Compiler error instance of final class [duplicate]

Question

The following code compiles fine:

interface Flyer{ }
class Bat { }

public class App {

    public static void main(String[] args) { 
        Bat b = new Bat();
        if(b instanceof Flyer) System.out.println("b is a Bird");
    }

} 

If we make the Bat class final, the code doesn't compile:

final class Bat { } 

If the final class implements Flyer, it compiles fine:

final class Bat implements Flyer { } 

Anyone care to explain the logic behind this?

Answer 1

When you make class Bat final, you're saying that this class can't be sub-classed. Since Bat does not implement the interface Flyer, the compiler is able to determine that b instanceof Flyer can never be true and raises an error.

This is specified in JLS section 15.20.2:

If a cast (§15.16) of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.

Additionally, from section 15.16 about cast expressions:

It is a compile-time error if the compile-time type of the operand may never be cast to the type specified by the cast operator according to the rules of casting conversion (§5.5).

In this case, Bat can never be cast to Flyer: it doesn't implement it and final ensures that there can't be sub-classes that would implement it.

---

As you found out, the fixes are:

• Make Bat implement Flyer: in this case, the instanceof operator would always return true.
• Remove the final identifier, implying that there could be sub-classes of Bat implementing Flyer.

Answer 2

Well, if Bat is a final class and it doesn't implement Flyer, it can't have any sub-class that would implement Flyer either, so instanceof can never return true. In such cases the compiler doesn't allow this expression (i.e. (x instanceof Y) is only allowed if there's a chance that x would contain a reference to an instance that implements or extends Y).

In your second snippet Bat already implements Flyer, so b instanceof Flyer would always return true, regardless of whether Bat is final or not.