Signature of Collections.min/max method

Question

In Java, the Collections class contains the following method:

public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> c)

Its signature is well-known for its advanced use of generics, so much that it is mentioned in the Java in a Nutshell book and in the official Sun Generics Tutorial.

However, I could not find a convincing answer to the following question:

Why is the formal parameter of type Collection<? extends T>, rather than Collection<T>? What's the added benefit?

Answer 1

Type inference is a tricky topic that I'll admit that I don't know that much about. However, examine this example:

public class ScratchPad {
   private static class A implements Comparable<A> {
     public int compareTo(A o) { return 0; }
   }
   private static class B extends A {}
   private static class C extends B {}

   public static void main(String[] args)
   {
     Collection<C> coll = null;
     B b = Scratchpad.<B>min(coll);
   }

   public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> c)  {
     return null;
   }

   //public static <T extends Object & Comparable<? super T>> T min(Collection<T> c) {
   //  return null;
   //}
}

Consider that the first signature of min() allows the call to compile whereas the second does not. This isn't a very practical example, since one must ask why I would be explicitly typing the method to <B>, but perhaps there is an implicit inference where B would be the inferred type.