Stack Program using Arrays in Java!

Stack :

A stack is a container of objects that are inserted and removed according to the last-in first-out (LIFO) principle. In the pushdown stacks only two operations are allowed: pushthe item into the stack, and pop the item out of the stack. A stack is a limited access data structure - elements can be added and removed from the stack only at the top. push adds an item to the top of the stack, pop removes the item from the top. A helpful analogy is to think of a stack of books; you can remove only the top book, also you can add a new book on the top.

Applications :

1. The simplest application of a stack is to reverse a word. You push a given    word to stack - letter by letter - and then pop letters from the stack.
2. Another application is an "undo" mechanism in text editors; this operation is accomplished by keeping all text changes in a stack.
3. Infix to Postfix conversion.

Operations :

1. Push : To push an element into the stack.
2. Pop   : To pop an element from the stack.
3. Peek  : Shows the top most element in the stack.
4. isEmpty : Determines whether the stack is empty or not.

Java Program!

package adsa;
import java.util.Scanner;
interface StackAdt{
    void push() throws Exception;
    void pop();
    void display();
}
public class StackArr implements StackAdt{
    int top;
    int stack[];
    int size;
    Scanner s=new Scanner(System.in);
    public StackArr() throws Exception{
        System.out.println("Enter the stack size :");
        size=s.nextInt();
        stack=new int[size];
        top=-1;
    }
    public void push() throws Exception{
        if(top==stack.length-1)
            System.out.println("Stack is full!!");
        else{
            System.out.println("Enter the element into the stack :");
            int ele=s.nextInt();
            top++;
            stack[top]=ele;
        }
    }
    public void pop(){
        if(top==-1)
            System.out.println("Stack underflow!!");
        else{
            System.out.println("Popped item is :"+stack[top]);
            top--;
        }
        
    }
    public void display(){
        System.out.println("The elements in the stack are...");
        if(top==-1){
            System.out.println("No elements to display!!");
        }else
        {
        for(int i=top;i>=0;i--){
            System.out.print(stack[i]);
        }
        }System.out.println();
    }
    public static void main(String args[])throws Exception{
        Scanner s=new Scanner(System.in);
        StackArr st=new StackArr();
        String ch="y";
        while(ch.equalsIgnoreCase("y")){
            System.out.println("1. Push 2. Pop 3. Display 4. Exit\n Enter the option :");
            int opt=s.nextInt();
            switch(opt){
                case 1:st.push();
                        break;
                case 2: st.pop();
                        break;
                case 3:st.display();
                        break;
                case 4:System.exit(0);
                default:System.out.println("Invalid input");
            }
            System.out.println("Do you want to continue (y/N):");
            ch=s.next();
        }
    }
}