Always something new to learn

It seems that you can override a concrete method and make it abstract. I actually came across some code that did this, so I had to check how it worked. Consider the following:

abstract class Grandfather
{
   protected String speak ()
   {
      return ( "I'm old" );
   }
}

abstract class Father
   extends Grandfather
{
   @Override
   protected abstract String speak ();
}

public class Child
   extends Father
{
   @Override
   protected String speak ()
   {
      return ( "I'm young" );
   }

   public static void main ( final String [] args )
   {
      System.out.println ( new Child ().speak () );
   }
}

It prints "I'm young" (unsurprisingly) but the interesting part is that despite Grandfather defining a speak method, Father is forcing Child to re-implement it. Grandfather's speak method can't be called by Child because Father has made it abstract. I had to test that this worked by commenting out some of the speak methods. It really is the case that Grandfather has an implementation but Father doesn't.