CODEWITHSUNDEEP
javagenericsinterface
Matt Magnusson
Matt Magnusson

Reputation: 11

Implementing a nested generic in java

I have the following class

public class Element <T1 extends Comparable<T1>,T2> {
   public T1 key;
   public T2 value;
}

This compiles and works as I'd like.

I have an interface that I want to guarantee that all Elements in it have the same type. But I want to specify that type in the class implementing the interface. So as an example, I might want the class that implements the interface to all be of the type Element<String,Integer>.

However, this won't compile

public interface D <Element<T1,T2>>  {
  ArrayList <Element<T1,T2>> getVertices();
}

This does compile

public interface D <Element>  {
    ArrayList <Element> getVertices();
}

When I run this code

public class G<Element> implements D<Element> {
    public ArrayList<Element> getVertices(){return null;}

    public static void main(String[] args) {
        G <Element<String,Integer>> g = new G<>();
    }
}

I get this error. 'Error:(7, 12) java: non-static type variable Element cannot be referenced from a static context'

I'm not sure how to specify in the interface that I want all elements to have to be of the same type.

Upvotes: 1

Views: 85

Answers (1)

tsolakp
tsolakp

Reputation: 5948

The T1 and T2 in your D interface are undefined, thats why you are getting compile error. The second D example is using row type of Element and thats why your G <Element<String,Integer>> declaration is wrong. Also you dont need to parametrize G at all.

Here is modified code that uses proper generics: 

public interface D < T1 extends Comparable<T1>, T2  >  {

    ArrayList < Element<T1, T2> > getVertices();
}

public static class G implements D< String, Integer > {
    public ArrayList< Element<String, Integer> > getVertices(){return null;}

    public static void main(String[] args) {
        G g = new G();
    }
}

Upvotes: 2

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