Testing (junit) the default case in a switch statement?

I need to know how to test the default case in a switch statement with junit. I can't change the code itself and I'm trying for 100% coverage but I don't know how to test my default. Helps?

public Hello helloSwitch() {
        Hello hi = Hello.A;
        switch (this) {
        case A:
            hi = Hello.B;
            break;
        case B:
            hi = Hello.C;
            break;
        case C:
            hi = Hello.A;
            break;
        default:
            hi = Hello.A;
            break;
        }

I had to modify the code a fair bit so sorry that it looks silly. I just need to know how to write a junit to test the default, I've tested everything else.

I can't change this code.

Edit: changed

Edit: This code isn't important I jut need to know how to write the unit test for the default