It's BitmapCache for ImageLoader.ImageCache of Volley. It rarely use memory through WeakReference. And it's thread-safety.

gistfile1.java

/*
 * Copyright 2013 Hi Zeon
 * Copyright 2011 The Android Open Source Project
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
 
public class WeakImageCache implements ImageLoader.ImageCache
{
	private final static int MAX_SIZE = 4096;
	private ConcurrentHashMap<String, WeakReference<Bitmap>> map
            = new ConcurrentHashMap<String, WeakReference<Bitmap>>();

	@Override
	public void putBitmap(String k, Bitmap v){
		if(map.size() > MAX_SIZE){
			map.clear();
		}
		map.put(k, new WeakReference<Bitmap>(v));
	}

	@Override
	public Bitmap getBitmap(String key){
		WeakReference<Bitmap> ref = map.get(key);
		
		if(ref == null){
			return null;
		}
		
		return ref.get();
	}
}