Mastering Stacks in Java: A Step-by-Step Implementation (Array Based)

In this tutorial, we’ll implement an Array-Based Stack in Java. The code includes detailed comments to explain what each part does.

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Stack Class

// Stack class manages all operations using an array
public class Stack {
    private int[] stackArray;  // Array to hold stack elements
    private int top;           // Index of the top element
    private int capacity;      // Maximum size of the stack

    // Constructor initializes the stack with given capacity
    public Stack(int capacity) {
        this.capacity = capacity;
        this.stackArray = new int[capacity];
        this.top = -1;  // Indicates empty stack
    }

    // Check if the stack is empty
    public boolean isEmpty() {
        return this.top == -1;
    }

    // Check if the stack is full
    public boolean isFull() {
        return this.top == this.capacity - 1;
    }

    // Push an element onto the stack
    public void push(int data) {
        if (isFull()) {
            System.out.println("Stack Overflow: Cannot push " + data);
        } else {
            this.stackArray[++this.top] = data;  // Increment top and add element
        }
    }

    // Pop the top element from the stack
    public int pop() {
        if (isEmpty()) {
            System.out.println("Stack Underflow: Cannot pop from empty stack");
            return -1;  // Or throw exception in real scenarios
        } else {
            return this.stackArray[this.top--];  // Return top and decrement
        }
    }

    // Peek at the top element without removing it
    public int peek() {
        if (isEmpty()) {
            System.out.println("Stack is empty: No element to peek");
            return -1;
        } else {
            return this.stackArray[this.top];  // Return top element
        }
    }

    // Print the Stack
    public void printStack() {
        if (isEmpty()) {
            System.out.println("Stack is Empty");
        } else {
            System.out.print("Stack (top to bottom): ");
            for (int i = this.top; i >= 0; i--) {  // Traverse from top to bottom
                System.out.print(this.stackArray[i] + " ");
            }
            System.out.println();
        }
    }
}

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Main Class – Testing the Stack

// Main class to test Stack operations
public class Main {
    public static void main(String[] args) {
        Stack myStack = new Stack(5);  // Create stack with capacity 5

        // Push elements onto the stack
        myStack.push(10);
        myStack.push(20);
        myStack.push(30);
        myStack.printStack();  // Output: Stack (top to bottom): 30 20 10

        // Push one more element
        myStack.push(40);
        myStack.printStack();  // Output: Stack (top to bottom): 40 30 20 10

        // Peek at top
        System.out.println("Top element: " + myStack.peek());  // Output: Top element: 40

        // Pop elements
        myStack.pop();
        myStack.printStack();  // Output: Stack (top to bottom): 30 20 10

        myStack.pop();
        myStack.printStack();  // Output: Stack (top to bottom): 20 10

        // Try to pop from near empty
        myStack.pop();
        myStack.printStack();  // Output: Stack (top to bottom): 10

        // Pop last element
        myStack.pop();
        myStack.printStack();  // Output: Stack is Empty
    }
}

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Final Output

Stack (top to bottom): 30 20 10 
Stack (top to bottom): 40 30 20 10 
Top element: 40
Stack (top to bottom): 30 20 10 
Stack (top to bottom): 20 10 
Stack (top to bottom): 10 
Stack is Empty

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Conclusion

With comments explaining each step, it’s easier to understand how an Array-Based Stack works internally. By practicing pushes, pops, and peeks, you’ll strengthen your foundation in Data Structures, which is essential for advanced topics like expression evaluation, recursion, and undo mechanisms.