The compiler is giving you an Unreachable statement error because your System.out.print("inside while");
code can never be reached with
while (false) { // error
System.out.print("inside while");
}
The compiler knows that while (false)
will never be true, and warns you (with an error) about the dead code.
In contrast, if you put a variable in the while
instead, because the nature of variables is that they can vary (change), the compiler's static analysis doesn't identify unreachable code. (Even in many situations where you or I could look at it and say "that code's never going to be run"; the compiler's analysis is fairly shallow, that's not it's main job. There are more beefy analysis and code-coverage tools you can use.)
In regards if (false)
, here's what the JLS has to say about it:
As an example, the following statement results in a compile-time error:
while (false) { x=3; }
because the statement x=3
; is not reachable; but the superficially similar case:
if (false) { x=3; }
does not result in a compile-time error. An optimizing compiler may realize that the statement x=3;
will never be executed and may choose to omit the code for that statement from the generated class file, but the statement x=3;
is not regarded as "unreachable" in the technical sense specified here.
The rationale for this differing treatment is to allow programmers to define "flag variables" such as:
static final boolean DEBUG = false;
and then write code such as:
if (DEBUG) { x=3; }
The idea is that it should be possible to change the value of DEBUG
from false to true or from true to false and then compile the code correctly with no other changes to the program text.