Why is an if/else if/else for a simple boolean not giving an "unreachable code" error

Question

Why is this code not giving an "unreachable code" error? Since a boolean can only be true or false.

public static void main(String args[]) {
    boolean a = false;
    if (a == true) {

    } else if (a == false) {

    } else {
        int c = 0;
        c = c + 1;
    }
}

Turbit

See the second half of this question: #9276878 – Othaothe

Also interesting: #20300414 – Quarantine

A check in the JLS gives the answer (as always): "The else-statement is reachable iff the if-then-else statement is reachable. ". Since both if and else if are reachable, the else statement is it as well. – Quarantine

Refer this docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.21 – Isomerous

@Quarantine This should be the answer :) – Farthermost

can't boolean values be set to null? – Trapezohedron

@Trapezohedron no, you confuse with Boolean which is object. boolean is primitive. – Alvie

There are loads of examples of code that is not unreachable but can never be reached - e.g. default cases in switches where all possibilities already covered. Why should boolean be special just because there are only 2 values? – Finalism

it is possible in multithreaded environment that first it is false when it just reaches else if variable is again true – Valse

if (a == true), if (a == false) <-- Don't ever do this when a is a boolean. Just do if (a) and if (!a). – Scalp

@Scalp I am 99% sure both generate the same bytecode. Use what is more readable in the context you need it in. – Tracheostomy

@Tracheostomy It is a matter of style and presentation. Seeing the code, a == true without there being a very good reason why screams that the author lacks understanding about what == does. I admit that there are rare cases where it might make sense, but these are extremely rare. (They might be a bit more common in weakly or dynamically typed languages, but this is Java.) Certainly, in this example, it's completely unnecessary. – Scalp

@Scalp I'm not opposed to your general statement, just to the "ever" part of it ;-) – Tracheostomy

@Tracheostomy To be fair, my comment includes the if and the parentheses and that a is a boolean (primitive). It never makes sense in the full context (a single boolean check on a non-nullable type). – Scalp

This might help: thedailywtf.com/articles/What_Is_Truth_0x3f_ – Klepht

Answer 1

From JLS 14.21. Unreachable Statements

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

and

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

Your if-then-else statement is reachable. So, by the definition the compiler thinks that the else-statement is reachable.

Note: Interestingly the following code also compiles

// This is ok
if (false) { /* do something */ }

This is not true for while

// This will not compile
while (false) { /* do something */ }

because the reachability definition for while is different (emphasis mine):

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

Physiologist

I don't know if this is what the Java designers had in mind, but it can be useful to have if (COMPILE TIME CONSTANT EXPRESSION) { ... } constructs where the COMPILE TIME CONSTANT EXPRESSION might be either true or false depending on the environment. – Cult

@Cult that's exactly the reason why such behavior exists. I also think to remember that in the very first version of JVM if(false) was not compilable because clearly unreachable. Also note that this behavior comes in handy to put if(true){return;} in the middle of a method while developing for debug purpose. Compiler perfectly knows that everything that comes after is unreachable, but it still allows you to write it since it's just useful to have it. – Pentarchy