Lower bound elevation in sub classes

Question

I have a hierarchy of classes that can be simplified to the following example

public interface I<T> {
  T get();
}

public interface II<T extends Number> extends I<T> {
}

public class A implements II<Integer> {
  @Override
  public Integer get() {
    return null;
  }
}

public class B implements II<Double> {
  @Override
  public Double get() {
    return null;
  }
}

Now when I'm trying to compile the following code

    II ii = new A();
    Number n = ii.get();

I get a compiler error (at the 2nd line)

java: incompatible types: java.lang.Object cannot be converted to java.lang.Number

I understand that it is because the it takes the lower bound of I which is Object (since method get is declared in I) but I was expecting that it will take the lower bound of II. How can I refactor this hierarchy such that the compiler will consider Number as lower bound. The only restriction is that interface I cannot be modified because it is part of a library.

UPDATE: The idea is that I would like a generic solution

    II ii = <any instance of classes that implements interface II>
    Number n = ii.get();

What happens if you declare `T get();` also in the interface `II`? — Emil L, Nov 19 '19 at 10:10
compiles without errors but in the real problem, interface `I` has 10 methods and I don't want to redeclare them all — Radu Dumbrăveanu, Nov 19 '19 at 10:12

Sukhpal Singh · Answer 1 · 2019-11-19T10:16:43.070

2

Everything else is ok, declaration need to be changed to II<Integer> ii = new A();, then it works.

For both A and B, you can use II<? extends Number> = new A();

edited Nov 19 '19 at 10:16

answered Nov 19 '19 at 10:13

Sukhpal Singh

2,130
9
20

I would like to have a generic solution, which will work for both `ii = new A()` and `ii = new B()` and so on, `A` and `B` are dependencies – Radu Dumbrăveanu Nov 19 '19 at 10:15
Edited answer to reflect this – Sukhpal Singh Nov 19 '19 at 10:16

score 0 · Accepted Answer · answered Nov 20 '19 at 14:21

0

You shouldn't use raw type II. Replace it with II<?>, and everything will work fine:

II<?> ii = new A();

answered Nov 20 '19 at 14:21

IlyaMuravjov

2,352
1
9
27

Lower bound elevation in sub classes

2 Answers2