Java generics extends in type parameter

Question

i have problem in my java project recently with generics. the following is the simplified version of my actual problem.

abstract class Value{
    public int value;

    Value(int v){ value = v; }
}

class Wrap<T> {
    T data;

    Wrap(T data){ this.data = data; }    

    public void add(T value){
        data = actualAdd(data, value);
    }

    public T actualAdd(T p1, T p2){/* other code. don't worry */}
}

easy to see, i want to inherit Value afterwards.

but when it comes to combine both, i.e.:

Wrap<? extends Value> wrapValue;

i really wish add recieves Value, i.e.:

wrapValue.add(Value value)

instead of

wrapValue.add(? extends Value value)

since i just want to add up the value, the other fields are irrelevant.

i know how to do it in static method, that should be:

public static void staticAdd(Wrap<? extends T> wrapper, T value){/**/}

but how can i have the same trick in method member?

Don't use `Wrap extends Value>`, you won't be allowed to set anything. — Luiggi Mendoza, Dec 18 '14 at 17:18
@LuiggiMendoza i don't need to set anything internally. i just want the `add` method functioning. — Jason Hu, Dec 18 '14 at 17:20
The `add` method is setting something. Thus, you cannot use it. I recommend you to read here: [What is PECS?](http://stackoverflow.com/q/2723397/1065197) — Luiggi Mendoza, Dec 18 '14 at 17:21
@LuiggiMendoza so i can't do the similar thing like `staticAdd` to `Wrap` itself? — Jason Hu, Dec 18 '14 at 17:27
You can do it but for consumers, which is not really your case. In fact, that method definition doesn't make any sense. — Luiggi Mendoza, Dec 18 '14 at 17:27
@LuiggiMendoza i've thought a while but it seems i am still confused a little bit. could you show me how to make `Wrap extends Value>` have method with signature of `Value`? i am curious. — Jason Hu, Dec 18 '14 at 17:50
@LuiggiMendoza oh, now i understand what you were talking about. thank you! — Jason Hu, Dec 18 '14 at 18:13

score 0 · Answer 1 · answered Dec 18 '14 at 17:22

0

PECS (Producer Extends and Consumer Super) is the way to go here. Try using ? super Value.

Here is one post, which might help you with more context on this.

answered Dec 18 '14 at 17:22

nitishagar

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Java generics extends in type parameter

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