What's the error in 'for' loop?

Question

Why do the following raise an error?

for(; 0   ;)     System.out.println("guess");  // or
for(;false;)     System.out.println("guess");  // or
for(; 1   ;)     System.out.println("guess");

But the following runs okay (infinitely):

for(;true;)      System.out.println("guess");

Why does it work for true but not for false?

Answer 1

The condition (i.e. the bit between the ;s) must be a boolean, so this immediately rules out the first and third variants in your first snippet.

Now, the second variant, in which you have used a boolean, doesn't compile because the compiler realizes that the loop will never be entered and hence issues an error:

Untitled.java:3: error: unreachable statement
        for(;false;)     System.out.println("guess");
                         ^
1 error

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Note that the JLS mandates that errors be issued for unreachable statements (see §14.21):

It is a compile-time error if a statement cannot be executed because it is unreachable.

...

The contained statement is reachable iff the for statement is reachable and the condition expression is not a constant expression whose value is false.