Why does a Java Compiler not produce an unreachable statement error for an unreachable then statement?

Question

If I try to compile

for(;;)
{

}
System.out.println("End");

The Java compiler produces an error saying Unreachable statement. But if I add another "unreachable"(according to me) break statement and make it:

for(;;)
{
    if(false) break;
}
System.out.println("End");

It compiles. Why does it not produce an error?

Answer 1

The behaviour is defined in the JLS description of unreachable statements:

The then-statement is reachable iff the if-then statement is reachable.

So the compiler determines that the then-statement (break;) is reachable, regardless of the condition in the if.

And a bit further, emphasis mine:

A basic for statement can complete normally iff at least one of the following is true:

• The for statement is reachable, there is a condition expression, and the condition expression is not a constant expression (§15.28) with value true.
• There is a reachable break statement that exits the for statement.

So the for can complete normally because the then-statement contains a break. As you noticed, it would not work if you replaced break with return.

---

The rationale is explained towards the end of the section. In substance, if has a special treatment to allow constructs such as:

if(DEBUG) { ... }

where DEBUG may be a compile time constant.

Answer 2

As explained in my answer to a similar question, the specific construct if(compile-time-false) is exempt from the unreachability rules as an explicit backdoor. In this case, the compiler treats your break as reachable because of that.