Java Inheritance and Generics

Question

I have some classes that look like this:

MODEL

public abstract class BaseEntity<O extends Object> { ... }

public class Person extends BaseEntity<Person> { ... }

COMMAND

public abstract class BaseCommand<BE extends BaseEntity<BE>> { ... }

public class PersonCommand extends BaseCommand<Person> { ... }

SERVICE

public interface BaseService<BE extends BaseEntity<BE>> {
    public BE create(BaseCommand<BE> command);
}

public interface PersonService extends BaseService<Person> { ... }

SERVICE IMPL

public abstract class BaseServiceImpl<BE extends BaseEntity<BE>> implements BaseService<BE> { }

public class PersonServiceImpl extends BaseServiceImpl<Person> implements PersonService {
    public Person create(PersonCommand personCommand) { ... }
}

The PersonServiceImpl class won't compile. It's not recognizing that the create() method is implementing the create() method from the BaseService interface. Can anyone tell why PersonCommand isn't being recognized as a BaseCommand<BE> (in the parameter list)?

I don't understand why you're parametrizing the class with itself... public class Person extends BaseEntity<Person> { ... } — I82Much
– I82Much, Commented Aug 10, 2011 at 1:59
Basically I do it so that methods in BaseEntity can refer to the type. public BE method(); etc — Snowy Coder Girl
– Snowy Coder Girl, Commented Aug 10, 2011 at 2:01
The return type is correct. When I use Eclipse to override/implement, it uses Person as the return and it works correctly. It won't accept PersonCommand instead of BaseCommand<Person>. — Snowy Coder Girl
– Snowy Coder Girl, Commented Aug 10, 2011 at 2:02
@Rachel: Correct, it shouldn't accept a PersonCommand instead of BaseCommand<Person>. (See my answer for more details. :-)) — C. K. Young
– C. K. Young, Commented Aug 10, 2011 at 2:03

C. K. Young · Accepted Answer · 2011-08-10 02:00:11Z

7

When overriding, method parameters are not covariant (that is, subclasses have to accept a type that the superclass also accepts, not anything narrower).

This is because people can use your PersonServiceImpl via the PersonService interface, which will accept an argument of type BaseCommand<Person> that is not necessarily a PersonCommand (imagine if you created a second class that extended BaseCommand<Person>).

If you make your method take a parameter of type BaseCommand<Person>, your code should compile correctly.

answered Aug 10, 2011 at 2:00

C. K. Young

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2 Comments

Snowy Coder Girl Over a year ago

Makes sense. The BaseServiceImpl class is forcing you to accept all BaseCommand<Person> objects. Thanks for the help. +1 for a good explanation and example.

Snowy Coder Girl Over a year ago

Won't let me accept for 4 minutes apparently, but I will. Thanks again.

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