Android LRUCache

 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. 
一个cache缓冲区，维护有限个值的强引用。当一个值被命中时，它被移到队列的头部？？？确定有移动么
当一个值被加入一个已满的cache中时，队列最后一个值被舍弃。可能会被垃圾回收
  * <p>If your cached values hold resources that need to be explicitly released,
  * override {@link #entryRemoved}.
如果你缓存的值中有必须被释放的资源，需要override  entryRemoved方法
  * <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.
如果一次cache未命中时，同样需要返回。 需要override create()方法。
这使调用简化：不管是不是命中，都返回一个值。

因为当前的create方法返回的是个null。当你没get到的时候，会调用create方法。在当前情况下，返回null会直接return null。
不会往后执行之后的创建键值对，并返回的操作。

但是如果你重写了create()，使其返回的不是null，则后边的操作会执行。调用get(key)，value值被返回。
  * <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:
默认情况下，一个cache的大小,是以实体的个数来衡量的。因为当前的sizeOf() 方法返回的是1，也就是每个entry大小是1
你可以override  sizeof方法，改变cache 的最小单元大小。
例如，下边这个cache 限制为最大4MiB的bitmap cache
每个单元的大小就不是1了，而是bitmap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>

LRUCache类是线程安全的。多个cache操作时候。要synchronize这个cache

  * <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>

这个类不允许传入的key和value是空。
get put remove返回null，代表这个key不再cache中

  * <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.
  *
  * <p>This class appeared in Android 3.1 (Honeycomb MR1); it's available as part
  * of <a href="http://developer.android.com/sdk/compatibility-library.html">Android's
  * Support Package</a> for earlier releases.
  */
 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;
     private int maxSize;

     private int putCount;
     private int createCount;
     private int evictionCount;
     private int hitCount;
     private int missCount;

     /**
      * @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);
     }

     /**
      * Sets the size of the cache.
      * @param maxSize The new maximum size.
      *
      * @hide
      */
     public void resize(int maxSize) {
         if (maxSize <= 0) {
             throw new IllegalArgumentException("maxSize <= 0");
         }

         synchronized (this) {
             this.maxSize = maxSize;
         }
         trimToSize(maxSize);
     }

     /**
      * 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.
      */
     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++;
                 return mapValue;
             }
             missCount++;
         }

         /*
          * 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.
          */

         V createdValue = create(key);
         if (createdValue == null) {
             return null;
         }

         synchronized (this) {
             createCount++;
             mapValue = map.put(key, createdValue);

             if (mapValue != null) {
                 // There was a conflict so undo that last 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) {
                 size -= safeSizeOf(key, previous);
             }
         }

         if (previous != null) {
             entryRemoved(false, key, previous, value);
         }

         trimToSize(maxSize);
         return previous;
     }

     /**
      * Remove the eldest entries until the total of remaining entries is at or
      * below the requested size.
      *
      * @param maxSize the maximum size of the cache before returning. May be -1
      *            to evict even 0-sized elements.
      */
     public 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.
      *
      * @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.
      *
      * <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}.
      * @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.
      *
      * <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.
      *
      * <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.
      */
     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.
      */
     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.
      */
     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);
     }
 }