Switch statement constant and case values

[Jason Attin]

Hi guys, I'm doing some mock tests from another book and I came across this question:

Which of these combinations of switch expression types and case label value types are legal within a switch statement?
Select 1 option
A. switch expression of type int and case label value of type char.
B. switch expression of type float and case label value of type int.
C. switch expression of type byte and case label value of type float.
D. switch expression of type char and case label value of type byte.
E. switch expression of type boolean and case label value of type boolean.

Apparently the correct answer is A. I would have thought that they were all wrong, but apparently I was wrong.
Any idea as I just wouldn't have a clue. I thought that the switch expression and the constant had to be of the same type.

[Henry Wong]

A quick look at the Java Language Specification, seems to show that implicit casting is allowed from the case type to the switch type -- and since a char may be implicitly cast to an int, it is allowed.

Henry

[Jason Attin]

OK, so by implicit casting you mean casting without using "cast", correct?

[Liutauras Vilda]

Jason Attin wrote:OK, so by implicit casting you mean casting without using "cast", correct?

Pretty much. That means Java language does casting internally.

[Ganesh Patekar]

JLS 14.11 The switch Statement wrote:Every case constant associated with the switch statement must be assignment compatible with the type of the switch statement's Expression

Since Java does implicit casting between them then I think A and D be the correct answers?

[Henry Wong]

A byte does not implicitly cast from a byte to a char (or vice versa) -- so, D should not be correct.  Having said that though, the assignment conversion rules do allow for compile time constant cases. I wonder if those rules apply? ... meaning would it be allowed, if *all* the cases are positive bytes? Or not allowed because the "default" case can't be handled?

Henry

[Ganesh Patekar]

Hope I tried correctly what an option D says: switch expression of type char and case label value of type byte

	private void testSwitch() {

		final byte bValA = 65;
		final byte bValB = 66;

		switch ('B') {
		case bValA:// Or 0b01000001 nothing but 65
			System.out.println("A");
			break;
		case 0b01000010:// Or bValB nothing but 66
			System.out.println("B");
			break;
		default:
			System.out.println("default");
		}
	}

It prints B. Since It runs successfully, shouldn't It be considered as legal combinations within a switch statement?

[Henry Wong]

Interesting. So, the compile time constant rules do apply for case statements. Makes sense.

Anyway, I doubt that it will work for negative byte case statements though.

Henry

[Ganesh Patekar]

Henry Wong wrote: Anyway, I doubt that it will work for negative byte case statements though.

Ah! yes, since char is unsigned so negative byte case statement doesn't compile, means D can't be correct answer.