缩小继承的返回类型（涉及泛型） [英] Narrowing return types on inheritance (generics involved)

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：

  public class AbstractIndex {
} 
 
 public class TreeIndex extends AbstractIndex {
} 
 
 public interface IService< T extends AbstractIndex> {
} 
 
公共接口ITreeService扩展IService< TreeIndex> {
} 
 
公共抽象类AbstractServiceTest< T扩展AbstractIndex> {
 abstract< V extends IService< T>> V getService（）; 
} 
 
 public class TreeServiceTest extends AbstractServiceTest< TreeIndex> {
 @Override 
 ITreeService getService（）{
 return null; 
 
 
 $ / code> 

问题在于，当我尝试缩小时，Java会发出警告返回类型 getService 到 ITreeService 。警告是：
$ b

类型安全性：来自类型TreeServiceTest的getService（）的返回类型ITreeService需要未经检查的转换以符合类型V AbstractServiceTest

为什么ITreeService不是 getService ？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

Type safety: The return type ITreeService for getService() from the type TreeServiceTest needs unchecked conversion to conform to V from the type AbstractServiceTest

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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