java 8: difference between class.getName() and String literal [duplicate]

Answer 1

The reason this does not work is that the value of the case to switch on needs to be known at compile-time (because it is hard-coded and inlined in the bytecode).

String.class.getName() is a method call that gets evaluated at runtime. Yes, static final guarantees that it does not change after the class is initially loaded. But all that is long after compile-time.

The compiler never calls any code from application source code (with the possible exception of annotation processing). It only compiles it.

Answer 2

Every case label must be a "constant expression". What is "constant expression" is defined in Java Language Standard, §15.28 Constant Expressions:

A compile-time constant expression is an expression denoting a value of primitive type or a String that does not complete abruptly and is composed using only the following:

• Literals of primitive type and literals of type String

...

• Simple names that refer to constant variables

No method calls are listed there, so method call result cannot be the constant expression even if the method is trivial. But "Simple names that refer to constant variables" are listed here, so reference to the constant variable is also a constant.

Answer 3

classObject.getName() is a method call, and the results of method calls are by definition not compile-time constants. A string literal is a compile-time constant.

Note that while many situations could take a static final reference as a constant for the lifetime of the program, a switch has to have its options hard-coded at compile-time. The value of a case target must be either an enum value or a (compile-time) ConstantExpression.