title: Secondary data type
body: <p>Consider the following statement:
  "A superclass provides a secondary data type to objects of its subclass"</p>
  
  <p>Similarly,
  "An interface provides a secondary data type to objects of classes that implement that interface."</p>
  
  <p>What is a secondary data type and what does it mean in these statements? An example would be much appreciated. Thanks. </p>