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>          


        

Answer 1

Since there is no relationship between static methods and the class’ 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 a a Class parameter, as otherwise, the implementation has no chance to get hands on the actual type arguments:

public interface DbValuesEnumIface> {

   public static
   &DbValuesEnumIface> T fromId(ID id, Class 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’ type parameter. You may consider giving them different names for clarity.

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

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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&DbValuesEnumIface> {

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