缩小继承的返回类型（涉及泛型）

问题描述

我正在琢磨一些关于在子类化时能够缩小返回类型的奇怪泛型行为。我设法将问题减少到以下一组类：

 的有效缩小类型？ p> 
 
 
编辑：将错误更改为警告
解决方案
因为我认为你的意思是这样说：
  public abstract class AbstractServiceTest< T extends AbstractIndex> {
抽象IService< T>的getService（）; 
} 
  
制作单独的 V 类型变量，而不是添加您的子类无法实现的约束。 :-P 
 
I'm wrestling with a bit of weird generics behavior regarding being able to "narrow" return types when subclassing. I managed to reduce the problem to the following set of classes:
public class AbstractIndex {
}

public class TreeIndex extends AbstractIndex {
}

public interface IService<T extends AbstractIndex> {
}

public interface ITreeService extends IService<TreeIndex> {
}

public abstract class AbstractServiceTest<T extends AbstractIndex> {
    abstract <V extends IService<T>> V getService();
}

public class TreeServiceTest extends AbstractServiceTest<TreeIndex> {
    @Override
    ITreeService getService() {
        return null;
    }
}
The problem is that Java warns when I try to narrow the return type of getService to ITreeService. The warning is
Why is not ITreeService a valid narrowing type for getService?
EDIT: changed error to warning
 解决方案 
Because I think you meant to say this:
public abstract class AbstractServiceTest<T extends AbstractIndex> {
    abstract IService<T> getService();
}
There's no purpose to making the separate V type variable, other than adding constraints that your subclasses can't fulfil. :-P
                        
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