Inheritance and polymorphism are powerful language features to develop extensible and modifiable code.
Inheritance mechanism is used to share common code among the related classes.
Inheritance is different from the Java interface, which is used to share common behavior among unrelated classes.
The third visibility modifier is the protected modifier.
If no instances are created from a superclass, then define the superclass as an abstract class.
Polymorphic messages tell us that the method executed in response to the message will vary according to the class to which the object belongs.
The first statement in a constructor of a subclass must be a call to a constructor of the superclass. If the required statement is not made explicitly, then the statement to call the default constructor of the superclass is inserted automatically by the Java compiler.
The standard class described or used in this chapter is StringTokenizer.
