Generis method got error cannot be referenced from a static context

Question

I try to make the generic method for other class. But I got a error
cannot be referenced from a static context
How to do the generics method for GetInstance

public class Instance<T> {

    private static final Instance<?> mInstance = new Instance<>(null);

    @SuppressWarnings("unchecked")
    public static Instance<T> GetInstance() { //Got error
        // Make generic static instance.
        // Strategy used similar to Collections.emptyList() implementation
        return (Instance<T>) mInstance; //Got Error
    }

    protected Instance(Context context) {

    }
}

 public static DataController GetDataController(@Nullable  Context context) {
    DataController dataController = (DataController) DataController.GetInstance();
    if(dataController == null) {
        return new DataController(context);
    }

    return dataController;
}

score 0 · Answer 1 · answered May 11 '18 at 10:26

0

Try this. It's the ancient druid magic

public static <T> Instance<T> GetInstance() {
    return (Instance<T>) mInstance;
}

answered May 11 '18 at 10:26

Zeon

535
8
23

Thanks all for quick reply. – ManLok Wong May 11 '18 at 10:32
@ManLokWong, L.Spillner answer is more info, mark it as correct :) – Zeon May 11 '18 at 10:34

score 0 · Answer 2 · answered May 11 '18 at 10:27

Static methods are poperty of the class rather than of an instance of the class and therefore the generic of the class cannot be referenced.

However you can still create a generic method by adding a type reference directly to the method:

public static <T> Instance<T> GetInstance() {...}

And even bound it:

public static <T extends Comparable<T>> Instance<T> GetInstance() {...}

However to call such a method you maybe need to pass the type argument if the compiler can't infer it by itself:

Instance.<Integer> GetInstance();

Thanks all for quick reply. – ManLok Wong May 11 '18 at 10:32 — ManLok Wong, May 11 '18 at 10:32

Generis method got error cannot be referenced from a static context

2 Answers2