Static Method in Interface with Generic signature

Question

As of Java 8 you can have default or static methods implemented in Interfaces as the below

public interface DbValuesEnumIface<ID, T extends Enum<T>> {
   T fromId(ID id);

   ID getId();
   static String getDescriptionKey(){
      return "this is a test";
   }
}

I would like to declare the above with the static method having a signature that uses bounds defined by the implementing classes since the method's implementation should be the same for all,with the only thing different should be the generics declared, as such:

public interface DbValuesEnumIface<ID, T extends Enum<T>> {

   public static T fromId(ID id) {
        if (id == null) {
            return null;
        }
        for (T en : T.values()) {
            if (en.getId().equals(id)) {
                return en;
            }
        }
    }

    ID getId();

    String getDescriptionKey();
}
...
public enum Statuses implements DbValuesEnumIface<Integer,Statuses>

which breaks because T and ID are not static and cant be referenced from a static context.

So, how should the above be modified to compile successfully and if thats not possible, how the above should be implemented to achieve the desired purpose while avoiding code duplication within implementing classes .

Answer 1

Since there is no relationship between static methods and the class’s type parameters, which describe how instances are parameterized, you have to make the static method generic on its own. The tricky part is to get the declarations right to describe all needed constraints. And, as this answer already explained, you need to add a Class parameter, as otherwise, the implementation has no chance to get hands on the actual type arguments:

public interface DbValuesEnumIface<ID, T extends Enum<T>> {

   public static <ID, T extends Enum<T> & DbValuesEnumIface<ID, T>>
   T fromId(ID id, Class<T> type) {

        if (id == null) {
            return null;
        }
        for (T en : type.getEnumConstants()) {
            if (en.getId().equals(id)) {
                return en;
            }
        }
        throw new NoSuchElementException();
    }

    ID getId();

    String getDescriptionKey();
}

Note that the type parameters of the static method are independent from the class’s type parameter. You may consider giving them different names for clarity.

So now, given you enum Statuses implements DbValuesEnumIface<Integer,Statuses> example, you can use the method like Statuses status = DbValuesEnumIface.fromId(42, Statuses.class);

---

Note that for default methods, it is possible to access the actual type, as a method providing the enum type will be provided by the implementation. You only have to declare the presence of the method within the interface:

public interface DbValuesEnumIface<ID, T extends Enum<T>&DbValuesEnumIface<ID,T>> {

    public default T fromId(ID id) {
        if (id == null) {
            return null;
        }
        for (T en : getDeclaringClass().getEnumConstants()) {
            if (en.getId().equals(id)) {
                return en;
            }
        }
        throw new NoSuchElementException();
    }

    //no needed to implement it, inherited by java.lang.Enum
    Class<T> getDeclaringClass();

    ID getId();

    String getDescriptionKey();
}

However, the obvious disadvantage is that you need a target instance to invoke the method, i.e. Statuses status = Statuses.SOME_CONSTANT.fromId(42);