Why can’t a “Class” variable be passed to instanceof?

The instanceof operator works on reference types, like Integer, and not on objects, like new Integer(213). You probably want something like

clazz.isInstance(obj)

Side note: your code will be more concise if you write

public boolean isOf(Class clazz, Object obj){
    return clazz.isInstance(obj)
}

Not really sure if you need a method anymore ,though.