Abstract Superclasses & Concrete Classes

• In the Shape class, we referred to a draw method, even though its draw method never was invoked, only the draw methods overridden by its subclasses.
• In this case, the draw method must be specified in the Shape class, but it need not have any body; it is never actually used.
• Classes such as these are called abstract.
• To declare a class abstract, we insert the modifer abstract in the class declaration:

  public abstract class Shape {

  ...

  }

• Abstract classes cannot instantiate objects, they only can be subclassed.
• Their sole purpose is to provide an interface (and/or partial implementation) for an inheritance hierarchy of subclasses.

  The abstract Shape class ensures that each subclass implements a draw method.

(lest it too be an abstract class).

• If a class has a method whose body is undefined, the class is abstract, and must be declared as such.
• Classes that are not abstract are said to be concrete: they can instantiate objects.