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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Javas 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