Question

I have a question about generics in Java and instanceof operator.

It’s immposible to make such instanceof check:

if (arg instanceof List<Integer>)  // immposible due to
                                   // loosing parameter at runtime

but it’s possible to run this one:

if (arg instanceof List<?>)

Now comes my question – is there any difference between arg instanceof List and arg instanceof List<?> ?

Answer 1

Java Generics are implemented by erasure, that is, the additional type information (<...>) will not be available at runtime, but erased by the compiler. It helps with static type checking, but not at runtime.

Since instanceof will perform a check at runtime, not at compile time, you can not check for Type<GenericParameter> in an instanceof ... expression.

As to your question (you probably seem to know already that the generic parameter is not available at runtime) there is no difference between List and List<?>. The latter is a wildcard which basically expresses the same thing as the type without parameters. It’s a way of telling the compiler “I know that I don’t know the exact type in here”.

instanceof List<?> boils down to instanceof List – which are exactly the same.