001 /*
002 * Copyright 1994-2005 Sun Microsystems, Inc. All Rights Reserved.
003 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
004 *
005 * This code is free software; you can redistribute it and/or modify it
006 * under the terms of the GNU General Public License version 2 only, as
007 * published by the Free Software Foundation. Sun designates this
008 * particular file as subject to the "Classpath" exception as provided
009 * by Sun in the LICENSE file that accompanied this code.
010 *
011 * This code is distributed in the hope that it will be useful, but WITHOUT
012 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
013 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
014 * version 2 for more details (a copy is included in the LICENSE file that
015 * accompanied this code).
016 *
017 * You should have received a copy of the GNU General Public License version
018 * 2 along with this work; if not, write to the Free Software Foundation,
019 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
020 *
021 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
022 * CA 95054 USA or visit www.sun.com if you need additional information or
023 * have any questions.
024 */
025
026 package java.util;
027
028 /**
029 * The <code>Stack</code> class represents a last-in-first-out
030 * (LIFO) stack of objects. It extends class <tt>Vector</tt> with five
031 * operations that allow a vector to be treated as a stack. The usual
032 * <tt>push</tt> and <tt>pop</tt> operations are provided, as well as a
033 * method to <tt>peek</tt> at the top item on the stack, a method to test
034 * for whether the stack is <tt>empty</tt>, and a method to <tt>search</tt>
035 * the stack for an item and discover how far it is from the top.
036 * <p>
037 * When a stack is first created, it contains no items.
038 *
039 * <p>A more complete and consistent set of LIFO stack operations is
040 * provided by the {@link Deque} interface and its implementations, which
041 * should be used in preference to this class. For example:
042 * <pre> {@code
043 * Deque<Integer> stack = new ArrayDeque<Integer>();}</pre>
044 *
045 * @author Jonathan Payne
046 * @version 1.36, 05/05/07
047 * @since JDK1.0
048 */
049 public class Stack<E> extends Vector<E> {
050 /**
051 * Creates an empty Stack.
052 */
053 public Stack() {
054 }
055
056 /**
057 * Pushes an item onto the top of this stack. This has exactly
058 * the same effect as:
059 * <blockquote><pre>
060 * addElement(item)</pre></blockquote>
061 *
062 * @param item the item to be pushed onto this stack.
063 * @return the <code>item</code> argument.
064 * @see java.util.Vector#addElement
065 */
066 public E push(E item) {
067 addElement(item);
068
069 return item;
070 }
071
072 /**
073 * Removes the object at the top of this stack and returns that
074 * object as the value of this function.
075 *
076 * @return The object at the top of this stack (the last item
077 * of the <tt>Vector</tt> object).
078 * @exception EmptyStackException if this stack is empty.
079 */
080 public synchronized E pop() {
081 E obj;
082 int len = size();
083
084 obj = peek();
085 removeElementAt(len - 1);
086
087 return obj;
088 }
089
090 /**
091 * Looks at the object at the top of this stack without removing it
092 * from the stack.
093 *
094 * @return the object at the top of this stack (the last item
095 * of the <tt>Vector</tt> object).
096 * @exception EmptyStackException if this stack is empty.
097 */
098 public synchronized E peek() {
099 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 }
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