[英]Java wildcarding with multiple types in variable
I know it is possible to wildcard with multiple types in case of methods and classes but what about variables? 我知道在方法和类的情况下可以通配多种类型,但是变量呢? Eg can I require an ArrayList to only take elements that implement both of two different interfaces that are not in the same type hierarchy?
例如,我是否可以要求ArrayList仅接受实现两个不在同一类型层次结构中的两个不同接口的元素? See code below for what I am trying to do.
请参阅下面的代码以了解我要执行的操作。
import java.util.ArrayList;
interface A {}
interface B{}
interface AB extends A, B{}
class D <T extends A & B> { //This works but this is a class
T variable;
}
public class C {
ArrayList<AB> myList1 = new ArrayList<AB>(); // compiles
ArrayList<? extends AB> myList3 = new ArrayList<AB>(); //compiles
//The following does not compile.
ArrayList<? extends A & B> myList4 = new ArrayList<AB>();
//This works but this is a method:
public static <T extends A & B> T someMethod(ArrayList<? extends T> list) {
return null;
}
}
Yes: 是:
class C<T extends A & B> {
ArrayList<T> field;
public <T2 extends A & B> void method() {
ArrayList<T2> localVar;
}
}
It looks a bit odd that you have to define an alias for the generic "type" outside of the scope where you use it but it works. 您必须在使用它的范围之外为通用“类型”定义一个别名,这看起来有些奇怪,但是它可以工作。 I find it's easiest to see with the method:
T2
is never used in the method declaration. 我发现用该方法最容易看到:方法声明中从未使用过
T2
。 It's solely used inside of the method. 它仅在方法内部使用。
It's unfortunate that these inner type limits become part of the public API this way but that's the only way I know to make this work. 不幸的是,这些内部类型限制以这种方式成为了公共API的一部分,但这是我知道实现此工作的唯一方法。
If you don't want this, then try an inner, private interface ( AB
) in your example. 如果您不希望这样做,请在您的示例中尝试一个内部专用接口(
AB
)。
Java does not support multiple inheritance of classes - a class can extend only one single class (although a class can implement multiple interfaces). Java不支持类的多重继承-一个类只能扩展一个类(尽管一个类可以实现多个接口)。 Hence I believe the
因此,我相信
ArrayList<? extends A & B>
doesn't work. 不起作用。
Solution would be create a super type that extends both A & B and then pass the type to your method. 解决方案是创建一个同时扩展A和B的超级类型,然后将该类型传递给您的方法。 Something like
就像是
inteface C extends A,B{}
ArrayList<? extends C>
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