举例说明在 Java 中重载和覆盖的情况下协变和逆变的函数? [英] Give examples of functions which demonstrate covariance and contravariance in the cases of both overloading and overriding in Java?

问题描述

请举例说明 Java 中的协变和逆变.

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(但满足 Super.getSomething() 的契约)

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

逆变

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

Sub#doSomething 是逆变的，因为它采用 Super#doSomething 参数的超类的参数(但是，同样，满足 Super#doSomething 的契约)

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:此示例不适用于 Java.Java 编译器将重载而不覆盖 doSomething() 方法.其他语言确实支持这种逆变风格.

泛型

这也适用于泛型:

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

您现在可以访问 covariantList 的所有不带泛型参数的方法(因为它必须是扩展对象")，但是 getter 可以正常工作(因为返回的对象将始终是对象"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 的情况正好相反: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)，但不能使用 getter (返回的类型可能是 String 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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