Java类型参数是隐藏一个泛型？

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：

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：

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:

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:

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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