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      1 /**
      2  * Copyright (c) 2011, Google Inc.
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *     http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package com.android.mail.utils;
     18 
     19 import java.util.LinkedHashMap;
     20 import java.util.Map;
     21 
     22 /**
     23  * A simple in-memory LRU cache, which is trivial to implement on top
     24  * of JDK's {@link LinkedHashMap}.
     25  *
     26  * LRU policy is ensured by the underlying LinkedHashMap functionality.
     27  */
     28 public final class LruCache<K, V> extends LinkedHashMap<K, V> {
     29     private static final long serialVersionUID = 1L;
     30     private final int maxCapacity;
     31 
     32     /**
     33      * Creates an instance of LRUCache, with given capacity.
     34      *
     35      * @param capacity maximum number of elements in the cache. This is also
     36      * used as initialCapacity i.e. memory is allocated upfront.
     37      */
     38     public LruCache(int capacity) {
     39         this(capacity, capacity);
     40     }
     41 
     42     /**
     43      * Creates an instance of LRUCache, with given capacity.
     44      *
     45      * @param initialCapacity initial capacity of the cache.
     46      * @param maxCapacity maximum number of elements in the cache.
     47      */
     48     private LruCache(int initialCapacity, int maxCapacity) {
     49         super(initialCapacity, (float) 0.75, true);
     50         this.maxCapacity = maxCapacity;
     51     }
     52 
     53     // These methods are needed because the default implementation of LinkedHashMap is *not*
     54     // synchronized.
     55     /**
     56      * Gets an element from the cache, returning the element if found, or null if not
     57      * @param key
     58      * @return
     59      */
     60     public synchronized V getElement(K key) {
     61         return get(key);
     62     }
     63 
     64     /**
     65      * Puts an element into the cache.
     66      * @param key
     67      * @param value, a non-null value.
     68      */
     69     public synchronized void putElement(K key, V value) {
     70         put(key, value);
     71     }
     72 
     73     /**
     74      * Removes an element from the cache. Returns the removed element, or null if no such element
     75      * existed in the cache.
     76      * @param key
     77      * @return
     78      */
     79     public synchronized V removeElement(K key) {
     80         return remove(key);
     81     }
     82 
     83     /**
     84      * {@inheritDoc}
     85      * <p>
     86      * Necessary to override because HashMap increases the capacity of the
     87      * hashtable before inserting the elements. However, here we call put() which
     88      * checks if we can remove the eldest element before increasing the capacity.
     89      */
     90     @Override
     91     public synchronized void putAll(Map<? extends K, ? extends V> m) {
     92         for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
     93             put(e.getKey(), e.getValue());
     94         }
     95     }
     96 
     97     /**
     98      * This method is called by LinkedHashMap to check whether the eldest entry
     99      * should be removed.
    100      *
    101      * @param eldest
    102      * @return true if element should be removed.
    103      */
    104     @Override
    105     protected synchronized boolean removeEldestEntry(Map.Entry<K, V> eldest) {
    106         return size() > maxCapacity;
    107     }
    108 }
    109