Data Structures Using Java1 Chapter 2 Inheritance and Exception Handling.

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Chapter 2

Inheritance and Exception Handling

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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

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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

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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

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Inheritance

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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

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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)

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Defining Constructor of Subclass

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

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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

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The class Object

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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

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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

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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

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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

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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

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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

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Java Exception Hierarchy

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The class Throwable

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The class Exception and its Subclasses from java.lang

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The class Exception and its Subclasses from java.util

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The class Exception and its Subclasses from java.io

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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

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The class Exception and its Constructors

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Java Exception Classes

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Exceptions Thrown by Methods

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Exceptions Thrown by Methods

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Checked Exceptions

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

• Example– IOExceptions

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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

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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

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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

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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

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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

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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

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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);

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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

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Programming Example: Grade Report

• Components: student, course

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

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Programming Example: Grade Report

• 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

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Chapter Summary

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

• The class Object– Constructors– Rules

• Abstract methods and classes

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Chapter Summary

• Composition

• Exception

• Checked and Unchecked Exceptions

• Creating your own exception classes

• Exception handling techniques

• Handling exceptions within a program