Java Generics with wildcard

问题:

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,

回答1:

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);