/* * Copyright (C) 2008 Google Inc. * * 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 com.google.gson;import java.util.Stack;/** * A stack data structure that only does a memory reference comparison * when looking for a particular item in the stack.  This stack does * not allow {@code null} values to be added. * * @author Joel Leitch */final class MemoryRefStack<T> {  private final Stack<T> stack = new Stack<T>();  /**   * Adds a new element to the top of the stack.   *   * @param obj the object to add to the stack   * @return the object that was added   */  public T push(T obj) {    Preconditions.checkNotNull(obj);    return stack.push(obj);  }  /**   * Removes the top element from the stack.   *   * @return the element being removed from the stack   * @throws java.util.EmptyStackException thrown if the stack is empty   */  public T pop() {    return stack.pop();  }  /**   * Retrieves the item from the top of the stack, but does not remove it.   *   * @return the item from the top of the stack   * @throws java.util.EmptyStackException thrown if the stack is empty   */  public T peek() {    return stack.peek();  }  /**   * Performs a memory reference check to see it the {@code obj} exists in   * the stack.   *   * @param obj the object to search for in the stack   * @return true if this object is already in the stack otherwise false   */  public boolean contains(T obj) {    if (obj == null) {      return false;    }    for (T stackObject : stack) {      if (obj == stackObject) {        return true;      }    }    return false;  }}