Java covariance | 易学教程

问题

I'm having a hard time trying to figure this out. Say I have the following code:

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? The error is something like:

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. What am I missing?

回答1:

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. You can't be allowed to add a Zebra to a list of Giraffe, can you?

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

回答2:

Well, it doesn't work like that unfortunately.

When you declare getMammals() to return List<? extends Mammal> it means it can return List<Mammal> or List<Giraffe> but not List<Animal>

Yours main() should look like this:

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:

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.

来源：https://stackoverflow.com/questions/1184295/java-covariance

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java

covariance