I have this very simple code snippet:
static String getInput() throws IOException{
if(in.ready()){
return in.readLine().trim();
}
System.err.println("Please provide more input in order to execute the program.");
System.exit(0);
return "";
}
By what I think I know, there is no possible way that the JVM will execute the return statement at the end of the code. But if I comment this line out, java will complain about a missing return statement. Why doesn't the JVM recognize that a System.exit(0) will not allow any other code to execute, but complains about unreachable statements if a return will not allow code to be executed?
I think the return statement at the end is redundant and might be confusing to other devs, so why won't java let me get rid of it?
Why doesn't the JVM recognize that a System.exit(0) will not allow any other code to execute, but complains about unreachable statements if a return will not allow code to be executed?
It's not the JVM - it's the compiler. And the compiler doesn't know what library calls will do - it only knows the language rules. (In particular, JLS section 14.21, Unreachable Statements.)
So for example:
public int foo() {
alwaysThrow();
// This is required.
return 10;
}
private static void alwaysThrow() {
throw new RuntimeException();
}
vs
public int foo() {
throw new RuntimeException();
// Error: unreachable statement
return 10;
}
That simple inlining changes the meaning of the code as far as the compiler's concerned.
This could be "fixed" by having a return type of "never" - to indicate "this method never returns normally - it either hangs or throws an exception" but that's simply not part of the language (and would have its own complications). If you're interested, Eric Lippert has a couple of blog posts on this topic with regard to C# (which is in a similar position): part one, part two.
