Overriding method with generic return type

Question

Let's say I have a super-class that defines the following abstract method

public abstract <T extends Interface> Class<T> getMainClass();

Now if I want to override it in some sub-class

public Class<Implementation> getMainClass(){
    return Implementation.class;
}

I get a warning about type safety and unchecked conversion:

Type safety: The return type Class<Implementation> for getMainClass() from the type SubFoo needs unchecked conversion to conform to Class<Interface> from the type SuperFoo

Doesn't Class<Implementation> fall under Class<T> if <T extends Interface>? Is there any way to properly get rid of the warning?

Answer 1

the overriding method's return type must be a subtype of the overridden method's return type.

Class<Impl> is not a subtype of Class<T> where <T extends Interface>. T is unknown here.

Class<Impl> is a subtype of Class<? extends Interface>, per subtyping rules.

---

some subtyping rules regarding wildcards:

for any type X

• A<X> is a subtype of A<? extends X>

• A<X> is a subtype of A<? super X>

if S is subtype of T

• A<? extends S> is a subtype of A<? extends T>

• A<? super T> is a subtype of A<? super S>

More concisely, ( <: means "is a subtype of" )

A<S>    <:    A<? extends S>    <:    A<? extends T>

A<T>    <:    A<?  super  T>    <:    A<?  super  S>

Answer 2

Consider the following scenario similar to yours:

public class SuperFoo {
     public abstract <T extends Interface> List<T> getList();
} 

public class SubFoo extends SuperFoo {
     private List<Implementation> l = new ArrayList<Implementation>();

     public List<Implementation> getList() {
          return l;
     }

     public void iterate() {
          for (Implementation i: l) ...;
     }
}

SubFoo subFoo = new SubFoo();
SuperFoo superFoo = subFoo;
superFoo.getList().add(new AnotherImplementation()); // Valid operation!
subFoo.iterate(); // Unexpected ClassCastException!

In this case unchecked conversion warning warns you about possibility of unexpected ClassCastException.

However, in your case, when return type is Class<...>, it's not a problem (as far as I understand), so you can legally suppress a warning:

@SuppressWarnings("unchecked")
public Class<Implementation> getMainClass(){ ... }  

Another option is to make SuperFoo itself generic:

public class SuperFoo<T extends Interface> {
    public abstract Class<T> getMainClass(); 
}

public class SubFoo extends SuperFoo<Implementation> {
    public Class<Implementation> getMainClass() { ... }
}

For yet another (and perhaps the best) option see Stas Kurilin's answer.

Answer 3

try this

public abstract Class<? extends Interface> getMainClass();

reorganized example by such warnings java tried prevents cases like this

class OtherImpl implements Interface{ 
} 
A a = new B();//where A - your abstract class and B - implementation
Class<OtherImpl> other = a.<OtherImpl>getMainClass();//some broken think, But _without_ runtime exception

As @axtavt mentioned example was broken. I reorganized it.