Thanks Vivien.. the K isn't a class, but the way to indicate a generic
type (so K could be an Integer, a String or any class you are using at
the moment).
However, at the end of the facts, in my case the K is always
equivalent to a String, so i decide to implement(?).. re-write
everything using the type String (I think that i could write a
comparator class that accept the generic K and then switch into the
right compare() method in the case this K were an Integer, a
Character, a String, etc.. etc...)

Thanks again Vivien

Ciao

Giordano wrote, quoted or indirectly quoted someone who said :

What do you mean by "implement Comparable in a generic way"?

Here's a prototypical use of Collections.sort(), with T = String,
output:

tangled: { first, last, always, never, maybe, }
 sorted: { always, first, last, maybe, never, }

<sscce name="SortOf.java">
package testit;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class SortOf
{
  private static final String [] TANGLED = new String[]
       { "first", "last", "always", "never", "maybe", };

  static <T> void show( List <T> list )
  {
    System.out.print( "{ " );
    for ( T t : list )
    {
        System.out.print( t );
        System.out.print( ", " );
    }
    System.out.println( "}" );
  }

  public void demonstrate()
  {
    List<String> strangs = new ArrayList <String> ();
    strangs.addAll( Arrays.asList( TANGLED ));

    System.out.print( "tangled: " );
    show( strangs );

    Collections.sort( strangs );

    System.out.print( " sorted: " );
    show( strangs );
  }

  public static void main( String[] args )
  {
    (new SortOf()).demonstrate();
  }
}
</sscce>