instanceof operator in java with interfaces

Question

Can anyone explain the following behavior in Java using instanceof operator .

Runnable r = new Thread();

eventhough the type of variable r is Runnable and the instanceof comparison is done on classes that do not reside in the same class hierarchy

System.out.println(r instanceof String); // This line does not compile 

System.out.println(r instanceof Vector); // This line compiles

System.out.println(r instanceof FileNotFoundException); // This line compiles

score 3 · Answer 1 · edited Aug 10 '17 at 09:57

3

String class is final -- that means it can't be subclassed. Moreover, it doesn't implement Runnable. All of this is known at compile time; hence, compilation error.

edited Aug 10 '17 at 09:57

Adeel Ansari

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answered Aug 10 '17 at 09:26

Maurice Perry

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More to the point: `String` doesn't implement `Runnable`, and it can't have *subclasses* that do (because it's `final`). – T.J. Crowder Aug 10 '17 at 09:28
@T.J.Crowder Yes +1 this would be good to edit into the answer to make it more useful – Erwin Bolwidt Aug 10 '17 at 09:31
@ErwinBolwidt: Yes. I was leaving it to Maurice. But the whole thing is a dupe anyway, so... – T.J. Crowder Aug 10 '17 at 09:33

score 1 · Answer 2 · answered Aug 10 '17 at 09:28

An example:

static class First {

}

static final class Second {

}

And than :

    Runnable r = new Thread();

    System.out.println(r instanceof First);

    System.out.println(r instanceof Second);

The compiler sees that Second is final, thus it can not have any sub-classes, thus it can not implement Runnable.

score 0 · Answer 3 · answered Aug 10 '17 at 09:28

The String class is final and it doesn't implement the Runnable interface. Hence, r instanceof String can never return true (since there cannot be any sub-class of String that would implement Runnable), which is why the compiler doesn't allow it.

On the other hand, there may be sub-classes of the Vector class or the FileNotFoundException class that implement the Runnable interface, so r instanceof Vector and r instanceof FileNotFoundException may return true as far as the compiler can tell.

This is covered by JLS 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.

instanceof operator in java with interfaces

3 Answers3