WeakValueHashMapFrom http://stackoverflow.com/questions/13889051/weak-hashmap-with-weak-references-to-the-values

WeakValueHashMap.java

package tools.cache;

import java.lang.ref.WeakReference;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

public class WeakValueHashMap<K,V> extends HashMap<K,V> implements Map<K, V> {
    private HashMap<K,WeakReference<V>> mDatabase=new HashMap<K, WeakReference<V>>();

    @Override
    public V get(Object key) {
        WeakReference<V> weakRef=mDatabase.get(key);
        if(weakRef!=null) {
            return weakRef.get();
        }
        this.mDatabase.remove(key);
        return null;
    }

    @Override
    public V put(K key, V value) {
        WeakReference<V> weak = new WeakReference<V>(value);
        mDatabase.put(key, weak);
        return value;
    }

    public void removeInvalidKeyPairs() {
        Iterator<Entry<K,WeakReference<V>>> it = mDatabase.entrySet().iterator();

        while(it.hasNext()) {
            Entry<K,WeakReference<V>> e = it.next();
            if(e.getValue()==null) {
                it.remove();
            }
        }
    }
}