title: Is it possible and meaningful to make the superclass abstract?
body: <p>I am told that: a class can extend a concrete superclass and override an implemented method to make it abstract. </p>
  
  <p>If the method becomes abstract, then the corresponding superclass should become abstract too.</p>
  
  <p>Is this correct? If so, is it meaningful to make the superclass or the method in superclass to become abstract?</p>