View Javadoc
1   /*
2    * Copyright (c) 1994, 2010, Oracle and/or its affiliates. All rights reserved.
3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4    *
5    * This code is free software; you can redistribute it and/or modify it
6    * under the terms of the GNU General Public License version 2 only, as
7    * published by the Free Software Foundation.  Oracle designates this
8    * particular file as subject to the "Classpath" exception as provided
9    * by Oracle in the LICENSE file that accompanied this code.
10   *
11   * This code is distributed in the hope that it will be useful, but WITHOUT
12   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14   * version 2 for more details (a copy is included in the LICENSE file that
15   * accompanied this code).
16   *
17   * You should have received a copy of the GNU General Public License version
18   * 2 along with this work; if not, write to the Free Software Foundation,
19   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20   *
21   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22   * or visit www.oracle.com if you need additional information or have any
23   * questions.
24   */
25  
26  package java.util;
27  
28  /**
29   * The <code>Stack</code> class represents a last-in-first-out
30   * (LIFO) stack of objects. It extends class <tt>Vector</tt> with five
31   * operations that allow a vector to be treated as a stack. The usual
32   * <tt>push</tt> and <tt>pop</tt> operations are provided, as well as a
33   * method to <tt>peek</tt> at the top item on the stack, a method to test
34   * for whether the stack is <tt>empty</tt>, and a method to <tt>search</tt>
35   * the stack for an item and discover how far it is from the top.
36   * <p>
37   * When a stack is first created, it contains no items.
38   *
39   * <p>A more complete and consistent set of LIFO stack operations is
40   * provided by the {@link Deque} interface and its implementations, which
41   * should be used in preference to this class.  For example:
42   * <pre>   {@code
43   *   Deque<Integer> stack = new ArrayDeque<Integer>();}</pre>
44   *
45   * @author  Jonathan Payne
46   * @since   JDK1.0
47   */
48  public
49  class Stack<E> extends Vector<E> {
50      /**
51       * Creates an empty Stack.
52       */
53      public Stack() {
54      }
55  
56      /**
57       * Pushes an item onto the top of this stack. This has exactly
58       * the same effect as:
59       * <blockquote><pre>
60       * addElement(item)</pre></blockquote>
61       *
62       * @param   item   the item to be pushed onto this stack.
63       * @return  the <code>item</code> argument.
64       * @see     java.util.Vector#addElement
65       */
66      public E push(E item) {
67          addElement(item);
68  
69          return item;
70      }
71  
72      /**
73       * Removes the object at the top of this stack and returns that
74       * object as the value of this function.
75       *
76       * @return  The object at the top of this stack (the last item
77       *          of the <tt>Vector</tt> object).
78       * @throws  EmptyStackException  if this stack is empty.
79       */
80      public synchronized E pop() {
81          E       obj;
82          int     len = size();
83  
84          obj = peek();
85          removeElementAt(len - 1);
86  
87          return obj;
88      }
89  
90      /**
91       * Looks at the object at the top of this stack without removing it
92       * from the stack.
93       *
94       * @return  the object at the top of this stack (the last item
95       *          of the <tt>Vector</tt> object).
96       * @throws  EmptyStackException  if this stack is empty.
97       */
98      public synchronized E peek() {
99          int     len = size();
100 
101         if (len == 0)
102             throw new EmptyStackException();
103         return elementAt(len - 1);
104     }
105 
106     /**
107      * Tests if this stack is empty.
108      *
109      * @return  <code>true</code> if and only if this stack contains
110      *          no items; <code>false</code> otherwise.
111      */
112     public boolean empty() {
113         return size() == 0;
114     }
115 
116     /**
117      * Returns the 1-based position where an object is on this stack.
118      * If the object <tt>o</tt> occurs as an item in this stack, this
119      * method returns the distance from the top of the stack of the
120      * occurrence nearest the top of the stack; the topmost item on the
121      * stack is considered to be at distance <tt>1</tt>. The <tt>equals</tt>
122      * method is used to compare <tt>o</tt> to the
123      * items in this stack.
124      *
125      * @param   o   the desired object.
126      * @return  the 1-based position from the top of the stack where
127      *          the object is located; the return value <code>-1</code>
128      *          indicates that the object is not on the stack.
129      */
130     public synchronized int search(Object o) {
131         int i = lastIndexOf(o);
132 
133         if (i >= 0) {
134             return size() - i;
135         }
136         return -1;
137     }
138 
139     /** use serialVersionUID from JDK 1.0.2 for interoperability */
140     private static final long serialVersionUID = 1224463164541339165L;
141 }