ConcurrentStack.java

/*
 * Copyright (c) 2001-2017, Zoltan Farkas All Rights Reserved.
 *
 * This library 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 library 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 General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 *
 * Additionally licensed with:
 *
 * 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.spf4j.concurrent;

import java.util.concurrent.atomic.AtomicReference;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import javax.annotation.concurrent.ThreadSafe;

/**
 * A Treiber stack.
 * @author Zoltan Farkas
 * @param <E>
 */
@ThreadSafe
public final class ConcurrentStack<E> {

  private final AtomicReference<Node<E>> top = new AtomicReference<Node<E>>();

  public void push(@Nonnull final E item) {
    Node<E> oldHead;
    Node<E> newHead;
    do {
      oldHead = top.get();
      newHead = new Node<E>(item, oldHead);
    } while (!top.compareAndSet(oldHead, newHead));
  }

  @Nullable
  public E pop() {
    Node<E> oldHead;
    Node<E> newHead;
    do {
      oldHead = top.get();
      if (oldHead == null) {
        return null;
      }
      newHead = oldHead.next;
    } while (!top.compareAndSet(oldHead, newHead));
    return oldHead.item;
  }

  private static class Node<E> {

    private final E item;
    private final Node<E> next;

    Node(final E item, @Nullable final Node<E> next) {
      this.item = item;
      this.next = next;
    }
  }

  @Override
  public String toString() {
    return "ConcurrentStack{" + "top=" + top + '}';
  }

}