| /******************************************************************************* |
| * Copyright (c) 2009, 2011 Oracle. All rights reserved. |
| * This program and the accompanying materials are made available under the |
| * terms of the Eclipse Public License v1.0, which accompanies this distribution |
| * and is available at http://www.eclipse.org/legal/epl-v10.html. |
| * |
| * Contributors: |
| * Oracle - initial API and implementation |
| ******************************************************************************/ |
| package org.eclipse.jpt.common.utility.internal; |
| |
| import java.io.Serializable; |
| import java.util.NoSuchElementException; |
| |
| /** |
| * Interface defining the classic queue behavior, |
| * without the backdoors allowed by {@link java.util.Queue}. |
| * |
| * @param <E> the type of elements contained by the queue |
| */ |
| public interface Queue<E> { |
| |
| /** |
| * "Enqueue" the specified item to the tail of the queue. |
| */ |
| void enqueue(E o); |
| |
| /** |
| * "Dequeue" an item from the head of the queue. |
| */ |
| E dequeue(); |
| |
| /** |
| * Return the item on the head of the queue |
| * without removing it from the queue. |
| */ |
| E peek(); |
| |
| /** |
| * Return whether the queue is empty. |
| */ |
| boolean isEmpty(); |
| |
| |
| final class Empty<E> |
| implements Queue<E>, Serializable |
| { |
| @SuppressWarnings("rawtypes") |
| public static final Queue INSTANCE = new Empty(); |
| @SuppressWarnings("unchecked") |
| public static <T> Queue<T> instance() { |
| return INSTANCE; |
| } |
| // ensure single instance |
| private Empty() { |
| super(); |
| } |
| public void enqueue(E o) { |
| throw new UnsupportedOperationException(); |
| } |
| public E dequeue() { |
| throw new NoSuchElementException(); |
| } |
| public E peek() { |
| throw new NoSuchElementException(); |
| } |
| public boolean isEmpty() { |
| return true; |
| } |
| @Override |
| public String toString() { |
| return StringTools.buildSingletonToString(this); |
| } |
| private static final long serialVersionUID = 1L; |
| private Object readResolve() { |
| // replace this object with the singleton |
| return INSTANCE; |
| } |
| } |
| } |