java - Why is return needed even after System.exit(0);

Question

Consider this function :

public boolean foo(){
   System.exit(1);
   //The lines beyond this will not be read
   int bar = 1;                  //L1
   //But the return statement is required for syntactically correct code
   return false;                 //L2

   //error here for unreachable code
   //int unreachable = 3;        //L3

}

Can someone please explain why L1 and L2 visibly not reachable does not give warnings but L3 does.

Answer 1

Because as far as the compiler is concerned, System.exit() is just another method call.

The fact that what it does is end the process can only be found out from the implementation (which is native code, not that it makes any difference).

If you have to put System.exit() in your code (usually it's best to avoid it, unless you want to return a code other than 0), it should really be in a method that returns void, main() for example. It's nicer that way.

As for the reachability, the explanation is the same: return is a keyword of the Java language, so the compiler or the parser the IDE uses can tell that it's theoretically impossible for code after the return statement to be executed. These rules are defined here.