Feedback needed on suggestion for enhancing Collections.unmodifiableList()

Question

What is the difference the following two method signatures?
public static <T> List<T> unmodifiableList ( List<? extends T> list )
public static <T, S extends T> List<T> unmodifiableList(final List<S> source)Isn't the latter a superset of the former? That is, it allows you to do the following:
List<Animal> animals = Collections.unmodifiableList(dogs);If the signature of Collections.unmodifiableList() were modified to this signature would it be backwards compatible?
Thanks,
Gili

Answer 1

Here is my test program:
import java.util.*;
public class TestSignature
  public class A {  }
  public class B extends A { }
  public static <T> List<T> unmodifiableList ( List<? extends T> list )
  {  return null;  }
  public static <T, S extends T> List<T> unmodifiableList2(List<S> source)
  {  return null;  }
  public static void main(String[] argv)
    List<? extends A> dogs = null;
    List<A> animals = unmodifiableList(dogs);
    List<A> animals2 = unmodifiableList2(dogs);
    List<B> cats = null;
    List<A> animals3 = unmodifiableList(cats);
    List<A> animals4 = unmodifiableList2(cats);
}Errors:
TestSignature.java:19: incompatible types
found   : java.util.List<capture#159 of ? extends TestSignature.A>
required: java.util.List<TestSignature.A>
    List<A> animals2 = unmodifiableList2(dogs);
                                        ^
TestSignature.java:22: incompatible types
found   : java.util.List<TestSignature.B>
required: java.util.List<TestSignature.A>
    List<A> animals3 = unmodifiableList(cats);
                                       ^
TestSignature.java:23: incompatible types
found   : java.util.List<TestSignature.B>
required: java.util.List<TestSignature.A>
    List<A> animals4 = unmodifiableList2(cats);
                                        ^
3 errors