summary of Key Concepts
■ Inheritance is the process of deriving a new class from an existing one.
■ One purpose of inheritance is to reuse existing software.
■ Inheritance creates an is-a relationship between the parent and child classes.
■ Protected visibility provides the best possible encapsulation that permits inheritance.
■ A parent’s constructor can be invoked using the super reference.
■ A child class can override (redefine) the parent’s definition of an inherited
method.
■ The child of one class can be the parent of one or more other classes, cre- ating a class hierarchy.
■ Common features should be located as high in a class hierarchy as is rea- sonably possible.
■ All Java classes are derived, directly or indirectly, from the Object class.
■ The toString and equals methods are inherited by every class in every
Java program.
■ An abstract class cannot be instantiated. It represents a concept on which other classes can build their definitions.
■ A class derived from an abstract parent must override all of its parent’s abstract methods, or the derived class will also be considered abstract.
■ Inheritance can be applied to interfaces so that one interface can be derived from another.
■ Private members are inherited by the child class, but cannot be referenced directly by name. They may be used indirectly, however.
■ Software design must carefully and specifically address inheritance.
■ The final modifier can be used to restrict inheritance.
■ The classes that represent Java GUI components are organized into a class hierarchy.
■ A listener class can be created by deriving it from an event adapter class.
■ A Timer object generates action events at regular intervals and can be used to control an animation.