Generics subtle differences

Question

  class P {

   }
  class Q extends P {

   }
  class R extends Q { 

   }
  class S extends R {

  }

  public class Manager {

    public static void main(String[] args) {

      X<? super R> x4 = null;
      x4.test(new S());
      x4.test(new R());
      x4.test(new P());//compile time error

     }
}

    class X<A> {
      void test(A obj) {

       }
    }

//means that R or superclass of R i.e P, Q or Object but then why I get a compile time error on line as specified in the program.

score 4 · Answer 1 · edited May 23 '17 at 11:44

Here are some rules from the JLS

A type argument T1 is said to contain another type argument T2, written T2 <= T1, if the set of types denoted by T2 is provably a subset of the set of types denoted by T1 under the reflexive and transitive closure of the following rules (where <: denotes subtyping (§4.10)):

? super T <= ? super S if S <: T

? super T <= ?

? super T <= ? extends Object

T <= ? super T

The notation

X<? super R> x4 = null;

declares X with a generic wildcard type argument that has a lower bound of R.

With the rules above and your declaration of the variable, we can determine that the wildcard covers any sub types of R and R itself. S is a sub type of R, R is R, P is not a sub type of R.

The confusion may come in the following. You can do

X<? super R> x4 = new X<P>();

since P is a superclass of R. However, when you want to use the type parameter of the class X, all you know is that it must at least be an R.

Generics subtle differences

2 Answers2