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/*******************************************************************************
* Copyright (c) 2009, 2010 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;
}
private static final long serialVersionUID = 1L;
private Object readResolve() {
// replace this object with the singleton
return INSTANCE;
}
}
}
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