Java covariance

Question

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?

Daniel Earwicker · Accepted Answer · 2009-07-26 11:26:02Z

18

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

answered Jul 26, 2009 at 11:26

Daniel Earwicker

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Comments

Brian Agnew · Accepted Answer · 2009-07-26 11:31:56Z

6

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.

edited Jul 26, 2009 at 11:31

Brian Agnew

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answered Jul 26, 2009 at 11:27

Gregory Mostizky

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4 Comments

Yngve Sneen Lindal Over a year ago

+ 1, but could you explain what "List or List but not List" means?! :p

Brian Agnew Over a year ago

Reformatted to remove the confusing 'List or List but not List' :-)

Rich Seller Over a year ago

The generics were being treated as html, escaped

Daniel Earwicker Over a year ago

I don't understand this answer - the OP doesn't want to treat it as a List<Animal>.

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