Why “t instanceof T” is not allowed where T is a type parameter and t is a variable?

Question

Eclipse says that the instanceof operation is not allowed with Type Parameter due to generic type eraser.

I agree that at runtime, no type information stays. But con

Answer 1

Because of type erasure, this never works. At runtime, you only know that your class has a type parameter T, but not which type it is for a given instance. So you can't determine whether an object is of type T to begin with, because you don't know what T is, not because it would cause some sort of trouble.

If you need to do this sort of runtime check, pass a type token to your object explicitly:

SomeClass(Object o, Class type) {
    System.out.println(type.isInstance(o));
}