Implementation of Stack using ArrayList in Java

Stack follows LIFO ie., Last In First Out. In this data structure the elements are arranged in such a way that the last element added will be the first one to pop out of it.

Let’s implement this Stack data structure using ArrayList. We will implement the following APIs.

push – To push the elements(Objects) into the Stack.
pop – To pop the top Object from the Stack
peek – To view the Top Object . This will not modify the Stack.
size – Get the size of the Stack
isEmpty – to check if the Stack is empty or not.

For our example, we will create a String Stack which will have String Objects.

Java Code
package com.java.datastructure;
import java.util.ArrayList;
import java.util.List;
public class Stack {
	@Override
	public String toString() {
		return "Stack [elements=" + elements + "]";
	}
	private List<String> elements = new ArrayList<>();
	public String peek() {
		if (elements.isEmpty()) {
			return null;
		}
		return elements.get(elements.size() - 1);
	}
	
	public String pop() {
		if (elements.isEmpty()) {
			return null;
		}
		String top = elements.get(elements.size() - 1);
		elements.remove(elements.size() - 1);
		return top;
	}
	public void push(String element) {
		elements.add(element);
	}
	public int size() {
		return elements.size();
	}
	public boolean isEmpty() {
		return elements.isEmpty();
	}
	
	public static void main(String[] args) {
		
		Stack stack = new Stack();
		System.out.println("Is Stack Empty:"+stack.isEmpty());
		stack.push("Gyan");
		stack.push("Vivek");
		stack.push("Rochit");
		stack.push("Panda");
		System.out.println("Is Stack Empty:"+stack.isEmpty());
		System.out.println(stack);
		System.out.println("Stack Size:"+stack.size());
		System.out.println("Peek Top Element:"+stack.peek());
		System.out.println("After peek:"+stack);
		System.out.println("Pop Top Element:"+stack.pop());
		System.out.println("After pop:"+stack);
		System.out.println("Stack Size now:"+stack.size());
		
	}
}

Output
Is Stack Empty:true
Is Stack Empty:false
Stack [elements=[Gyan, Vivek, Rochit, Panda]]
Stack Size:4
Peek Top Element:Panda
After peek:Stack [elements=[Gyan, Vivek, Rochit, Panda]]
Pop Top Element:Panda
After pop:Stack [elements=[Gyan, Vivek, Rochit]]
Stack Size now:3