Programming and Problem Solving with Java

(^366) | Inheritance, Polymorphism, and Scope

Summary

Object-oriented languages such as Java organize class data types into a hierarchy. At

the top of Java’s hierarchy is a class called Objectthat provides a few basic

operations. Using inheritance, other classes extend Objectand are said to be derived

from it. Derived classes inherit all of the publicand protectedfields and methods of

their superclass, except for its constructors. We must explore the entire inheritance

hierarchy for a class to determine its full interface.

Instance methods can override superclass instance methods, and class methods

and fields can hide superclass class methods and fields. Overriding and hiding

enable us to change the meaning of a method or field when extending a superclass

with a derived class. In this way, the derived class can retain the same form of inter-

face, but operate in a different manner.

Constructors are not inherited by derived classes. The first statement in a

constructor must be a call to a constructor for the superclass. Otherwise, Java will

automatically insert such a call to the default constructor for the superclass.

Java allows us to declare multiple methods with the same name as long as they

have different signatures. The method name is then said to be overloaded. The

signature consists of the method name plus the types of its parameters in a particu-

lar order. The return type and modifiers are not part of the signature. Java

determines which version of an overloaded method to call by examining the types in

the argument list and selecting the method with the matching parameter list.

Sometimes we need to access a method or field in a superclass that has been

overridden or hidden. We can use the superkeyword to refer to the superclass

version of a field or method instead of the local version. Similarly, a method can de-

fine a local variable or parameter with the same name as a field in the class; we can

then use thisto refer to the instance version instead of the local version.

Scope rules determine the range of code that has access to an identifier. Internal

scope rules specify where class members can be accessed within a class. In Java,

members can be used anywhere within a class, with two exceptions: Class variables

13.For instances of a class to be written on a file, the class must implement the

Serializableinterface.

14.When an instance of a class is read from a file, the object must be type cast

back into its original class. For this reason, the mainmethod of an application

that uses the method readObjectmust include ClassNotFoundExceptionin its

throwsclause.