/**
* Copyright 2010 Google Inc.
*
* 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.
*/
package org.waveprotocol.wave.model.adt;
/**
* Encapsulates a value of which there is either zero or one canonical instance.
*
* @author anorth@google.com (Alex North)
* @param <V> type of the value
* @param <I> type of a value initializer
*/
public interface Singleton<V, I> {
/**
* Checks whether the singleton has a value.
*/
boolean hasValue();
/**
* Gets the singleton value, or null if it has no value.
*/
V get();
/**
* Sets the singleton value.
*
* It is implementation-dependent whether null is a valid initial state.
*/
V set(I initialState);
/**
* Clears the singleton value.
*/
void clear();
}