Stack in Java

Stack in Java

In Java, a stack is a data structure that follows the LIFO (Last In, First Out) principle. This means the most recently added element is the first one to be removed.

1. Using java.util.Stack

Java provides a built-in Stack class in java.util package.

Example:

import java.util.Stack;

public class StackExample {
    public static void main(String[] args) {
        Stack<Integer> stack = new Stack<>();

        // Push elements onto the stack
        stack.push(10);
        stack.push(20);
        stack.push(30);

        System.out.println("Stack: " + stack);

        // Peek at the top element
        System.out.println("Top element: " + stack.peek());

        // Pop elements
        System.out.println("Popped: " + stack.pop());
        System.out.println("Stack after pop: " + stack);

        // Check if empty
        System.out.println("Is stack empty? " + stack.isEmpty());
    }
}

Output:

Stack: [10, 20, 30]
Top element: 30
Popped: 30
Stack after pop: [10, 20]
Is stack empty? false

2. Common Methods in Stack

  • push(E item) → Inserts an item onto the top of the stack.
  • pop() → Removes and returns the top item.
  • peek() → Returns the top item without removing it.
  • isEmpty() → Returns true if stack is empty.
  • search(Object o) → Returns 1-based position from top of stack (or -1 if not found).

3. Implementing Stack with Deque (Recommended)

Although Stack is available, it’s considered legacy. It’s better to use Deque (like ArrayDeque) for stack operations.

Example with ArrayDeque:

import java.util.ArrayDeque;
import java.util.Deque;

public class DequeStackExample {
    public static void main(String[] args) {
        Deque<Integer> stack = new ArrayDeque<>();

        stack.push(100);
        stack.push(200);
        stack.push(300);

        System.out.println("Stack: " + stack);

        System.out.println("Top: " + stack.peek());

        System.out.println("Popped: " + stack.pop());
        System.out.println("Stack after pop: " + stack);
    }
}

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