272: Highlights

• Class hierarchies reflect subclass–superclass relations among classes.
• You have several reasons to arrange classes in hierarchies:
  • To parallel natural categories
  • To prevent avoidable duplication and to simplify maintenance
  • To avoid introducing bugs into previously debugged code
  • To use purchased code
• When a subclass–superclass relation is direct, with no intervening classes, the subclass is called the direct subclass, and the superclass is called the direct superclass.
• A class inherits public instance variables and public instance methods from all its superclasses.
• When a subclass–superclass chain contains multiple instance methods with the same name, argument number, and argument types, the one closest to the class of the target instance in the subclass–superclass chain is the one executed. All others are shadowed.
• If you want to create a class hierarchy, then draw a diagram that reflects natural categories. Populate the classes in that class hierarchy with public instance variables and public instance methods such that there is no needless duplication of an instance variable or instance method, and such that each public instance variable and public instance method is useful to instances of every subclass.
• If you want to create a direct-subclass–direct-superclass relation, then instantiate the following pattern:

  public class subclass name extends superclass name { 
   ... 
  }