How to Implement Stack in Java Using Array and Generics?

// Java Program to Implement Stack in Java Using Array and
// Generics

// Importing input output classes
import java.io.*;
// Importing all utility classes
import java.util.*;

// user defined class for generic stack
class stack<T> {

    // Empty array list
    ArrayList<T> A;

    // Default value of top variable when stack is empty
    int top = -1;

    // Variable to store size of array
    int size;

    // Constructor of this class
    // To initialize stack
    stack(int size)
    {
        // Storing the value of size into global variable
        this.size = size;

        // Creating array of Size = size
        this.A = new ArrayList<T>(size);
    }

    // Method 1
    // To push generic element into stack
    void push(T X)
    {
        // Checking if array is full
        if (top + 1 == size) {

            // Display message when array is full
            System.out.println("Stack Overflow");
        }
        else {

            // Increment top to go to next position
            top = top + 1;

            // Over-writing existing element
            if (A.size() > top)
                A.set(top, X);

            else

                // Creating new element
                A.add(X);
        }
    }
    // Method 2
    // To return topmost element of stack
    T top()
    {
        // If stack is empty
        if (top == -1) {

            // Display message when there are no elements in
            // the stack
            System.out.println("Stack Underflow");

            return null;
        }

        // else elements are present so
        // return the topmost element
        else
            return A.get(top);
    }

    // Method 3
    // To delete last element of stack
    void pop()
    {
        // If stack is empty
        if (top == -1) {

            // Display message when there are no elements in
            // the stack
            System.out.println("Stack Underflow");
        }

        else

            // Delete the last element
            // by decrementing the top
            top--;
    }

    // Method 4
    // To check if stack is empty or not
    boolean empty() { return top == -1; }

    // Method 5
    // To print the stack
    // @Override
    public String toString()
    {

        String Ans = "";

        for (int i = 0; i < top; i++) {
            Ans += String.valueOf(A.get(i)) + "->";
        }

        Ans += String.valueOf(A.get(top));

        return Ans;
    }
}
// Main Class
public class GFG {

    // main driver method
    public static void main(String[] args)
    {

        // Integer Stack

        // Creating an object of Stack class
        // Declaring objects of Integer type
        stack<Integer> s1 = new stack<>(3);

        // Pushing elements to integer stack - s1

        // Element 1 - 10
        s1.push(10);
        // Element 2 - 20
        s1.push(20);
        // Element 3 - 30
        s1.push(30);

        // Print the stack elements after pushing the
        // elements
        System.out.println(
            "s1 after pushing 10, 20 and 30 :\n" + s1);

        // Now, pop from stack s1
        s1.pop();

        // Print the stack elements after popping few
        // element/s
        System.out.println("s1 after pop :\n" + s1);

        // String Stack

        // Creating an object of Stack class
        // Declaring objects of Integer type
        stack<String> s2 = new stack<>(3);

        // Pushing elements to string stack - s2

        // Element 1 - hello
        s2.push("hello");
        // Element 2 - world
        s2.push("world");
        // Element 3 - java
        s2.push("java");

        // Print string stack after pushing above string
        // elements
        System.out.println(
            "\ns2 after pushing 3 elements :\n" + s2);

        System.out.println(
            "s2 after pushing 4th element :");

        // Pushing another element to above stack

        // Element 4 - GFG
        s2.push("GFG");

        // Float stack

        // Creating an object of Stack class
        // Declaring objects of Integer type
        stack<Float> s3 = new stack<>(2);

        // Pushing elements to float stack - s3

        // Element 1 - 100.0
        s3.push(100.0f);
        // Element 2 - 200.0
        s3.push(200.0f);

        // Print string stack after pushing above float
        // elements
        System.out.println(
            "\ns3 after pushing 2 elements :\n" + s3);

        // Print and display top element of stack s3
        System.out.println("top element of s3:\n"
                           + s3.top());
    }
}