多个类的 Java 泛型通配符
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Java Generics Wildcarding With Multiple Classes
提问by Alex Beardsley
I want to have a Class object, but I want to force whatever class it represents to extend class A and implement interface B.
我想要一个 Class 对象，但我想强制它代表的任何类扩展类 A 并实现接口 B。
I can do:
我可以：
Class<? extends ClassA>
Or:
或者：
Class<? extends InterfaceB>
but I can't do both. Is there a way to do this?
但我不能两者兼得。有没有办法做到这一点？
采纳答案by Eddie
Actually, you cando 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:
其实，你可以为所欲为。如果你想提供多个接口或一个类加接口，你必须让你的通配符看起来像这样：
<T extends ClassA & InterfaceB>
See the Generics Tutorialat sun.com, specifically the Bounded Type Parameterssection, at the bottom of the page. You can actually list more than one interface if you wish, using & InterfaceNamefor each one that you need.
请参阅sun.com 上的泛型教程，特别是页面底部的有界类型参数部分。如果您愿意，您实际上可以列出多个接口，& InterfaceName为您需要的每个接口使用。
This can get arbitrarily complicated. To demonstrate, see the JavaDoc declaration of Collections#max, which (wrapped onto two lines) is:
这可以变得任意复杂。为了演示，请参阅 的 JavaDoc 声明Collections#max，其中（包含在两行中）是：
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.
为什么这么复杂？正如 Java 泛型常见问题解答中所说：为了保持二进制兼容性。
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:
看起来这不适用于变量声明，但在类上放置通用边界时确实有效。因此，要做您想做的事，您可能需要跳过几个环节。但是你可以做到。你可以做这样的事情，在你的类上放置一个通用边界，然后：
class classB { }
interface interfaceC { }
public class MyClass<T extends classB & interfaceC> {
Class<T> variable;
}
to get variablethat 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.
得到variable你想要的限制。有关更多信息和示例，请查看Java 5.0 中的泛型的第 3 页。注意，在 中<T extends B & C>，类名必须在前，接口在后。当然，您只能列出一个类。
回答by user2861738
You can't do it with "anonymous" type parameters (ie, wildcards that use ?), but you can do it with "named" type parameters. Simply declare the type parameter at method or class level.
您不能使用“匿名”类型参数（即使用 的通配符?）来执行此操作，但可以使用“命名”类型参数来执行此操作。只需在方法或类级别声明类型参数。
import java.util.List;
interface A{}
interface B{}
public class Test<E extends B & A, T extends List<E>> {
T t;
}
