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      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 }
    142