1. Data Structures Using Java1 Chapter 2 Inheritance and Exception Handling

2. Data Structures Using Java2 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

3. Data Structures Using Java3 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

4. Data Structures Using Java4 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

5. Data Structures Using Java5 Inheritance

6. Data Structures Using Java6 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

7. Data Structures Using Java7 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)

8. Data Structures Using Java8 Defining Constructor of Subclass Call to constructor of superclass –must be first statement –specified by: super parameter list

9. Data Structures Using Java9 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

10. Data Structures Using Java10 The class Object

11. Data Structures Using Java11 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

12. Data Structures Using Java12 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

13. Data Structures Using Java13 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

14. Data Structures Using Java14 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

15. Data Structures Using Java15 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

16. Data Structures Using Java16 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

17. Data Structures Using Java17 Java Exception Hierarchy

18. Data Structures Using Java18 The class Throwable

19. Data Structures Using Java19 The class Exception and its Subclasses from java.lang

20. Data Structures Using Java20 The class Exception and its Subclasses from java.util

21. Data Structures Using Java21 The class Exception and its Subclasses from java.io

22. Data Structures Using Java22 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

23. Data Structures Using Java23 The class Exception and its Constructors

24. Data Structures Using Java24 Java Exception Classes

25. Data Structures Using Java25 Exceptions Thrown by Methods

26. Data Structures Using Java26 Exceptions Thrown by Methods

27. Data Structures Using Java27 Checked Exceptions Checked Exception: any exception that can be analyzed by the compiler Example –IOExceptions

28. Data Structures Using Java28 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

29. Data Structures Using Java29 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

30. Data Structures Using Java30 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

31. Data Structures Using Java31 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

32. Data Structures Using Java32 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

33. Data Structures Using Java33 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

34. Data Structures Using Java34 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);

35. Data Structures Using Java35 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

36. Data Structures Using Java36 Programming Example: Grade Report Components: student, course Operations on course –Set course information –Print course information –Show credit hours –Show course number –Show grade

37. Data Structures Using Java37 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

38. Data Structures Using Java38 Chapter Summary Inheritance –Single and multiple –Rules and Uses –Superclasses/subclasses (objects) –Overriding/overloading methods The class Object –Constructors –Rules Abstract methods and classes

39. Data Structures Using Java39 Chapter Summary Composition Exception Checked and Unchecked Exceptions Creating your own exception classes Exception handling techniques Handling exceptions within a program