给出一些函数示例，这些函数在Java中的重载和重载情况下表现出协方差和矛盾.

问题描述

请给出一个有关Java中协方差和逆方差的好例子.

Please show a good example for covariance and contravariance in Java.

推荐答案

协方差:

class Super {
  Object getSomething(){}
}
class Sub extends Super {
  String getSomething() {}
}

Sub#getSomething是协变的，因为它返回Super#getSomething的返回类型的子类(但完全履行了Super.getSomething()的约定)

Sub#getSomething is covariant because it returns a subclass of the return type of Super#getSomething (but fullfills the contract of Super.getSomething())

矛盾性

class Super{
  void doSomething(String parameter)
}
class Sub extends Super{
  void doSomething(Object parameter)
}

Sub#doSomething是互变的，因为它采用了Super#doSomething的参数的超类的参数(但同样，它满足了Super#doSomething的约定)

Sub#doSomething is contravariant because it takes a parameter of a superclass of the parameter of Super#doSomething (but, again, fullfills the contract of Super#doSomething)

注意:此示例在Java中不起作用. Java编译器将重载并且不会覆盖doSomething()-Method.其他语言也支持这种类型的逆变.

泛型

这对于泛型也是可能的:

This is also possible for Generics:

List<String> aList...
List<? extends Object> covariantList = aList;
List<? super String> contravariantList = aList;

现在，您可以访问所有不带通用参数的covariantList方法(因为必须是扩展对象")，但是getter可以正常工作(作为返回的对象)将始终为对象"类型)

You can now access all methods of covariantList that doesn't take a generic parameter (as it must be something "extends Object"), but getters will work fine (as the returned object will always be of type "Object")

contravariantList的情况相反:您可以使用泛型参数访问所有方法(您知道它必须是"String"的超类，因此您始终可以传递一个)，但是没有吸气剂(返回的类型可能是字符串的任何其他超类型)

The opposite is true for contravariantList: You can access all methods with generic parameters (you know it must be a superclass of "String", so you can always pass one), but no getters (The returned type may be of any other supertype of String)

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