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 1.0
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 }