Most confusing generic error

[Pat Farrell]

I thought I understood Java Generics.... I can't figure out what the compiler is complaining about in a recent code snippet that I wrote.

I've got a class that wants a class as its argument.

  
public class ContainedPboList<T extends PatentElement, V extends PatentBase> implements Serializable {
public ContainedPboList(Class<T> elementType) {
}

In the sample below, the "Inventor" class implements PatentBase. So the result of the two calls to the constructor are identical, to my eyes. But the compiler thinks differently.

        Inventor wizard = new Inventor();
        ContainedPboList<Inventor, Patent> instance = null;
        instance = new ContainedPboList<Inventor, Patent>(wizard.getClass());  // compiler complains
        instance = new ContainedPboList<Inventor, Patent>(Inventor.class);      // compiler is happy

The specific error from the compiler is:

/Users/pfarrell/sandbox/wayserver/wayfinderlib/test/com/wayfinderdigital/struct/ContainedPboListTest.java:67: cannot find symbol
symbol : constructor ContainedPboList(java.lang.Class<capture#480 of ? extends com.wayfinderdigital.busobj.Inventor>)
location: class com.wayfinderdigital.struct.ContainedPboList<com.wayfinderdigital.busobj.Inventor,com.wayfinderdigital.busobj.Patent>
instance = new ContainedPboList<Inventor, Patent>(wizard.getClass());
1 error

I think that lines 3 and 4 are functionally identical in the second code segment. How is "Inventor.class" different from "wizard.getClass()" when the wizard is an Inventor?

[James Sabre]

      public ContainedPboList(Class<? extends T> elementType)
        {
        }

[Stephan van Hulst]

To further clarify, wizard.getClass() returns Class<? extends Inventor>, whereas Inventor.class returns Class<Inventor>.

[Pat Farrell]

Thanks for the quick replies, but I'm not understanding this.

public class Inventor extends Human implements PatentElement {

We have the constructor argument taking <T extends PatentElement>
So Inventor matches the generic template.

Inventor instanceof PatentElement is true.

[Stephan van Hulst]

Yes, but generics are invariant. Your constructor requires a Class<Inventor>, while you're trying to provide it a Class<? extends Inventor>. Check out the documentation for Object.getClass().

Class<? extends T> is *not* a subtype of Class<T>.

Let's compare it to the cage example:

Cage<Lion> lionCage = new Cage<Lion>();
Cage<? extends Animal> someCage = lionCage // legal, Cage<Lion> is a subtype of Cage<? extends Lion>;
Cage<Animal> animalCage = someCage;        // illegal, Cage<? extends Animal> is not a subtype of Cage<Animal>
animalCage.put(new ButterFly());           // legal, because of the Liskov substitution principle
Lion lion = lionCage.get();                // Class cast exception!

If line 3 were allowed, we would be able to add butterflies to a cage that was designed for lions only.