Java Stack class

The java.util.Stack class implements the standard Stack data structure. The class is based on the basic principle of last-in-first-out (LIFO).

It extends class Vector with five operations that allow a vector to be treated as a stack. The usual push() and pop() operations are provided, as well as a method to peek() at the top item on the stack, a method to test for whether the stack is empty(), and a method to search() the stack for an item and discover how far it is from the top.

Stack only defines the default constructor, which creates an empty stack object:

Stack()

Stack Methods

Stack includes all of the methods defined by Vector, and adds some new methods of its own. −

Example

The following program illustrates different methods supported by the Stack class−

StackDemo.java

import java.util.Iterator;
import java.util.Stack;

public class StackDemo {

    public static void main(String[] args) {
        // Create Stack object
        Stack stack = new Stack<>();
        Iterator iterator;

        // Push operation
        custom_push(stack);

        // Iterate through the elements of stack
        iterator = stack.iterator();
        System.out.println("\nStack elements: ");
        while (iterator.hasNext()) {
            System.out.print(iterator.next() + " ");
        }
        System.out.println();

        // Pop operation
        custom_pop(stack);

        // Check empty or not
        System.out.println(custom_empty(stack));

        // Push operation
        custom_push(stack);

        // Iterate through the elements of stack
        iterator = stack.iterator();
        System.out.println("\nStack elements: ");
        while (iterator.hasNext()) {
            System.out.print(iterator.next() + " ");
        }
        System.out.println();

        // Peek operation
        custom_peek(stack);

        // Search operations
        custom_search(stack, 4); 
        custom_search(stack, 7);

        // Check empty or not
        System.out.println(custom_empty(stack));
    }

    // Pushing element onto the top of the stack 
    static void custom_push(Stack st) {
        System.out.println("\nPush operation:~ ");
        for (int i = 1; i <= 5; i++) {
            System.out.println("Item pushed: " + i);
            st.push(i);
        }
    }

    // Popping element from the top of the stack 
    static void custom_pop(Stack st) {
        System.out.println("\nPop operation:~ ");

        for (int i = 1; i <= 5; i++) {
            Integer item = (Integer) st.pop();
            System.out.println("Item popped: "+ item);
        }
    }

    // Displaying element on the top of the stack 
    static void custom_peek(Stack st) {
        System.out.println("\nPeek operation:~ ");
        Integer element = (Integer) st.peek();
        System.out.println("Element on stack top : " + element);
    }

    // Searching element in the stack 
    static void custom_search(Stack st, int element) {
        System.out.println("\nSearch operation:~ ");
        Integer pos = (Integer) st.search(element);

        if (pos == -1) {
            System.out.println("Element " + element + " is not found");
        } else {
            System.out.println("Element " + element + " is found at position " + pos);
        }
    }

    // Checking stack is empty or not
    static String custom_empty(Stack st) {
        System.out.println("\nChecking stack is empty or not:~ ");
        if(st.empty() == true)
            return "Stack is empty!";
        else
            return "Stack is not empty!"; 
    }

}

Output:

Push operation:~ 
Item pushed: 1
Item pushed: 2
Item pushed: 3
Item pushed: 4
Item pushed: 5

Stack elements: 
1 2 3 4 5

Search operation:~ 
Element 2 is found at position 4

Pop operation:~ 
Item popped: 5
Item popped: 4
Item popped: 3
Item popped: 2
Item popped: 1

Checking stack is empty or not:~ 
Stack is empty!

Push operation:~ 
Item pushed: 1
Item pushed: 2
Item pushed: 3
Item pushed: 4
Item pushed: 5

Stack elements: 
1 2 3 4 5

Peek operation:~ 
Element on stack top : 5

Search operation:~ 
Element 4 is found at position 2

Search operation:~ 
Element 7 is not found

Checking stack is empty or not:~ 
Stack is not empty!

Use ArrayDeque In place of Stack

The Stack class provides the direct implementation of the stack data structure. However, it is recommended not to use it. Instead, better use the ArrayDeque class (implements the Deque interface) to implement the Stack data structure in Java.

For example:

Deque<Integer> stack = new ArrayDeque<Integer>();