1 /* 2 * Copyright (c) 1994, 2022, 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} class represents a last-in-first-out 30 * (LIFO) stack of objects. It extends class {@code Vector} with five 31 * operations that allow a vector to be treated as a stack. The usual 32 * {@code push} and {@code pop} operations are provided, as well as a 33 * method to {@code peek} at the top item on the stack, a method to test 34 * for whether the stack is {@code empty}, and a method to {@code search} 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 * @param <E> Type of component elements 46 * 47 * @author Jonathan Payne 48 * @since 1.0 49 */ 50 public class Stack<E> extends Vector<E> { 51 /** 52 * Creates an empty Stack. 53 */ Stack()54 public Stack() { 55 } 56 57 /** 58 * Pushes an item onto the top of this stack. This has exactly 59 * the same effect as: 60 * <blockquote><pre> 61 * addElement(item)</pre></blockquote> 62 * 63 * @param item the item to be pushed onto this stack. 64 * @return the {@code item} argument. 65 * @see java.util.Vector#addElement 66 */ push(E item)67 public E push(E item) { 68 addElement(item); 69 70 return item; 71 } 72 73 /** 74 * Removes the object at the top of this stack and returns that 75 * object as the value of this function. 76 * 77 * @return The object at the top of this stack (the last item 78 * of the {@code Vector} object). 79 * @throws EmptyStackException if this stack is empty. 80 */ pop()81 public synchronized E pop() { 82 E obj; 83 int len = size(); 84 85 obj = peek(); 86 removeElementAt(len - 1); 87 88 return obj; 89 } 90 91 /** 92 * Looks at the object at the top of this stack without removing it 93 * from the stack. 94 * 95 * @return the object at the top of this stack (the last item 96 * of the {@code Vector} object). 97 * @throws EmptyStackException if this stack is empty. 98 */ peek()99 public synchronized E peek() { 100 int len = size(); 101 102 if (len == 0) 103 throw new EmptyStackException(); 104 return elementAt(len - 1); 105 } 106 107 /** 108 * Tests if this stack is empty. 109 * 110 * @return {@code true} if and only if this stack contains 111 * no items; {@code false} otherwise. 112 */ empty()113 public boolean empty() { 114 return size() == 0; 115 } 116 117 /** 118 * Returns the 1-based position where an object is on this stack. 119 * If the object {@code o} occurs as an item in this stack, this 120 * method returns the distance from the top of the stack of the 121 * occurrence nearest the top of the stack; the topmost item on the 122 * stack is considered to be at distance {@code 1}. The {@code equals} 123 * method is used to compare {@code o} to the 124 * items in this stack. 125 * 126 * @param o the desired object. 127 * @return the 1-based position from the top of the stack where 128 * the object is located; the return value {@code -1} 129 * indicates that the object is not on the stack. 130 */ search(Object o)131 public synchronized int search(Object o) { 132 int i = lastIndexOf(o); 133 134 if (i >= 0) { 135 return size() - i; 136 } 137 return -1; 138 } 139 140 /** use serialVersionUID from JDK 1.0.2 for interoperability */ 141 @java.io.Serial 142 private static final long serialVersionUID = 1224463164541339165L; 143 } 144