Chapter 8 Additional Inheritance Concepts and Techniques

Chapter 8 - Additional Inheritance Concepts and Techniques 1

Chapter 8

Additional Inheritance Concepts and Techniques


Chapter 8 Topics

• Require a subclass to override a superclass method by including an abstract method in the superclass

• Creating a Java interface and using an interface to require a class to implement methods

• Creating and using custom exceptions that provide detailed information about an error by extending the Exception class

• How all Java classes implicitly extend the Object class


Introducing the Initial Lease Class

• Bradshaw Marina Case Study– Includes three

generalization/specialization hierarchies that require inheritance

• Boat (Sail, Power), Slip (Covered), Lease (Annual, Daily)

– Lease hierarchy• Introduces additional inheritance concepts

– Introduces problem domain classes that contain attributes that hold references to other objects


Adding an Abstract Method to Lease

• Abstract Method (Figure 8-5)– Method without any statements– Requires that a subclass include or

implement the method– If a class has an abstract method:

• The class must also be abstract

– Keyword• abstract


Implementing the AnnualLease Subclass

• AnnualLease Class (Figure 8-6)– Extends Lease class– Adds attributes– Constructors and methods more

complex– Imports java.util package for Date and

Calendar classes• Both subclass and superclass must do this


Implementing the DailyLease Subclass

• DailyLease Class (Figure 8-7)– Extends Lease class– Adds attribute– Constructors and methods more

complex– Imports java.util package for Date and

Calendar classes• Both subclass and superclass must do this


Testing the AnnualLease and DailyLease Classes

• TesterTwo Class (Figure 8-8)– Tests AnnualLease and DailyLease

classes• Instantiates objects of both classes• Instantiates date objects• Retrieves information about each lease

object


Understanding and Using Java Interfaces

• Two methods to require that class override a method:– Add an abstract method to the superclass

that the class derives from– Define an interface that the class

implements• Interface

– Java component that defines abstract methods and constants that must be included by implementing classes



• Concept– Knowing how to use something means

knowing how to interface with it– The methods an instance can respond to

can be defined as the interface to the instance

– To assure instance has a defined set of methods:• Define an interface and have class implement it



• Component-based development– System components interact using well-

defined interface built from a variety of technologies• If interface is known can be used in

system• Do not need to know technology used or

internal structure of component



• Multiple inheritance– Ability to “inherit” from more than one

class– Java allows only single inheritance

• Using extends keyword

– Interfaces allow Java an alternative mechanism to emulate the concept of multiple inheritance



• Creating a Java Interface– Created much the same way as a class

• Header– Use interface keyword:

» public interface LeaseInterface

• Abstract methods

– Implement using implements keyword:• public class AnnualLease extends Lease

implements LeaseInterface



• Implementing More Than One Interface– Java classes can implement more than

one interface• Use comma separated list:

– Interfaces can include static final variables (constants) – Figure 8-11• All classes that implement the interface have

the constants available

– DailyLease in Figure 8-12



• Testing the Complete Interface Example– TesterThree class (Figure 8-13)

• Tests:– AnnualLease with one interface– DailyLease with two interfaces


Using Custom Exceptions

• Custom exception– Exception written specifically for a

particular application– Extends a built-in class

• Any class that is not declared final can be extended



• Defining the LeasePaymentException (Fig. 8-15)– Extend custom exception from Exception class– Add appropriate components:

• Attributes• Constructors• Methods

– Sequence diagram• Shows interaction between the

LeasePaymentException class and the AnnualLease class



• Throwing a Custom Exception– AnnualLease class (Figure 8-17)

• Uses LeasePaymentException class– Use throws keyword in method declaration– Use throw keyword in method body



• Testing the LeasePaymentException – TesterFour class

• Tests:– Revised AnnualLease class– LeasePaymentException class

– Sequence diagram• Shows interactions for TesterFour class



• Handling Payments as a Batch– Batch processing

• Takes a collection of transactions and processes them one after the other

– If successful:» Success message is displayed

– If unsuccessful:» Exception message is displayed (but

processing continues)

• Useful in testing validation and exceptions


Understanding the Object Class and Inheritance

• All Java classes extend from Object class– Object class

• Defines basic functionality that any other class of objects need in Java

– All built-in class hierarchies have Object as common ancestor

– All custom problem domain classes extend Object as well (implicitly)

Chapter 8 Additional Inheritance Concepts and Techniques

Documents

Transcript of Chapter 8 Additional Inheritance Concepts and Techniques