oop - Is there a way to determine what type a class is an instance of in Java?

Question

Say I have 3 classes like so:

class A {}
class B extends A {}
class C extends A {}

Would it then be possible to determine whether a particular object was an instance of A, B, or C?

I thought that something like this might work:

if (myObject.getClass().isInstance(B.class)) {
    // do something for B
} else (myObject.getClass().isInstance(C.class)) {
    // do something for C
} else {
    // do something for A
}

but after reading a little I think it would always evaluate as B since it's just testing if a cast would work and there's no substantial differences between them.

Answer 1

The simpler and faster code is:

if (myObject instanceof B) {
} else if (myObject instanceof C) {
} else if (myObject instanceof A) {
}

Note that the order is important: you have to have the test for A last, as that will succeed for instances of B and C as well.

However, your original code would nearly work. Class.isInstance checks whether the value really is an instance of the given class or any superclass. So if myObject is an instance of C, then B.class.isInstance(myObject) will return false. All you've got wrong is that you're calling getClass() on myObject unnecessarily, instead of using B.class etc.

This is the approach you would take if you didn't know which classes you were interested in at compile time - the instanceof operator only works when you can specify the type statically in code.

Now if you want to find out whether myObject is an instance of exactly B (and not a subclass) then just use:

if (myObject.getClass() == B.class)

(This will blow up if myObject is a null reference, of course.)