/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

package java.util;

/**
 * This class provides skeletal implementations of some {@link Queue}
 * operations. The implementations in this class are appropriate when
 * the base implementation does <em>not</em> allow <tt>null</tt>
 * elements.  Methods {@link #add add}, {@link #remove remove}, and
 * {@link #element element} are based on {@link #offer offer}, {@link
 * #poll poll}, and {@link #peek peek}, respectively but throw
 * exceptions instead of indicating failure via <tt>false</tt> or
 * <tt>null</tt> returns.
 *
 * <p> A <tt>Queue</tt> implementation that extends this class must
 * minimally define a method {@link Queue#offer} which does not permit
 * insertion of <tt>null</tt> elements, along with methods {@link
 * Queue#peek}, {@link Queue#poll}, {@link Collection#size}, and a
 * {@link Collection#iterator} supporting {@link
 * Iterator#remove}. Typically, additional methods will be overriden
 * as well. If these requirements cannot be met, consider instead
 * subclassing {@link AbstractCollection}.
 *
 * @since 1.5
 * @author Doug Lea
 */
public abstract class AbstractQueue<E>
    extends AbstractCollection<E>
    implements Queue<E> {

    /**
     * Constructor for use by subclasses.
     */
    protected AbstractQueue() {
    }


    /**
     * Adds the specified element to this queue. This implementation
     * returns <tt>true</tt> if <tt>offer</tt> succeeds, else
     * throws an IllegalStateException. 
     * th
     * @param o the element
     * @return <tt>true</tt> (as per the general contract of
     *         <tt>Collection.add</tt>).
     *
     * @throws NullPointerException if the specified element is <tt>null</tt>
     * @throws IllegalStateException if element cannot be added
     */
    public boolean add(E o) {
        if (offer(o))
            return true;
        else
            throw new IllegalStateException("Queue full");
    }

    /**
     * Retrieves and removes the head of this queue.
     * This implementation returns the result of <tt>poll</tt>
     * unless the queue is empty.
     *
     * @return the head of this queue.
     * @throws NoSuchElementException if this queue is empty.
     */
    public E remove() {
        E x = poll();
        if (x != null)
            return x;
        else
            throw new NoSuchElementException();
    }


    /**
     * Retrieves, but does not remove, the head of this queue.  
     * This implementation returns the result of <tt>peek</tt>
     * unless the queue is empty.
     *
     * @return the head of this queue.
     * @throws NoSuchElementException if this queue is empty.
     */
    public E element() {
        E x = peek();
        if (x != null)
            return x;
        else
            throw new NoSuchElementException();
    }

    /**
     * Removes all of the elements from this collection.
     * The collection will be empty after this call returns.
     * <p>This implementation repeatedly invokes {@link #poll poll} until it
     * returns <tt>null</tt>.
     */
    public void clear() {
        while (poll() != null)
            ;
    }
}