Data Structures Using Java1 Chapter 2 Inheritance and Exception Handling.

Data Structures Using Java 1

Chapter 2

Inheritance and Exception Handling


Chapter Objectives

• Learn about inheritance• Learn about sub- and superclasses• Explore how to override the methods of a

superclass• Examine how constructors of super- and

subclasses work• Examine abstract classes• Learn about composition


Chapter Objectives

• Learn about exceptions• Become aware of exception classes and their

hierarchy • Learn about checked and unchecked exceptions• Learn how to handle exceptions within a program• Examine try/catch blocks• Discover how to throw and rethrow an exception


Inheritance

• “is-a” relationship• Single inheritance

– subclass derived from one existing class (superclass)

• Multiple inheritance– Subclass derived from more than one superclass

– Not supported by Java

– In Java, a class can only extend definition of one class


Inheritance


Inheritance

• Private members of superclass– private to superclass; cannot be accessed

directly by subclass

• Subclass can override method of superclass; redefinition applies only to object of subclass


Inheritance

• To write method definition of subclass to specify call to public method of superclass– If subclass overrides public method of superclass, to

specify call to public method of superclass: super.MethodName(parameter list)

– If subclass does not override public method of superclass, to specify call to public method of superclass: MethodName(parameter list)


Defining Constructor of Subclass

• Call to constructor of superclass – must be first statement– specified by: super parameter list


The class Object

• Directly or indirectly superclass of every class in Java

• Public members of class Object can be overridden/invoked by object of any class type


The class Object


Objects of Superclasses and Subclasses

• Cannot automatically make reference variable of subclass type point to object of superclass

• Dynamic binding: method executed determined at execution time, not compile time

• Operator instanceof : determines whether reference variable that points to object is of particular class type

• ClassCastException thrown if class cast is not allowed


The Operator instanceof

• Reference of superclass type can point to objects of its subclass

• Can determine if a reference variable points to an object using operator instanceof


Abstract Methods and Classes

• Abstract method: method that has only the heading with no body– must be declared abstract

• Abstract class: class that is declared with the

reserved word abstract in its heading


Abstract Class

• Can contain instance variables, constructors, finalizer, abstract and nonabstract methods

• Cannot instantiate object of abstract class type; can only declare reference variable

• Can instantiate object of subclass of abstract class, but only if subclass gives definitions of all abstract methods of superclass


Composition

• Another way to relate two classes

• One or more members of a class are objects of another class type

• “has-a” relation between classes


Exception

• Definition: an occurrence of an undesirable situation that can be detected during program execution

• Examples– division by zero– trying to open an input file that does not exist is

an exception– an array index that goes out of bounds


Java Exception Hierarchy


The class Throwable


The class Exception and its Subclasses from java.lang


The class Exception and its Subclasses from java.util


The class Exception and its Subclasses from java.io


Java’s Exception Class• Class Exception

– Subclass of class Throwable – superclass of classes designed to handle exceptions

• Various types of exceptions– I/O exceptions– Number format exceptions– File not found exceptions– Array index out of bounds exception

• Various exceptions categorized into separate classes and contained in various packages


The class Exception and its Constructors


Java Exception Classes


Exceptions Thrown by Methods


Checked Exceptions

• Checked Exception: any exception that can be analyzed by the compiler

• Example– IOExceptions


Unchecked Exceptions• Unchecked Exception: exception that cannot be

analyzed when the program compiles (must be checked for by programmer)

• Examples– Division by zero– Array index out of hounds

• Syntax– throws ExceptionType1, ExceptionType2, …

*ExceptionType1, ExceptionType2, etc are names of exception classes


Handling Exceptions within a Program

• try/catch/finally block used to handle exceptions within a program

• try block:– Includes statements that may generate an exception

– Includes statements that should not be executed if an exception occurs

– followed by zero or more catch blocks

– May or may not be followed by finally block


Handling Exceptions within a Program

• Catch block:– heading specifies type of exception it can catch– contains exception handler; completely handles

exception– can catch all exceptions of a specific type or all types of

exceptions– may or may not be followed by a finally block

• Finally block– Code contained in this block always executes– try block with no catch block has finally block


Order of Catch Blocks

• If exception in try block caught by first catch block, reaming catch blocks ignored

• Must be careful about order catch blocks are listed


Rethrowing and Throwing an Exception

• Useful when – Catch block catches exception but is unable to

handle it– Catch block decides exception should be

handled by calling environment

• Allows programmer to provide exception handling code in one place


Exception Handling Techniques

• Terminate program– Output appropriate error message upon termination

• Fix error and continue– Repeatedly get user input/output appropriate error

message until valid value is entered

• Log error and continue– Write error messages to file and continue with program

execution


Creating Your Own Exception Classes

• Exception class you define extends class Exception or one of its subclasses

• Syntax to throw your own exception object– throw new ExceptionClassName(messageString);


Programming Example: Grade Report

• Main algorithm– Declare variables– Open input file– Open output file– Get number students registered and tuition rate– Load students’ data– Print grade reports



• Components: student, course

• Operations on course– Set course information– Print course information– Show credit hours– Show course number– Show grade



• Operations on student– Set student information– Print student information– Calculate number of credit hours taken– Calculate GPA– Calculate billing amount– Sort the courses according to the course number


Chapter Summary

• Inheritance– Single and multiple– Rules and Uses– Superclasses/subclasses (objects)– Overriding/overloading methods

• The class Object– Constructors– Rules

• Abstract methods and classes


Chapter Summary

• Composition

• Exception

• Checked and Unchecked Exceptions

• Creating your own exception classes

• Exception handling techniques

• Handling exceptions within a program

Data Structures Using Java1 Chapter 2 Inheritance and Exception Handling.

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