Queues.java

/*
 * Copyright (C) 2011 eXo Platform SAS.
 *
 * This is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation; either version 2.1 of
 * the License, or (at your option) any later version.
 *
 * This software is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this software; if not, write to the Free
 * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
 * 02110-1301 USA, or see the FSF site: http://www.fsf.org.
 */

package org.exoplatform.commons.utils;

import java.util.AbstractQueue;
import java.util.Collections;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;


/**
 * A LIFO stack implementing the {@link java.util.Queue} interface backed by an array.
 *
 * @author <a href="mailto:julien.viet@exoplatform.com">Julien Viet</a>
 */
public class Queues {

    /** . */
    private static final Queue<Object> EMPTY = new AbstractQueue<Object>() {
        @Override
        public Iterator<Object> iterator() {
            return Collections.emptyList().iterator();
        }

        @Override
        public int size() {
            return 0;
        }

        public boolean offer(Object o) {
            return false;
        }

        public Object poll() {
            return null;
        }

        public Object peek() {
            return null;
        }
    };

    private static class LIFO<E> extends AbstractQueue<E> {
        /** . */
        private Object[] elements;

        /** . */
        private int size;

        public LIFO(int initialCapacity) {
            this.elements = new Object[initialCapacity];
            this.size = 0;
        }

        @Override
        public Iterator<E> iterator() {
            return new Iterator<E>() {
                int count = size;

                public boolean hasNext() {
                    return count > 0;
                }

                public E next() {
                    if (!hasNext()) {
                        throw new NoSuchElementException();
                    } else {
                        @SuppressWarnings("unchecked")
                        E element = (E) elements[--count];
                        return element;
                    }
                }

                public void remove() {
                    throw new UnsupportedOperationException();
                }
            };
        }

        @Override
        public int size() {
            return size;
        }

        public boolean offer(E e) {
            int length = elements.length;
            if (size == length) {
                Object[] tmp = new Object[(length * 3) / 2 + 1];
                System.arraycopy(elements, 0, tmp, 0, length);
                elements = tmp;
            }
            elements[size++] = e;
            return true;
        }

        public E poll() {
            if (size > 0) {
                int index = --size;
                @SuppressWarnings("unchecked")
                E element = (E) elements[index];
                elements[index] = null;
                return element;
            } else {
                return null;
            }
        }

        public E peek() {
            if (size > 0) {
                @SuppressWarnings("unchecked")
                E element = (E) elements[size - 1];
                return element;
            } else {
                return null;
            }
        }
    }

    public static <E> Queue<E> empty() {
        @SuppressWarnings("unchecked")
        Queue<E> queue = (Queue<E>) EMPTY;
        return queue;
    }

    public static <E> Queue<E> lifo() {
        return lifo(10);
    }

    public static <E> Queue<E> lifo(int initialCapacity) {
        return new LIFO<E>(initialCapacity);
    }
}