Switch statement: Why can't I have the same variable name in different cases? Java

Question

I still writing the code, and it doesn't make a very big difference in a project like mine, but if I was to make something bigger, it would be a pain. here it is:

        case 0:
            System.out.print("Insert the N: ");
            double N = in.nextDouble();
            double mol = N / Na;
            System.out.print("There are " + mol + " mol in that sample");
            break;

        case 1:
            System.out.print("Insert the m: ");
            double m = in.nextDouble();
            System.out.print("Insert the M: ");
            double M = in.nextDouble();
            double mol = m / M;
            System.out.print("There are " + mol + " mol in that sample");
            break;

        case 2:
            System.out.print("Insert the V: ");
            double V = in.nextDouble();
            double mol = V / Vm;
            System.out.print("There are " + mol + " mol in that sample");
            break;

The first "mol" has no problem, but in case 1 and case 2, it says "Duplicate local variable mol". If I use the If statement it works. Is Java just like this or there is a way around it?

Thanks

Answer 1

That's because a case doesn't create a scope. So, both the variables in the 2 cases are in the same scope. If you want to do this, you can add braces for each case, which will create a new scope for each case.

    case 0: {
        System.out.print("Insert the N: ");
        double N = in.nextDouble();
        double mol = N / Na;
        System.out.print("There are " + mol + " mol in that sample");
        break; 
    }

    case 1: {
        System.out.print("Insert the m: ");
        double m = in.nextDouble();
        System.out.print("Insert the M: ");
        double M = in.nextDouble();
        double mol = m / M;
        System.out.print("There are " + mol + " mol in that sample");
        break;
    }

But, ideally there is no need to declare a separate local variable for each case. If you are using a variable in all the cases, that clearly signifies that variable to be declared directly inside the switch statement:

switch (someVar) {
    double mol = 0.0;

    case 0: mol = n / Na;
            break;

    case 1: mol = m / M;
            break;
}

P.S.: Can I advice you to name your variables something apart from english alphabets - n, M, N?

Answer 2

Because those variables are there in single block may be you would have written this switch statement in some method. In a single method you can't have dup variables.