Java类型参数是隐藏一个泛型？ [英] Java type parameter is hiding a generic type?

问题描述

我有一个泛型类：

  public class Widget< COMMAND> {
 // ... 
} 
  

所以我可以创建实例它就像：

  Widget< Fizz> fizzWidget = new Widget< Fizz>（）; 
  

然后我有一个ABC BaseWidgetProcessor ，那我想接受任何类型的 Widget s：

  public abstract class BaseWidgetProcessor< ;窗口小部件<指挥GT;> {
 public abstract COMMAND process（）; 
} 
  

使用具体的impls：

  public abstract class AstroWidgetProcessor< Widget< COMMAND>> {
 @Override 
 public COMMAND process（）{
 // ... 
} 
} 
  

etc。

问题是，使用 BaseWidgetProcessor ：

  public abstract class BaseWidgetProcessor< Widget< COMMAND>> {
 public abstract COMMAND process（）; 
} 
  

我得到一个语法错误：

令牌上的语法错误，放错位置的结构

当我将其更改为：

  public abstract class BaseWidgetProcessor< Widget> {
 public abstract COMMAND process（）; 
} 
  

错误消失了，但现在我得到一个编译器警告：

类型参数Widget隐藏Widget类型

这里发生了什么？！？为什么错误，然后各自的警告？什么是正确的（语法）设置方法？

解决方案

我认为你在寻找的是：

  public abstract class BaseWidgetProcessor< W extends Widget< C>，C> {
 public abstract C process（W widget）; 
} 
  

我添加了 W小部件作为参数来显示更好的用法。

现在假设您有一个名为CoolWidget的窗口小部件：

  public class CoolWidget扩展Widget< Integer> {bb} {/ code> 

要声明一个CoolWidgetProcessor，您必须这样做：

  public class CoolWidgetProcessor扩展BaseWidgetProcessor< CoolWidget，Integer> {
 @Override 
 public Integer process（CoolWidget widget）{
 // ... 
} 
} 
  




I have a generic class:

public class Widget<COMMAND> {
    // ...
}

So I can create instances of it like:

Widget<Fizz> fizzWidget = new Widget<Fizz>();

Then I have an ABC BaseWidgetProcessor, that I want accepting Widgets of any type:

public abstract class BaseWidgetProcessor<Widget<COMMAND>> {
    public abstract COMMAND process();
}

With concrete impls:

public abstract class AstroWidgetProcessor<Widget<COMMAND>> {
    @Override
    public COMMAND process() {
        // ...
    }
}

etc.

The problem is, with BaseWidgetProcessor:

public abstract class BaseWidgetProcessor<Widget<COMMAND>> {
    public abstract COMMAND process();
}

I get a syntax error:

Syntax error on token(s), misplaced construct(s)

When I change it to:

public abstract class BaseWidgetProcessor<Widget> {
    public abstract COMMAND process();
}

The error goes away, but now I get a compiler warning:

The type parameter Widget is hiding the type Widget

What is going on here?!? Why the error and then respective warning? What's the correct (syntactically) way to set this up?

解决方案

I think what you are looking for is:

public abstract class BaseWidgetProcessor<W extends Widget<C>, C> {
    public abstract C process(W widget);
}

I added W widget as argument to show a bit better the usage.

Now let's say that you have a widget named CoolWidget:

public class CoolWidget extends Widget<Integer> {}

To declare a CoolWidgetProcessor you would have to do:

public class CoolWidgetProcessor extends BaseWidgetProcessor<CoolWidget, Integer> {
    @Override
    public Integer process(CoolWidget widget) {
        // ...
    }
}

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