What is the right direction of using “*.isInstance”?

Question

I am confused every time I read the Java Documentation again to that. So please try to help me in your own words.

List<Parent> list = new ArrayList<Parent>();
//Child extends Parent...
list.add(new Child());
...
...
for(Parent p: list){
    if(p.getClass().isInstance(Child)){
            Child c = (Child) p;
            c.execFuncNonExistingInParent();
    }
}

Just wanna proof the Objects inheritances from Parent, to avoid Cast Problems.

if(p.getClass().isInstance(Child.class))

or

if(Child.class.isInstance(p.getClass()))

Greatings Oekel

Answer 1

That's not checking what you want to check either way round. You want:

if (Child.class.isInstance(p))

That's equivalent to:

if (p instanceof Child)

... except you can specify the class to check dynamically, rather than it being fixed at compile-time. If you do know the class at compile-time (as in your example) then just use the instanceof operator instead.

For isInstance , it's easy to tell which way round it goes, once you've remembered it as being equivalent to instanceof , because of the signature:

// In Class
public boolean isInstance(Object obj)

You want the equivalent of obj instanceof clazz , where obj can be any object reference, and clazz must be a class... so you really have to call it as clazz.isInstance(obj) .