Stack Fundamentals in Java

What is a Stack?

Stack is one of the many data structures in programming. One of the best example of stack is books kept on top of each other on a table. The book kept on the top is picked first. For the same reason stack is also called Last In First Out (LIFO) data structure.

Important functions in Stack

If we talk about very simple stack, then there are very limited functions a stack has.

push(): This pushes the element at the top of the stack.

pop(): This removes and returns the element from the top of the stack.

size(): This returns the current size of the stack.

peek() This just returns the top value without removing it from the stack.

Stack Implementation

Stack can be implemented using array or linkedlist. We will be using LinkedList for the demonstration.

class Node {
    int data;
    Node next;
    public Node(int data) {
        this.data = data;
        this.next = null;
    }
}

public class Stack {
    Node top;
    int size;
    public Stack() {
        top = null;
        size = 0;
    }

    public void push(int data) {
        Node node = new Node(data);
        if(top == null) {
            top = node;
        } else {
            Node temp = top;
            top = node;
            node.next = temp;
        }
        size = size+1;
    }

    public int pop() {
        int val = top.data;
        top = top.next;
        size = size-1;
        return val;
    }

    public int peek() {
        return top.data;
    }

    public void printStack() {
        Node temp = top;
        while(temp != null) {
            System.out.println(temp.data);
            temp = temp.next;
        }
    }

    public int size() {
        return size;
    }

    public static void main(String[] args) {
        Stack stack = new Stack();
        stack.push(10);
        stack.push(20);
        stack.push(30);
        System.out.println("Current stack printed below with size:"+stack.size());
        stack.printStack();
        System.out.println("Popped value:"+ stack.pop());
        System.out.println("Current stack printed below with size:"+stack.size());
        stack.printStack();
        System.out.println("Peek the top value:"+stack.peek());
    }
}

Console output

Current stack printed below with size:3
30
20
10
Popped value:30
Current stack printed below with size:2
20
10
Peek the top value:20

Hope you liked this tutorial. You might also like other tutorials in this series here