src/main/java/com/googlecode/objectify/util/IdentityMultimapList.java
public boolean add(K key, V value) {

		List<V> list = this.get(key);
		if (list == null) {
			list = new ArrayList<>();
			this.put(key, list);
		}

		return list.add(value);
	}
public boolean add(K key, V value) {

		List<V> list = this.get(key);
		if (list == null) {
			list = new ArrayList<>();
			this.put(key, list);
		}

		return list.add(value);
	}
Adds a value to the set associated with the key.