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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      */
Stack()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      */
push(E item)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      */
pop()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      */
peek()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      */
empty()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      */
search(Object o)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