public class LruCache<K, V> {
 //缓存 map 集合，为什么要用LinkedHashMap
 //因为没错取了缓存值之后，都要进行排序，以确保
 //下次移除的是最少使用的值
 private final LinkedHashMap<K, V> map;
 //当前缓存的值
 private int size;
 //最大值
 private int maxSize;
 //添加到缓存中的个数
 private int putCount;
 //创建的个数
 private int createCount;
 //被移除的个数
 private int evictionCount;
 //命中个数
 private int hitCount;
 //丢失个数
 private int missCount;
 //实例化 Lru，需要传入缓存的最大值
 //这个最大值可以是个数，比如对象的个数，也可以是内存的大小
 //比如，最大内存只能缓存5兆
 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);
 }
 //重置最大缓存的值
 public void resize(int maxSize) {
  if (maxSize <= 0) {
   throw new IllegalArgumentException("maxSize <= 0");
  }
  synchronized (this) {
   this.maxSize = maxSize;
  }
  trimToSize(maxSize);
 }
 //通过 key 获取缓存值
 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++;
  }
  //如果没有，用户可以去创建
  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 {
    //缓存的大小改变
    size += safeSizeOf(key, createdValue);
   }
  }
  //这里没有移除，只是改变了位置
  if (mapValue != null) {
   entryRemoved(false, key, createdValue, mapValue);
   return mapValue;
  } else {
   //判断缓存是否越界
   trimToSize(maxSize);
   return createdValue;
  }
 }
 //添加缓存，跟上面这个方法的 create 之后的代码一样的
 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;
 }
 //检测缓存是否越界
 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!");
    }
    //如果没有，则返回
    if (size <= maxSize) {
     break;
    }
    //以下代码表示已经超出了最大范围
    Map.Entry<K, V> toEvict = null;
    for (Map.Entry<K, V> entry : map.entrySet()) {
     toEvict = entry;
    }
    if (toEvict == null) {
     break;
    }
    //移除最后一个，也就是最少使用的缓存
    key = toEvict.getKey();
    value = toEvict.getValue();
    map.remove(key);
    size -= safeSizeOf(key, value);
    evictionCount++;
   }
   entryRemoved(true, key, value, null);
  }
 }
 //手动移除，用户调用
 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;
 }
 //这里用户可以重写它，实现数据和内存回收操作
 protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}
 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;
 }
  //这个方法要特别注意，跟我们实例化 LruCache 的 maxSize 要呼应，怎么做到呼应呢，比如 maxSize 的大小为缓存的个数，这里就是 return 1就 ok，如果是内存的大小，如果5M，这个就不能是个数 了，这是应该是每个缓存 value 的 size 大小，如果是 Bitmap，这应该是 bitmap.getByteCount();
 protected int sizeOf(K key, V value) {
  return 1;
 }
 //清空缓存
 public final void evictAll() {
  trimToSize(-1); // -1 will evict 0-sized elements
 }
 public synchronized final int size() {
  return size;
 }
 public synchronized final int maxSize() {
  return maxSize;
 }
 public synchronized final int hitCount() {
  return hitCount;
 }
 public synchronized final int missCount() {
  return missCount;
 }
 public synchronized final int createCount() {
  return createCount;
 }
 public synchronized final int putCount() {
  return putCount;
 }
 public synchronized final int evictionCount() {
  return evictionCount;
 }
 public synchronized final Map<K, V> snapshot() {
  return new LinkedHashMap<K, V>(map);
 }
}

/**
 * Created by gyzhong on 15/4/5.
 */
public class LruPageAdapter extends PagerAdapter {
 private List<String> mData ;
 private LruCache<String,Bitmap> mLruCache ;
 private int mTotalSize = (int) Runtime.getRuntime().totalMemory();
 private ViewPager mViewPager ;
 public LruPageAdapter(ViewPager viewPager ,List<String> data){
  mData = data ;
  mViewPager = viewPager ;
  /*实例化LruCache*/
  mLruCache = new LruCache<String,Bitmap>(mTotalSize/5){
   /*当缓存大于我们设定的最大值时，会调用这个方法，我们可以用来做内存释放操作*/
   @Override
   protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue) {
    super.entryRemoved(evicted, key, oldValue, newValue);
    if (evicted && oldValue != null){
     oldValue.recycle();
    }
   }
   /*创建 bitmap*/
   @Override
   protected Bitmap create(String key) {
    final int resId = mViewPager.getResources().getIdentifier(key,"drawable",
      mViewPager.getContext().getPackageName()) ;
    return BitmapFactory.decodeResource(mViewPager.getResources(),resId) ;
   }
   /*获取每个 value 的大小*/
   @Override
   protected int sizeOf(String key, Bitmap value) {
    return value.getByteCount();
   }
  } ;
 }
 @Override
 public Object instantiateItem(ViewGroup container, int position) {
  View view = LayoutInflater.from(container.getContext()).inflate(R.layout.view_pager_item, null) ;
  ImageView imageView = (ImageView) view.findViewById(R.id.id_view_pager_item);
  Bitmap bitmap = mLruCache.get(mData.get(position));
  imageView.setImageBitmap(bitmap);
  container.addView(view);
  return view;
 }
 @Override
 public void destroyItem(ViewGroup container, int position, Object object) {
  container.removeView((View) object);
 }
 @Override
 public int getCount() {
  return mData.size();
 }
 @Override
 public boolean isViewFromObject(View view, Object object) {
  return view == object;
 }
}