Question about unreachable code compiler error

[Aline Galea]

Hello everyone, I feel a bit confused by this question from the Whizlabs mock tests for the OCAJP:

public class Main {
	public static void main(String[] args) {
		Loop1:for(int x = 0; x < 3; x++) {
			for(int y = 0; y < 3; y++) {
				if (y == 2) continue;
				if (y == 2 && x == 2) break Loop1;
					System.out.print(y);
			}
		}
	}
}

I thought this code would throw an unreachable code compiler error because of lines 5 and 6. The if condition on line 6 will never be true because line 5 will always prevent that line from being reached under conditions where it can be true. So the "break Loop1" statement will never be reached. However both the test results and my compiler say this is fine and does not lead to errors. I guess that's because "unreachable code" doesn't apply to statements inside an "if" block, even if the condition can never be true? But I've seen classes fail to compile because of a "while" condition that was never true. I don't understand this difference. What are the reasons? Am I missing something? And are there other exceptions to the "Unreachable Code" error?

I never expected the "junior" exam to be this hard.

[Paweł Baczyński]

The Java Language Specification states:

JLS wrote:Except for the special treatment of while, do, and for statements whose condition expression has the constant value true, the values of expressions are not taken into account in the flow analysis.

You think (and know) that the statement will never be reached. But for the compiler it is reachable! The reason is, as stated by JLS, that the compiler does not examine what is written inside if as a condition.

You could even write:

for(int y = 0; y < 3; y++) {
    if (true) break;
    System.out.println("Will never reach here"); // but the compiler does not care ;)
}

and for the compiler the statement would be reachable.

It is not hard. You are just overthinking it.

Of course this is also not super easy. The JLS states that while, do and for loops are treated differently.

This won't compile:

while(false) {
    System.out.println("Hello");
}

And this will compile:

if(false) {
    System.out.println("Hello");
}

[Aline Galea]

Paweł Baczyński wrote:The Java Language Specification states:

JLS wrote:Except for the special treatment of while, do, and for statements whose condition expression has the constant value true, the values of expressions are not taken into account in the flow analysis.

You think (and know) that the statement will never be reached. But for the compiler it is reachable! The reason is, as stated by JLS, that the compiler does not examine what is written inside if as a condition.

Thanks a lot for this bit of explanation that I was somehow not able to find on my own. I will do some more tests but I think I got it now.

Paweł Baczyński wrote:It is not hard. You are just overthinking it.

The problem is, there are other aspects on the exam that I was clearly underthinking ^^ Those questions sometimes throw absurd code at me just to see if I know of some rule that is never used by programmers in real life (like yes, apparently handling an error is legal, but apparently you shouldn't, so you'll never see code that does that... except on the exam). I find that difficult (and a bit frustrating at times) (but I am glad that I'm developing a deeper understanding of java).

[Paul Clapham]

Yes, the exams are full of code fragments which no reasonable programmer would ever write in real life. But if you become a professional programmer there's a good chance that you will find yourself working on code which was written by un-reasonable programmers, and you'll have to figure out what it's supposed to do. So if you look at it that way, this exam is testing your ability to do that.

[Roel De Nijs]

Aline Galea wrote:I guess that's because "unreachable code" doesn't apply to statements inside an "if" block, even if the condition can never be true? But I've seen classes fail to compile because of a "while" condition that was never true. I don't understand this difference. What are the reasons? Am I missing something? And are there other exceptions to the "Unreachable Code" error?

Luckily for you, the "unreachable code" topic is one of the more popular ones in this forum. So using the search function you'll find plenty of topics about "unreachable code". Here is a list which could probably be added to my certification book

return value for method.

Confused point about infinite loop

continue and break in a loop would generate compilation error?

Which of these is unreachable code?

Unreachable Code

Which is the first line to cause error?

Maybe "unreachable code" should be "dead code" in page 70, (Java OCA 8 Programmer I Study Guide)

Throwing a second exception question

System.exit() and unreachable code

good examples of unreachable code?

I think you know what to do this weekend

Hope it helps!
Kind regards,
Roel