sclass WeakValueMap<A, B> is AutoCloseable {
  // A value that has been stored with this key has been garbage
  // collected.
  event valueForKeyReleased(A key);
  
  // internal
  Map<A, WeakRef<B>> map = syncMap();
  RunnablesReferenceQueue queue;
  
  class MyRef extends WeakRef<B> is Runnable {
    A key;
    
    *(A *key, B value) { super(value, queue()!); }
    
    run {
      syncMapRemoveKeyAndValuePair(map, key, this);
      valueForKeyReleased(key);
    }
  }
  
  // public methods follow
  
  B get(A key) {
    ret getWeakRef(map.get(key));
  }
  
  B put(A key, B value) {
    synchronized(mutex()) {
      if (value == null) {
        B old = getWeakRef(map.get(key));
        map.remove(key);
        ret old;
      } else
        ret getWeakRef(map.put(key, new MyRef(key, value)));
    }
  }
  
  O mutex() { ret collectionMutex(map); }
  
  RunnablesReferenceQueue queue() {
    synchronized(mutex()) {
      if (queue == null) queue = new RunnablesReferenceQueue;
      ret queue;
    }
  }
  
  close {
    synchronized(mutex()) {  
      dispose queue;
    }
  }
}