Hello Java experts

I am learning Java 7 generics, reading Cay Horstmann, Core Java7, Volume I, on page 716.

I dont understand why the run time error (cast illegal) occurs, please see code below.

Can anyone explain it to me better than Cay does?

Thanks!

-------------------------------------

public class ProcessArgs {

public static <T extends Comparable> T[] minmax(T... a)
{
  Object[] mm = new Object[2];
 
  mm[0] = a[0];
  mm[1] = a[1];
  if (mm[0] instanceof Comparable)
  {
   System.out.println("Comparable"); // this is True, prints Comparable at run-time
  }
  return (T[]) mm;  // run-time error here

  /* Run-Time ERROR as below:
   ComparableException in thread "main"
   java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable;
    at ProcessArgs.minmax(ProcessArgs.java:13)
    at ProcessArgs.main(ProcessArgs.java:18)
   */

}

public static void main(String[] args)
{
  String[] sa = minmax("Hello","World"); // ERROR, illegal cast
  System.out.println(sa[0] + sa[1]);
  Object o = "Hello World"; //works - if we comment out the method call to minmax above
  Comparable<String> s = (Comparable) o; // works
  Comparable s2 = (Comparable) o; // works
  System.out.println(s + " " + (String) s2); // works
  return;
}

}