Question

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Why is this code giving an “Unreachable Statement” error?

This seems very easy question, I found this question in one book. If anyone help me to figure out why I’m getting error.

    do {
        System.out.print("inside do");
    } while (false);
    while (false) { // error
        System.out.print("inside while");
    }
    System.out.print("outside");

I thought, and according to me, output should be inside dooutside. But, it is showing Compiler Error : Unreachable Statement. Then, I tried to figure out, why, it is showing Compilation error : Unreachable Statement* . So, I change the above code like this

  boolean i = false;  
  do {
        System.out.print("inside do");
    } while (false);
    while (i) { // ok
        System.out.print("inside while");
    }
    System.out.print("outside");

Now, it is showing expected output i.e. inside dooutside . So, my question is – what makes difference in first and second case ?
Also, when I check

if(false){ 
  //something here
   }

Then, above code executes without any error.

Answer 1

The main difference between the first two examples is that in the first case, the condition is a constant, whereas in the second it is not.

For example, if you change boolean i = false; into final boolean i = false;, you will get the same compile error because i is now a constant.

The rules for unreachable statements are defined in the JLS 14.21. In particular there is a special treatment for if to allow if(DEBUG) structures where DEBUG might be a constant.

As for the do / while, the statement inside will be executed once so there is no problem.

More details about the constants in this related post.