A Stack is a linear data structure that operates on the LIFO (Last In, First Out) principle. This means the last element added to the stack is the first one to be removed.

Real-World Analogy

Think of a stack of plates in a cafeteria. You add new plates on top (push) and remove plates from the top (pop). You cannot remove a plate from the middle or bottom without first removing the ones on top.

Key Characteristics

• LIFO Principle: The last item entered is the first item accessed.
• Restricted Access: You can only add or remove elements from one end, known as the “Top”.

Core Operations

1. Push: Add an element to the top of the stack.
2. Pop: Remove and return the top element.
3. Peek/Top: View the top element without removing it.
4. isEmpty: Check if the stack is empty.
5. Size: Get the count of elements in the stack.

Java Implementation (Array-Based)

Here is a robust implementation of a Stack using an array in Java .

class Stack {
    private int maxSize;
    private int[] stackArray;
    private int top;

    // Constructor to initialize stack
    public Stack(int size) {
        this.maxSize = size;
        this.stackArray = new int[maxSize];
        this.top = -1; // Indicates empty stack
    }

    // Push operation: Add element to top
    public void push(int value) {
        if (isFull()) {
            System.out.println("Stack Overflow! Cannot push " + value);
            return;
        }
        stackArray[++top] = value;
        System.out.println("Pushed: " + value);
    }

    // Pop operation: Remove element from top
    public int pop() {
        if (isEmpty()) {
            System.out.println("Stack Underflow! Stack is empty");
            return -1;
        }
        int value = stackArray[top--];
        System.out.println("Popped: " + value);
        return value;
    }

    // Peek operation: View top element
    public int peek() {
        if (isEmpty()) {
            System.out.println("Stack is empty");
            return -1;
        }
        return stackArray[top];
    }

    // Helper methods
    public boolean isEmpty() {
        return (top == -1);
    }

    public boolean isFull() {
        return (top == maxSize - 1);
    }
}