Why does compiler build return unreachable code in some cases

Question

If I write

private void check(){
   if(true)
      return;

   String a = "test";
}

Above code works normally, but if I write

private void check(){
   return;

   String a = "test";
}

The compiler/gradle in Android studio doesn't let this one through even though it's the same, and it says that the code after return in example 2 is unreachable.

I don't have any issues regarding this but I am eager to know why?

Answer 1

javac compiler does very little optimizations, so it simply does not "see" that if(true) is always true(but you should get a warning); but C1/C2 JIT compilers will - so that code will simply be a return, without an if statement.

Answer 2

This is explained by the Unreachable Statements part of the Java Language Specification.

There are quite a few rules, with an interesting special case. This is a compile time error :

while (false) {
    // this code is unreachable
    String something = "";
}

while this is not :

if (false) {
    // this code is considered as reachable
    String something = "";
}

The given reason is to allow some kind of conditional compilation, like :

static final boolean DEBUG = false;
...
if (DEBUG) { x=3; }

So in your case :

private void check(){
    if(true)
        return;

    // NO compilation error
    // this is conditional code
    // and may be omitted by the compiler
    String a = "test";
}

is not an error because of the special if treatment, using while instead is not accepted :

private void check(){
    while(true)
        return;

    // "Unreachable statement" compilation error
    String a = "test";
}

This is also en error :

private void check(){
    if(true)
        return;
    else
        return;

    // "Unreachable statement" compilation error
    String a = "test";
}