Java generics abstract factory issue

Question

I am struggling to make this work:

public abstract class MapperFactory<M extends TaskMapper<? extends Message, ? extends Message, ? extends TaskForm>> {

    public static <M extends TaskMapper<? extends Message, ? extends Message, ? extends TaskForm>> MapperFactory<M> getMapperFactory(Message msgIn, Message msgOut) {

        if (msgIn.isMyMapper())
            return new MyTaskMapperFactory();

        throw new IllegalStateException("Mapper not found!");
    }

    public abstract TaskMapper<? extends Message, ? extends Message, ? extends TaskForm> getTaskMapper();

    public static class MyTaskMapperFactory extends MapperFactory<MyTaskMapper> {

        @Override
        public TaskMapper<? extends Message, ? extends Message, ? extends TaskForm> getTaskMapper() {
            return new MyTaskMapper();
        }

    }
}

public interface TaskMapper<I extends Message, O extends Message, F extends TaskForm> {

    public F fillForm(I msgIn, O msgOut, F taskForm);

    public O fillMsgOut(F taskForm);
}

public class MyTaskMapper implements TaskMapper<IncomingMessage, OutgoingMessage, MyTaskForm > {

    public MyTaskForm fillForm(IncomingMessage msgIn, OutgoingMessage msgOut,
            MyTaskForm taskForm) {
        return null;
    }

    public OutgoingMessage fillMsgOut(MyTaskForm taskForm) {
        return null;
    }

}

The problem is a compilation error:

Type mismatch: cannot convert from MapperFactory.MyTaskMapperFactory to MapperFactory

in my MapperFactory here:

if (msgIn.isMyMapper())
            return new MyTaskMapperFactory();

Any ideas how to fix this error?

Of course replacing:

public static <M extends TaskMapper<? extends Message, ? extends Message, ? extends TaskForm>> MapperFactory<M> getMapperFactory(Message msgIn, Message msgOut) {

        if (msgIn.isMyMapper())
            return new MyTaskMapperFactory();

        throw new IllegalStateException("Mapper not found!");
    }

with:

public static MapperFactory<?> getMapperFactory(Message msgIn, Message msgOut) {

        if (msgIn.isMyMapper())
            return new MyTaskMapperFactory();

        throw new IllegalStateException("Mapper not found!");
    }

would work, but that is not the answer that I am looking for.

This seems to be a problem with generic abstract factory pattern in general. Answers providing source samples using custom made-up objects are also welcomed.

`MAPPER` is not a good class name following Java conventions. — Hauke Ingmar Schmidt, Feb 13 '12 at 11:52
Please make your class names as small as possible (but still clear). And rename `MAPPER` to `Mapper`? Weird/long class names just confuse/clutter up the question *and* the answers. — Bohemian, Feb 13 '12 at 12:01
@his MAPPER is not a class name. It is the name of generic Type such as T in HashMap — Wojciech Owczarczyk, Feb 13 '12 at 12:06
It is still a type name. All capital is for constants in Java. — Hauke Ingmar Schmidt, Feb 13 '12 at 12:08
@his - no it is not, nevermind... The convention is to use only single letters as BalusC stated here http://stackoverflow.com/questions/2900881/generic-type-parameter-naming-convention-for-java-with-multiple-chars so I changed my code to follow the conventions. — Wojciech Owczarczyk, Feb 13 '12 at 12:13
It is a type parameter, a type variable: http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.1.2 For type parameters the convention is a single letter, that is right. — Hauke Ingmar Schmidt, Feb 13 '12 at 12:20

TDJoe · Accepted Answer · 2012-02-14T15:29:21.623

4

According to Effective Java, 2nd edition, item 28:

If a type parameter appears only once in a method declaration, replace it with a wildcard.

Your getMapperFactory method only uses the type parameter M in the return type. Following this advice gives the following method signature, and the method compiles:

public static MapperFactory<? extends TaskMapper<Message, ? extends Message, ? extends String>> getMapperFactory(Message msgIn, Message msgOut)

EDIT: The more I look at the code, the more I think MapperFactory shouldn't be parameterized. The parameter isn't used in the code here, getTaskMapper returns a TaskMapper.

edited Feb 14 '12 at 15:29

answered Feb 13 '12 at 18:10

TDJoe

1,388
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2

Even shorter, the signature can just be `public static MapperFactory> getMapperFactory()`. The method doesn't constrain any of the type parameters more than the class signature for `MapperFactory` does. The same goes for `public abstract TaskMapper, ?, ?> getTaskMapper()`. (It could also be `public abstract M getTaskMapper()`, can't tell what the intent behind the type parameter is.) *"When in doubt, use less generics"* is probably a good rule of thumb. – millimoose Feb 14 '12 at 15:39

score 1 · Answer 2 · answered Feb 08 '12 at 12:40

1

The return statement works fine with a typecast:

return (BpmMapperFactory<MAPPER>)new Bpm007PrepareDocTaskMapperFactory();

That code will never execute though in its current form, because Bpm007PrepareDocTaskMapper doesn't extend BpmCommonMessageDto, so msgIn cannot possibly be an instance of Bpm007PrepareDocTaskMapper.

answered Feb 08 '12 at 12:40

Boann

48,794
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I edited the `if` part -thanks for info, that wasn't finished yet and it's not relevant here. Any way to avoid casts? – Wojciech Owczarczyk Feb 08 '12 at 14:36

score 1 · Answer 3 · answered Feb 14 '12 at 15:45

My solution would be killing as much of the generics as possible with a fire:

abstract class MapperFactory<M extends TaskMapper<?, ?, ?>> {

    public static MapperFactory<?> getMapperFactory(Message msgIn, Message msgOut) {
        if (msgIn.isMyMapper()) return new MyTaskMapperFactory();
        throw new IllegalStateException("Mapper not found!");
    }

    public abstract M getTaskMapper();
}


class MyTaskMapperFactory extends MapperFactory<MyTaskMapper> {

    @Override
    public MyTaskMapper getTaskMapper() {
        return new MyTaskMapper();
    }

}


interface TaskMapper<I extends Message, O extends Message, F extends TaskForm> {

    public F fillForm(I msgIn, O msgOut, F taskForm); 

    public O fillMsgOut(F taskForm);

}

class MyTaskMapper implements TaskMapper<IncomingMessage, OutgoingMessage, MyTaskForm> {

    public MyTaskForm fillForm(IncomingMessage msgIn, OutgoingMessage msgOut, MyTaskForm taskForm) {
        return null;
    }

    public OutgoingMessage fillMsgOut(MyTaskForm taskForm) {
        return null;
    }

}

It's really not necessary to repeat the type parameters of a class in every method that uses it if you don't really care what they are or don't need to constrain them more than the class signature does.

Java generics abstract factory issue

3 Answers3