Java通用符与多个类 [英] Java Generics Wildcarding With Multiple Classes

问题描述

我想有一个Class对象，但我想强制它代表的任何类扩展类A和实现接口B.

我可以做： p>

 类< ;?扩展ClassA> 
  

或：

 code> Class< ;?扩展InterfaceB> 
  

但我不能这两者。有没有办法做这个？

解决方案

实际上，你可以如果你想提供多个接口或一个类加接口，你必须有你的通配符看起来像这样：

  T延伸ClassA& InterfaceB> 
  

请参阅泛型教程，特别是有界类型参数部分。如果你愿意，你可以列出多个接口，使用& InterfaceName 。

这可以随意复杂。要演示，请参阅 Collections＃max （包含两行）为：

  public static< T extends Object&可比较的超级T> T 
 max（Collection< ;? extends T> coll）
  

为什么这么复杂？正如Java通用常见问题解答中所述：保留二进制兼容性。 / p>

看起来这不适用于变量声明，但是当在类上放置一个通用边界时，它会工作。因此，要做你想要的，你可能需要跳过几个圈。但你可以做到。你可以这样做，把一个通用的边界放在你的类上，然后：

  class classB {} 
 interface interfaceC {} 
 
 public class MyClass< T extends classB& interfaceC> {
 Class< T>变量; 
} 
  

获取变量有你想要的限制。有关详情和示例，请查看泛型的第3页在Java 5.0 。注意，在< T extends B& C> ，类名必须先到，接口跟随。当然，你只能列出一个单一的类。

I want to have a Class object, but I want to force whatever class it represents to extend class A and implement interface B.

I can do:

Class<? extends ClassA>

Or:

Class<? extends InterfaceB>

but I can't do both. Is there a way to do this?

解决方案

Actually, you can do what you want. If you want to provide multiple interfaces or a class plus interfaces, you have to have your wildcard look something like this:

<T extends ClassA & InterfaceB>

See the Generics Tutorial at sun.com, specifically the Bounded Type Parameters section, at the bottom of the page. You can actually list more than one interface if you wish, using & InterfaceName for each one that you need.

This can get arbitrarily complicated. To demonstrate, see the JavaDoc declaration of Collections#max, which (wrapped onto two lines) is:

public static <T extends Object & Comparable<? super T>> T
                                           max(Collection<? extends T> coll)

why so complicated? As said in the Java Generics FAQ: To preserve binary compatibility.

It looks like this doesn't work for variable declaration, but it does work when putting a generic boundary on a class. Thus, to do what you want, you may have to jump through a few hoops. But you can do it. You can do something like this, putting a generic boundary on your class and then:

class classB { }
interface interfaceC { }

public class MyClass<T extends classB & interfaceC> {
    Class<T> variable;
}

to get variable that has the restriction that you want. For more information and examples, check out page 3 of Generics in Java 5.0. Note, in <T extends B & C>, the class name must come first, and interfaces follow. And of course you can only list a single class.

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