// 得到 應用程序 被分配的最大的內存    int maxMemory=(int) Runtime.getRuntime().maxMemory();    // 取處內存的 1/5 用來當 緩存 大小    int cachSize=maxMemory/5;    // 實例化 LruCache    lruCache=new lruCache<String, Bitmap>(cachSize){      //內部方法sizeOf設置每一張圖片的緩存大小      protected int sizeOf(String key, Bitmap value) {        //在每次存入緩存時調用，告訴系統這張緩存圖片有多大        // 相當于 為每次 要緩存的 資源 分配 大小空間        return value.getByteCount();      }    };     

public void putBitmapToCache(String url,Bitmap bitmap){    if (getBitmapfromCache(url)==null) {//判斷當前緩存是否存在      lruCache.put(url, bitmap);    }  }LurCache 的 get：public Bitmap getBitmapfromCache(String url){    return lruCache.get(url);//可將lruCache看成map  }

package android.util;  import java.util.LinkedHashMap; import java.util.Map;  /**  * A cache that holds strong references to a limited number of values. Each time  * a value is accessed, it is moved to the head of a queue. When a value is  * added to a full cache, the value at the end of that queue is evicted and may  * become eligible for garbage collection.  * Cache保存一個強引用來限制內容數量，每當Item被訪問的時候，此Item就會移動到隊列的頭部。 * 當cache已滿的時候加入新的item時，在隊列尾部的item會被回收。 * <p>If your cached values hold resources that need to be explicitly released,  * override {@link #entryRemoved}.  * 如果你cache的某個值需要明確釋放，重寫entryRemoved() * <p>If a cache miss should be computed on demand for the corresponding keys,  * override {@link #create}. This simplifies the calling code, allowing it to  * assume a value will always be returned, even when there's a cache miss.  * 如果key相對應的item丟掉啦，重寫create().這簡化了調用代碼，即使丟失了也總會返回。 * <p>By default, the cache size is measured in the number of entries. Override  * {@link #sizeOf} to size the cache in different units. For example, this cache  * is limited to 4MiB of bitmaps: 默認cache大小是測量的item的數量，重寫sizeof計算不同item的 * 大小。 * <pre>  {@code  *  int cacheSize = 4 * 1024 * 1024; // 4MiB  *  LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) {  *    protected int sizeOf(String key, Bitmap value) {  *      return value.getByteCount();  *    }  *  }}</pre>  *  * <p>This class is thread-safe. Perform multiple cache operations atomically by  * synchronizing on the cache: <pre>  {@code  *  synchronized (cache) {  *   if (cache.get(key) == null) {  *     cache.put(key, value);  *   }  *  }}</pre>  *  * <p>This class does not allow null to be used as a key or value. A return  * value of null from {@link #get}, {@link #put} or {@link #remove} is  * unambiguous: the key was not in the cache. * 不允許key或者value為null * 當get（），put（），remove（）返回值為null時，key相應的項不在cache中 */ public class LruCache<K, V> {   private final LinkedHashMap<K, V> map;    /** Size of this cache in units. Not necessarily the number of elements. */   private int size; //已經存儲的大小  private int maxSize; //規定的最大存儲空間   private int putCount; //put的次數  private int createCount; //create的次數  private int evictionCount; //回收的次數  private int hitCount; //命中的次數  private int missCount; //丟失的次數   /**    * @param maxSize for caches that do not override {@link #sizeOf}, this is    *   the maximum number of entries in the cache. For all other caches,    *   this is the maximum sum of the sizes of the entries in this cache.    */   public LruCache(int maxSize) {     if (maxSize <= 0) {       throw new IllegalArgumentException("maxSize <= 0");     }     this.maxSize = maxSize;     this.map = new LinkedHashMap<K, V>(0, 0.75f, true);   }    /**    * Returns the value for {@code key} if it exists in the cache or can be    * created by {@code #create}. If a value was returned, it is moved to the    * head of the queue. This returns null if a value is not cached and cannot    * be created. 通過key返回相應的item，或者創建返回相應的item。相應的item會移動到隊列的頭部，   * 如果item的value沒有被cache或者不能被創建，則返回null。   */   public final V get(K key) {     if (key == null) {       throw new NullPointerException("key == null");     }      V mapValue;     synchronized (this) {       mapValue = map.get(key);       if (mapValue != null) {         hitCount++; //命中        return mapValue;       }       missCount++; //丟失    }      /*      * Attempt to create a value. This may take a long time, and the map      * may be different when create() returns. If a conflicting value was      * added to the map while create() was working, we leave that value in      * the map and release the created value.      * 如果丟失了就試圖創建一個item     */      V createdValue = create(key);     if (createdValue == null) {       return null;     }      synchronized (this) {       createCount++;//創建++       mapValue = map.put(key, createdValue);        if (mapValue != null) {         // There was a conflict so undo that last put         //如果前面存在oldValue，那么撤銷put（）         map.put(key, mapValue);       } else {         size += safeSizeOf(key, createdValue);       }     }      if (mapValue != null) {       entryRemoved(false, key, createdValue, mapValue);       return mapValue;     } else {       trimToSize(maxSize);       return createdValue;     }   }    /**    * Caches {@code value} for {@code key}. The value is moved to the head of    * the queue.    *    * @return the previous value mapped by {@code key}.    */   public final V put(K key, V value) {     if (key == null || value == null) {       throw new NullPointerException("key == null || value == null");     }      V previous;     synchronized (this) {       putCount++;       size += safeSizeOf(key, value);       previous = map.put(key, value);       if (previous != null) { //返回的先前的value值        size -= safeSizeOf(key, previous);       }     }      if (previous != null) {       entryRemoved(false, key, previous, value);     }      trimToSize(maxSize);     return previous;   }    /**    * @param maxSize the maximum size of the cache before returning. May be -1    *   to evict even 0-sized elements.    * 清空cache空間   */   private void trimToSize(int maxSize) {     while (true) {       K key;       V value;       synchronized (this) {         if (size < 0 || (map.isEmpty() && size != 0)) {           throw new IllegalStateException(getClass().getName()               + ".sizeOf() is reporting inconsistent results!");         }          if (size <= maxSize) {           break;         }          Map.Entry<K, V> toEvict = map.eldest();         if (toEvict == null) {           break;         }          key = toEvict.getKey();         value = toEvict.getValue();         map.remove(key);         size -= safeSizeOf(key, value);         evictionCount++;       }        entryRemoved(true, key, value, null);     }   }    /**    * Removes the entry for {@code key} if it exists.    * 刪除key相應的cache項，返回相應的value   * @return the previous value mapped by {@code key}.    */   public final V remove(K key) {     if (key == null) {       throw new NullPointerException("key == null");     }      V previous;     synchronized (this) {       previous = map.remove(key);       if (previous != null) {         size -= safeSizeOf(key, previous);       }     }      if (previous != null) {       entryRemoved(false, key, previous, null);     }      return previous;   }    /**    * Called for entries that have been evicted or removed. This method is    * invoked when a value is evicted to make space, removed by a call to    * {@link #remove}, or replaced by a call to {@link #put}. The default    * implementation does nothing.    * 當item被回收或者刪掉時調用。改方法當value被回收釋放存儲空間時被remove調用，   * 或者替換item值時put調用，默認實現什么都沒做。   * <p>The method is called without synchronization: other threads may    * access the cache while this method is executing.    *    * @param evicted true if the entry is being removed to make space, false    *   if the removal was caused by a {@link #put} or {@link #remove}.    * true---為釋放空間被刪除；false---put或remove導致   * @param newValue the new value for {@code key}, if it exists. If non-null,    *   this removal was caused by a {@link #put}. Otherwise it was caused by    *   an eviction or a {@link #remove}.    */   protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}    /**    * Called after a cache miss to compute a value for the corresponding key.    * Returns the computed value or null if no value can be computed. The    * default implementation returns null.    * 當某Item丟失時會調用到，返回計算的相應的value或者null   * <p>The method is called without synchronization: other threads may    * access the cache while this method is executing.    *    * <p>If a value for {@code key} exists in the cache when this method    * returns, the created value will be released with {@link #entryRemoved}    * and discarded. This can occur when multiple threads request the same key    * at the same time (causing multiple values to be created), or when one    * thread calls {@link #put} while another is creating a value for the same    * key.    */   protected V create(K key) {     return null;   }    private int safeSizeOf(K key, V value) {     int result = sizeOf(key, value);     if (result < 0) {       throw new IllegalStateException("Negative size: " + key + "=" + value);     }     return result;   }    /**    * Returns the size of the entry for {@code key} and {@code value} in    * user-defined units. The default implementation returns 1 so that size    * is the number of entries and max size is the maximum number of entries.    * 返回用戶定義的item的大小，默認返回1代表item的數量，最大size就是最大item值   * <p>An entry's size must not change while it is in the cache.    */   protected int sizeOf(K key, V value) {     return 1;   }    /**    * Clear the cache, calling {@link #entryRemoved} on each removed entry.    * 清空cacke   */   public final void evictAll() {     trimToSize(-1); // -1 will evict 0-sized elements   }    /**    * For caches that do not override {@link #sizeOf}, this returns the number    * of entries in the cache. For all other caches, this returns the sum of    * the sizes of the entries in this cache.    */   public synchronized final int size() {     return size;   }    /**    * For caches that do not override {@link #sizeOf}, this returns the maximum    * number of entries in the cache. For all other caches, this returns the    * maximum sum of the sizes of the entries in this cache.    */   public synchronized final int maxSize() {     return maxSize;   }    /**    * Returns the number of times {@link #get} returned a value that was    * already present in the cache.    */   public synchronized final int hitCount() {     return hitCount;   }    /**    * Returns the number of times {@link #get} returned null or required a new    * value to be created.    */   public synchronized final int missCount() {     return missCount;   }    /**    * Returns the number of times {@link #create(Object)} returned a value.    */   public synchronized final int createCount() {     return createCount;   }    /**    * Returns the number of times {@link #put} was called.    */   public synchronized final int putCount() {     return putCount;   }    /**    * Returns the number of values that have been evicted.    * 返回被回收的數量   */   public synchronized final int evictionCount() {     return evictionCount;   }    /**    * Returns a copy of the current contents of the cache, ordered from least    * recently accessed to most recently accessed. 返回當前cache的副本，從最近最少訪問到最多訪問   */   public synchronized final Map<K, V> snapshot() {     return new LinkedHashMap<K, V>(map);   }    @Override public synchronized final String toString() {     int accesses = hitCount + missCount;     int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;     return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",         maxSize, hitCount, missCount, hitPercent);   } }