Class hierarchies

• Classes may be arranged in a hierarchy, with different classes in superclass-subclass relations.
• To create a class that is a subclass of another class, you use the identifier extends.
  class subclass-name extends superclass-name {
  class-definition
  }
• A subclass inherits the variables and methods of its superclass (unless they are declared private). Examples:
  • Since the definition for Thing includes a variable color and since Critter is a subclass of Animal, which is a subclass of Thing, all Critters (and all instances of subclasses of Critter) have a color.
  • Since Thing defines the method getCell(), all instances of subclasses of Thing, including Critter, can call this method.
• A method definition in a superclass is overridden by a definition for the same method (same name and same number and types of parameters) in a subclass. An instance of the subclass calling the method uses the definition that is lowest in the hierarchy of classes.
• A method definition may call a method in a superclass using the keyword super. For example, in the Animal class the definition for destroy overrides the definition in its superclass Thing by first doing what destroy in Thing does and then doing something additional.

  public void destroy() {
     super.destroy();
     closeMouth();
  }
  

• An abstract class, like Thing, is a class which cannot be instantiated directly. It must have at least one subclass which extends it and which is not abstract; the subclass may be instantiated directly. Even though an abstract class can never be instantiated directly, it can be used as a data type. For example, in Smarts, the Cell class has an instance variable contents which is some kind of Thing:
  private Thing contents;
• An abstract class may include abstract methods. An abstract method says nothing about what it does, but it must be overridden in any non-abstract class which extends the abstract class. An abstract class may also include non-abstract methods. For example, the abstract class Animal has an abstract method step which subclasses of Animal, that is, Predator and Critter, must define.

      public abstract void step();
  

  Note that an abstract method declaration has no body.
• Each Java class can have only one parent class; that is, Java does not implement multiple inheritance.

© 2004. Indiana University and Michael Gasser. www.indiana.edu/~gasser/Q530/Notes/hierarchy.html 11 Oct 2004