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