dschap02

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IAS1223 DATA STRUCTURES AND ALGORITHMS

Chapter 2

Inheritance and exception

handling

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Chapter Objectives

Data Structures Using Java

2

Learn about inheritance

Learn about sub- and superclasses

Explore how to override the methods of a superclass

Examine how constructors of super- and subclasseswork

Examine abstract classes

Learn about composition

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Chapter Objectives


3

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


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´is-aµ 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

Data Structures Using Java 5

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Inheritance


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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


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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


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Call to constructor of superclass

must be first statement

specified by: super parameter list

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The class Object


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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


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Cannot automatically make reference variable ofsubclass 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 classtype

ClassCastException thrown if class cast is not allowed

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The Operator instanceof


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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


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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


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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 abstractmethods of superclass

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Composition


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Another way to relate two classes

One or more members of a class are objects of

another class type

´has-aµ relation between classes

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Exception


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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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from java.util


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from java.io


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Java·s Exception Class


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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 separateclasses and contained in various packages

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The class Exception and its Constructors


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Java Exception Classes24


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Exceptions Thrown by Methods25


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Exceptions Thrown by Methods


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Checked Exceptions


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Checked Exception: any exception that can be

analyzed by the compiler

Example

IOExceptions

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Unchecked Exceptions


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Unchecked Exception: exception that cannot beanalyzed when the program compiles (must bechecked for by programmer)

Examples Division by zero

Array index out of hounds

Syntax

throws ExceptionType1, Exception

Type2, «

*ExceptionType1, ExceptionType2, etc are names of exception classes

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Handling Exceptions within a Program


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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


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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 ofexceptions

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


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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


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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


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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


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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


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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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Report


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Components: student, course

Operations on course

Set course information

Print course information

Show credit hours

Show course number

Show grade

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Report


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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


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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


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Composition

Exception

Checked and Unchecked Exceptions

Creating your own exception classes

Exception handling techniques

Handling exceptions within a program

dschap02

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