Instanceof for generic with <?> or without <?>

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.

Answer 2

List and List < ? > are not same but in this case when you using with instanceof operator they mean the same thing.

instanceof cann't be used with Generics as Generics doesn't store any type information at run time (due to erasure implementation).

This point gets cleared by below main method. I have declared two list (one of Integer type and other of String type). And instanceof behave same for List and List < ? >

public static void main(String[] args){
    List<Integer> l = new ArrayList<Integer>();
    List<String> ls = new ArrayList<String>();

    if(l instanceof List<?>){
        System.out.print("<?> ");
    }

    if(l instanceof List){
        System.out.print("l ");
    }

    if(ls instanceof List<?>){
        System.out.print("<?> ");
    }

    if(ls instanceof List){
        System.out.print("ls ");
    }
     }

output is : <?> l <?> ls

In above main method all if statements are true. So it clear that in this case List and List<?> are same.