5

我正在努力完成这项工作：

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;
    }

}

问题是编译错误：

类型不匹配：无法从 MapperFactory.MyTaskMapperFactory 转换为 MapperFactory

在我的 MapperFactory 中：

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

任何想法如何解决这个错误？

当然更换：

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!");
    }

和：

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

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

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

会工作，但这不是我正在寻找的答案。

一般来说，这似乎是通用抽象工厂模式的问题。也欢迎提供使用定制对象的源样本的答案。

4

3 回答 3

4

根据 Effective Java，第 2 版，第 28 项：

如果类型参数在方法声明中只出现一次，请将其替换为通配符。

您的 getMapperFactory 方法仅在返回类型中使用类型参数 M。遵循此建议会给出以下方法签名，并且该方法会编译：

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

编辑：我越看代码，我越认为 MapperFactory 不应该被参数化。此处代码中没有使用该参数，getTaskMapper 返回一个TaskMapper。

于 2012-02-13T18:10:20.213 回答
1

我的解决方案是用火杀死尽可能多的泛型：

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;
    }

}

如果您并不真正关心它们是什么或者不需要比类签名更多地约束它们，那么实际上没有必要在使用它的每个方法中重复类的类型参数。

于 2012-02-14T15:45:53.763 回答
1

return 语句适用于类型转换：

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

该代码将永远不会以其当前形式执行，因为 Bpm007PrepareDocTaskMapper 没有扩展 BpmCommonMessageDto，因此 msgIn 不可能是 Bpm007PrepareDocTaskMapper 的实例。

于 2012-02-08T12:40:42.103 回答