在Java泛型中如何使用逆变？ [英] How would contravariance be used in Java generics?

问题描述

在Java中，协方差允许API设计者指定一个实例可以被推广为某种类型或任何类型的子类型。例如：

In Java, covariance allows the API designer to specify that an instance may be generalised as a certain type or any of that type's subtypes. For example:

List<? extends Shape> shapes = new ArrayList<Circle>(); 
// where type Circle extends Shape

反向变换是另一种方式。它允许我们指定一个实例可以泛化为某种类型或超类型。

Contravariance goes the other way. It allows us to specify that an instance may be generalised as a certain type or supertype.

List<? super Shape> shapes = new ArrayList<Geometry>();
// where Shape extends Geometry

Java泛型的逆变是如何有用的？你什么时候选择使用它？

How is Java generic's contravariance useful? When would you choose to use it?

推荐答案

好吧，你的第二个例子可以让你编写：

Well, your second example would allow you to write:

Shape shape = getShapeFromSomewhere();
shapes.add(shape);

而第一种形式则无法做到这一点。它不像协变性那样有用，我会授予你的。

whereas you couldn't do that with the first form. It's not useful as often as covariance, I'll grant you.

可以用 的一个区域用于比较。例如，考虑：

One area where it can be useful is in terms of comparisons. For example, consider:

class AreaComparer implements Comparator<Shape>
...

您可以使用它来比较任何两种形状......因此，如果我们还可以 使用它对 List< Circle> 进行排序，那将会很好。幸运的是，我们可以通过逆变来做到这一点，这就是为什么 Collections.sort 的重载：

You can use that to compare any two shapes... so it would be nice if we could also use it to sort a List<Circle> for example. Fortunately, we can do that with contravariance, which is why there's an overload for Collections.sort of:

public static <T> void sort(List<T> list, Comparator<? super T> c)

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