[英]Java covariance
I'm having a hard time trying to figure this out. 我很难解决这个问题。 Say I have the following code:
说我有以下代码:
class Animal { }
class Mammal extends Animal { }
class Giraffe extends Mammal { }
...
public static List<? extends Mammal> getMammals() { return ...; }
...
public static void main(String[] args) {
List<Mammal> mammals = getMammals(); // compilation error
}
Why does the assignment result in a compilation error? 为什么分配会导致编译错误? The error is something like:
错误是这样的:
Type mismatch: cannot convert from List<capture#4-of ? extends Mammal> to List<Mammal>
According to my understanding of covariance, the getMammals()
method returns a list
that will always contain Mammal
objects so it should be assignable. 根据我对协方差的理解,
getMammals()
方法返回一个始终包含Mammal
对象的list
,因此它应该是可分配的。 What am I missing? 我想念什么?
Because getMammals
could return a List<Giraffe>
, and if that was convertable to List<Mammal>
then you'd be able to add a Zebra
to it. 因为
getMammals
可以返回List<Giraffe>
,并且如果可以将其转换为List<Mammal>
则可以向其中添加Zebra
。 You can't be allowed to add a Zebra
to a list of Giraffe
, can you? 您不可以将
Zebra
添加到Giraffe
列表中,可以吗?
class Zebra extends Mammal { }
List<Giraffe> giraffes = new List<Giraffe>();
List<Mammal> mammals = giraffes; // not allowed
mammals.add(new Zebra()); // would add a Zebra to a list of Giraffes
Well, it doesn't work like that unfortunately. 好吧,不幸的是,它不是那样工作的。
When you declare getMammals() to return List<? extends Mammal>
当您声明getMammals()返回
List<? extends Mammal>
List<? extends Mammal>
it means it can return List<Mammal>
or List<Giraffe>
but not List<Animal>
List<? extends Mammal>
这意味着它可以返回List<Mammal>
或List<Giraffe>
但不能返回List<Animal>
Yours main() should look like this: 您的main()应该如下所示:
public static void main(String[] args) {
List<? extends Mammal> mammals = getMammals();
Mammal mammal = mammals.get(0);
}
EDIT: Regarding covariance, that's what is usually meant by that: 编辑:关于协方差,这通常是指:
class Animal {
public Animal getAnimal() {return this;}
}
class Mammal extends Animal {
public Mammal getAnimal() {return this;}
}
class Giraffe extends Mammal {
public Giraffe getAnimal() {return this;}
}
As you can see it allows to overload return type of methods when you overriding the method. 如您所见,当您覆盖方法时,它允许重载方法的返回类型。
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