java - while(true); loop throws Unreachable code when isn't in a void

Question

I was doing some small programs in java. I know that if I write while(true); the program will freeze in this loop. If the code is like that:

Test 1:

public class While {
    public static void main(String[] args) {
        System.out.println("start");
        while (true);
        System.out.println("end");
    }
}

The compiler throws me the error:

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
    Unreachable code
    at While.main(While.java:6)

I didn't know that this error exists. But I got why it is thrown. Of course, line 6 was unreachable, causing a compilation problem. Then I tested this:

Test 2:

public class While {
    public static void main(String[] args) {
        System.out.println("start");
        a();
        b();
    }
    static void a() {
        while(true);
    }
    static void b() {
        System.out.println("end");
    }
}

For some reason the program ran normally (The console printed "start" and then froze). The compiler couldn't check inside of void a() and see that it isn't reachable. To be sure I tried:

Test 3:

public class While {
    public static void main(String[] args) {
        System.out.println("start");
        a();
        System.out.println("end");
    }
    static void a() {
        while(true);
    }
}

Same result as Test 2.

After some research I found this question. So, if the code inside the parentheses is a variable the compiler wouldn't throw the exception. That makes sense, but I don't think that the same applies to voids.

Q: So, why does the compiler just throw me the error at Test 1, if void b() (Test 2) and System.out.println("end"); (Test 3) isn't reachable?

EDIT: I tried Test 1 in C++:

#include <iostream>

using namespace std;

int main()
{
    cout << "start" << endl;
    while(true);
    cout << "end" << endl;
    return 0;
}

The compiler didn't throw any errors, then I got the same result as Test 2 and as Test 3. So I suppose this is a java thing?

question from:https://stackoverflow.com/questions/24130399/whiletrue-loop-throws-unreachable-code-when-isnt-in-a-void

Answer 1

The language spec has an exact definition what the compiler should treat as unreachable code, see also https://stackoverflow.com/a/20922409/14955.

In particular, it does not care about if a method completes, and it does not look inside other methods.

It won't do more than that.

However, you could get static code analysis tools like FindBugs to get a "deeper" analysis (not sure if they detect the pattern you described, though, either, and, as has been pointed out by others, the halting problem in all generality cannot be algorithmically solved anyway, so one has to draw the line at some reasonably definition of "best effort").