Create a stack using an ArrayList

Problem Statement: Create a stack using an ArrayList and implement push, pop, and peek operations.

Here’s a Java program that demonstrates how to create a stack using an ArrayList and implements the push, pop, and peek operations. I’ll provide an explanation of the program’s logic.

package com.javacodepoint.collection;

import java.util.ArrayList;
import java.util.List;

class Stack<T> {
	private List<T> list = new ArrayList<>();

	public void push(T item) {
		list.add(item);
	}

	public T pop() {
		if (isEmpty()) {
			throw new IllegalStateException("Stack is empty");
		}
		return list.remove(list.size() - 1);
	}

	public T peek() {
		if (isEmpty()) {
			throw new IllegalStateException("Stack is empty");
		}
		return list.get(list.size() - 1);
	}

	public boolean isEmpty() {
		return list.isEmpty();
	}

	public int size() {
		return list.size();
	}
}

public class StackWithArrayListDemo {

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

		stack.push(10);
		stack.push(20);
		stack.push(30);

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

		System.out.println("Pop: " + stack.pop());
		System.out.println("Pop: " + stack.pop());

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

OUTPUT:

Peek: 30
Stack size: 3
Pop: 30
Pop: 20
Is stack empty? false

Explanation:

1. Custom Stack Class: We define a generic Stack class that wraps an ArrayList to implement the stack operations. The generic type T allows the stack to hold elements of any type.
2. Push Operation: The push() method adds an item to the end of the ArrayList, effectively pushing it onto the stack.
3. Pop Operation: The pop() method removes and returns the last item from the ArrayList, simulating the pop operation of a stack. If the stack is empty, it throws an IllegalStateException.
4. Peek Operation: The peek() method returns the last item from the ArrayList without removing it, simulating the peek operation of a stack. If the stack is empty, it throws an IllegalStateException.
5. isEmpty Operation: The isEmpty() method checks if the ArrayList is empty, indicating whether the stack is empty.
6. size Operation: The size() method returns the number of items in the stack by querying the size of the ArrayList.
7. Main Method: We create an instance of the Stack class with Integer type. We perform push operations to add integers to the stack and demonstrate peek, size, and pop operations. We also check if the stack is empty after performing pop operations.

By encapsulating the stack operations within the Stack class, we can use an ArrayList as the underlying data structure to implement the stack behavior efficiently.

See also: Sorting a List: Sort an ArrayList of custom objects.