java - How to match and switch Strings against enums?

I want to transform a String into an enum. But how?

class Letter {
    A, B, C
}

String letter = "A";
switch (letter) {
    case Letter.A: //cannot convert from Letter to String

    case Letter.A.toString(): //case expressions must be constant expressions
    case Letter.C.name(): //case expressions must be constant expressions
    default:
}

Answer

First the Letter must be an enum:

enum Letter {
    A, B, C

}

Letter letter = Letter.valueOf("A")
// and just switch
switch (letter) {
    case A:
    case B:
    case C:
}