import java.util.*;
class Calculator
{
    public static void main(String[] args) 
    {
        boolean exit=false;
        Scanner input=new Scanner(System.in);
        System.out.print("Enter number1: ");
        double num1=input.nextDouble();
        System.out.print("Enter number2: ");
        double num2=input.nextDouble();
        System.out.println("1 Addition");
        System.out.println("2 Subtraction");
        System.out.println("3 Multiplication");
        System.out.println("4 Division");
        System.out.println("5 Exit");
        do{
            System.out.print("Enter your choice: ");
            int choice=input.nextInt();
            switch(choice){
                case 1: 
                    double add=num1+num2;
                    System.out.println("Addition="+add);
                    break;
                case 2: 
                    double sub=num1-num2;
                    System.out.println("Subtraction="+sub);
                    break;
                case 3: 
                    double mul=num1*num2;
                    System.out.println("Multiplication="+mul);
                    break;
                case 4: 
                    double div=num1/num2;
                    System.out.println("Division="+div);
                    break;
                case 5: 
                    exit=true;
                    break;
            }
        }
        while(!exit);
    }
}

Java number of days in given month