Java Generics with wildcard

Question

There is any way to fix this situation (I have try to simplyfy the scenario as much as i could):

public class Test {

    public static void main(String[] args) {

        /*
         * HERE I would like to indicate that the CollectionGeneric can be of
         * something that extends Animal (but the constructor doesn't allow
         * wildcards)
         */
        CollectionGeneric<? extends Animal> animalsCollectionGeneric = new CollectionGeneric<Animal>();
        List<? extends Animal> animals = getAnimals();
        /* Why I cannt do that? */
        animalsCollectionGeneric.setBeans(animals);
    }

    private static List<? extends Animal> getAnimals() {
        return new ArrayList<Dog>();
    }
}

class CollectionGeneric<T> {
    private List<T> beans;

    public List<T> getBeans() {
        return (beans != null) ? beans : new ArrayList<T>();
    }

    public void setBeans(List<T> beans) {
        this.beans = beans;
    }
}

interface Animal {}

class Dog implements Animal{}

this scenario is giving me the next error:

The method setBeans(List<capture#2-of ? extends Animal>) in the type    
CollectionGeneric<capture#2-of ? extends Animal> is not applicable for
the arguments (List<capture#3-of ? extends Animal>)*

I am not sure about if there is a way to do this with generics,

Out of interest, why would you use CollectionGeneric and not just use the List? — Nick Holt
– Nick Holt, Commented Jul 10, 2009 at 11:09
@Nick Holt: As I said at the beginning this a really simplyfied version of my problem, trust me, I have to use that CollectionGeneric class — David Santamaria
– David Santamaria, Commented Jul 10, 2009 at 11:13

Robert Munteanu · Accepted Answer · 2009-07-10 11:15:14Z

7

What this means is that the two collections can not be proved to have the same type bounds:

    CollectionGeneric<? extends Animal> animalsCollectionGeneric = 
             new CollectionGeneric<Animal>(); 
    List<? extends Animal> animals = getAnimals()

The first one might at runtime have CollectionGeneric<Tiger> and the second one List<Gnu>. Mixing those would mean you lose the type safety ( not to mention the carnage ).

Therefore you need to prove to the compiler that those two are related, so your generic signatures should be:

public void setBeans(List<? extends T> beans) {}
public List<T> getBeans();

and used as:

List<? extends Animal> beans = getBeans();
GenericCollection<Animal> animals = new GenericCollection<Animal>();
animals.add(beans);

answered Jul 10, 2009 at 11:15

Robert Munteanu

68.6k40 gold badges211 silver badges283 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

NamshubWriter Over a year ago

Good answer. Note that if you wanted to add an addBeans() method, it could simply take an object of type T. Josh Bloch gave a talk that went into generics and wildcards at Google I/O. See sites.google.com/site/io/effective-java-reloaded

Collectives™ on Stack Overflow

Java Generics with wildcard

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related