001    /*--------------------------------------------------------------------------+
002    $Id: LRUCacheBase.java 26268 2010-02-18 10:44:30Z juergens $
003    |                                                                          |
004    | Copyright 2005-2010 Technische Universitaet Muenchen                     |
005    |                                                                          |
006    | Licensed under the Apache License, Version 2.0 (the "License");          |
007    | you may not use this file except in compliance with the License.         |
008    | You may obtain a copy of the License at                                  |
009    |                                                                          |
010    |    http://www.apache.org/licenses/LICENSE-2.0                            |
011    |                                                                          |
012    | Unless required by applicable law or agreed to in writing, software      |
013    | distributed under the License is distributed on an "AS IS" BASIS,        |
014    | WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
015    | See the License for the specific language governing permissions and      |
016    | limitations under the License.                                           |
017    +--------------------------------------------------------------------------*/
018    package edu.tum.cs.commons.cache;
019    
020    import java.util.LinkedHashMap;
021    
022    import edu.tum.cs.commons.assertion.CCSMPre;
023    import edu.tum.cs.commons.error.NeverThrownRuntimeException;
024    
025    /**
026     * A cache with a fixed size using a last recently used (LRU) strategy. If
027     * identifiers itself are suitable hash keys, use class
028     * {@link edu.tum.cs.commons.cache.LRUStraightCacheBase}.
029     * 
030     * @author hummelb
031     * @author $Author: juergens $
032     * @version $Rev: 26268 $
033     * @levd.rating GREEN Hash: 7C20602D714E70624D8FF1B0F6BAAEFF
034     * 
035     * @param <I>
036     *            the index type of the cache
037     * @param <H>
038     *            the hash map key type
039     * @param <E>
040     *            the type stored in the cache
041     * @param <X>
042     *            the type of exception thrown by the {@link #obtainItem(Object)}
043     *            method. Use the {@link NeverThrownRuntimeException} if no
044     *            exception will be thrown.
045     */
046    public abstract class LRUCacheBase<I, H, E, X extends Exception> extends
047                    CacheBase<I, H, E, X> {
048    
049            /** The actual cache. */
050            private final LinkedHashMap<H, E> cache;
051    
052            /** Constructor. */
053            public LRUCacheBase(final int maxSize) {
054                    CCSMPre.isTrue(maxSize > 0, "Maximal size must be positive!");
055    
056                    cache = new LinkedHashMap<H, E>(2 * maxSize, .6f, true) {
057                            @Override
058                            protected boolean removeEldestEntry(java.util.Map.Entry<H, E> eldest) {
059                                    return size() > maxSize;
060                            }
061                    };
062            }
063    
064            /** {@inheritDoc} */
065            @Override
066            public E getItem(I identifier) throws X {
067                    H key = getHashKey(identifier);
068                    E value = cache.get(key);
069                    if (value == null) {
070                            value = obtainItem(identifier);
071                            cache.put(key, value);
072                    }
073                    return value;
074            }
075    }