Chapter 11: Handling Exceptions and Events

1. Chapter 11: Handling Exceptions and Events JavaProgramming: From Problem Analysis to Program Design, Fourth Edition

2. Objectives • Learn what an exception is. • See how a try/catch block is used to handle exceptions. • Become aware of the hierarchy of exception classes. • Learn about checked and unchecked exceptions. • Learn how to handle exceptions within a program. • Discover how to throw and rethrow an exception. • Learn how to handle events in a program. Java Programming: From Problem Analysis to Program Design, Second Edition

3. Exception • 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. • An array index that goes out of bounds. Java Programming: From Problem Analysis to Program Design, Second Edition

4. Handling Exceptions within a Program • Can use an if statement to handle an exception. • However, suppose that division by zero occurs in more than one place within the same block. • In this case, using ifstatements may not be the most effective way to handle the exception. Java Programming: From Problem Analysis to Program Design, Second Edition

5. Java’s Mechanism of Exception Handling • When an exception occurs, an object of a particular exception classis created. • Java provides a number of exception classes to effectively handle certain common exceptions, such as: • Division by zero • Invalid input • File not found Java Programming: From Problem Analysis to Program Design, Second Edition

6. Java’s Mechanism of Exception Handling • Division by zero is: • An arithmetic error. • Handled by the classArithmeticException. • When a division by zero exception occurs, the program creates an object of the classArithmeticException. Java Programming: From Problem Analysis to Program Design, Second Edition

7. Java’s Mechanism of Exception Handling • When a Scanner object is used to input data into a program, any invalid input errors are handled using the classInputMismatchException. • The classException (directly or indirectly) is the superclass of all the exception classes in Java. Java Programming: From Problem Analysis to Program Design, Second Edition

8. try/catch/finally Block • Statements that might generate an exception are placed in a tryblock. • The tryblock: • Might also contain statements that should not be executed if an exception occurs. • Is followed by zero or more catchblocks. • A catchblock: • Specifies the type of exception it can catch. • Contains an exception handler. Java Programming: From Problem Analysis to Program Design, Second Edition

9. try/catch/finallyBlock • The last catchblock may or may not be followed by a finallyblock. • Any code contained in a finallyblock always executes regardless of whether an exception occurs, except when the program exits early from a tryblock by calling the method System.exit. • If a tryblock has no catchblock, then it must have the finallyblock. Java Programming: From Problem Analysis to Program Design, Second Edition

10. try/catch/finallyBlock Java Programming: From Problem Analysis to Program Design, Second Edition

11. try/catch/finallyBlock • If no exception is thrown in a tryblock, all catchblocks associated with the tryblock are ignored and program execution resumes after the last catchblock. • If an exception is thrown in a tryblock, the remaining statements in the tryblock are ignored. • The program searches the catchblocks in the order in which they appear after the tryblock and looks for an appropriate exception handler. Java Programming: From Problem Analysis to Program Design, Second Edition

12. try/catch/finallyBlock • If the type of the thrown exception matches the parameter type in one of the catchblocks, the code of that catchblock executes and the remaining catchblocks are ignored. • If there is a finallyblock after the last catchblock, the finallyblock executes regardless of whether an exception occurs. Java Programming: From Problem Analysis to Program Design, Second Edition

13. Order of catch Blocks • The heading of a catchblock specifies the type of exception it handles. • A catchblock can catch either all exceptions of a specific type or all types of exceptions. • A reference variable of a superclass type can point to an object of its subclass. Java Programming: From Problem Analysis to Program Design, Second Edition

14. Order of catch Blocks • If you declare an exception using the classException in the heading of a catchblock, then that catchblock can catch all types of exceptions because the classException is the superclass of all exception classes. • In a sequence of catchblocks following a tryblock, a catchblock that declares an exception of a subclass type should be placed before catchblocks that declare exceptions of a superclass type. Java Programming: From Problem Analysis to Program Design, Second Edition

15. Order of catch Blocks Java Programming: From Problem Analysis to Program Design, Second Edition

16. Order of catch Blocks Java Programming: From Problem Analysis to Program Design, Second Edition

17. Order of catch Blocks Java Programming: From Problem Analysis to Program Design, Second Edition

18. Order of catch Blocks Java Programming: From Problem Analysis to Program Design, Second Edition

19. Order of catch Blocks Java Programming: From Problem Analysis to Program Design, Second Edition

20. Java’s Exception Class • classException: • Subclass of classThrowable. • 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 exceptions. • Various exceptions categorized into separate classes and contained in various packages. Java Programming: From Problem Analysis to Program Design, Second Edition

21. Java’s Exception Class Java Programming: From Problem Analysis to Program Design, Second Edition

22. Java’s Exception Class Java Programming: From Problem Analysis to Program Design, Second Edition

23. Java’s Exception Class Java Programming: From Problem Analysis to Program Design, Second Edition

24. Java’s Exception Class Java Programming: From Problem Analysis to Program Design, Second Edition

25. Java’s Exception Class Java Programming: From Problem Analysis to Program Design, Second Edition

26. Java’s Exception Class Java Programming: From Problem Analysis to Program Design, Second Edition

27. Checked Exceptions • Any exception that can be analyzed by the compiler. • Example: • FileNotFoundExceptions. Java Programming: From Problem Analysis to Program Design, Second Edition

28. Unchecked Exceptions • Exceptions that cannot be analyzed when the program compiles (must be checked for by programmer). • Examples: • Division by zero • Array index out of bounds • Syntax: throws ExceptionType1, ExceptionType2,... ExceptionType1, ExceptionType2, and so on are names of exception classes Java Programming: From Problem Analysis to Program Design, Second Edition

29. Exceptions Example Code public static void exceptionMethod() throws InputMismatchException, FileNotFoundException { //statements } • The method exceptionMethod throws exceptions of the type InputMismatchException and FileNotFoundException. Java Programming: From Problem Analysis to Program Design, Second Edition

30. The classException and the Operator instanceof • A reference of a superclass type can point to objects of its subclass. • You can determine if a reference variable points to an object using the operator instanceof. • You can combine catch blocks using this facility. Java Programming: From Problem Analysis to Program Design, Second Edition

31. The classException and the Operator instanceof try { System.out.print("Line 4: Enter the " + "dividend: "); dividend = console.nextInt(); System.out.println(); System.out.print("Line 7: Enter the " + "divisor: "); divisor = console.nextInt(); System.out.println(); quotient = dividend / divisor; System.out.println("Line 11: Quotient = " + quotient); } catch (Exception eRef) { if (eRef instanceof ArithmeticException) System.out.println("Line 14: Exception " + eRef.toString()); else if (eRef instanceof InputMismatchException) System.out.println("Line 16: Exception " + eRef.toString()); } Java Programming: From Problem Analysis to Program Design, Second Edition

32. Rethrowing and Throwing an Exception • When an exception occurs in a tryblock, control immediately passes to one of the catchblocks. • Typically, a catchblock does one of the following: • Completely handles the exception. • Partially processes the exception. • In this case, the catchblock either rethrows the same exception or throws another exception for the calling environment to handle the exception. • Rethrows the same exception for the calling environment to handle the exception. Java Programming: From Problem Analysis to Program Design, Second Edition

33. 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. • Syntax: throw exceptionReference; Java Programming: From Problem Analysis to Program Design, Second Edition

34. Rethrowing and Throwing an Exception import java.util.*; public class RethrowExceptionExmp1 { static Scanner console = new Scanner(System.in); public static void main(String[] args) { int number; try { number = getNumber(); System.out.println("Line 5: number = " + number); } catch (InputMismatchException imeRef) { System.out.println("Line 7: Exception " + imeRef.toString()); } } } Java Programming: From Problem Analysis to Program Design, Second Edition

35. Rethrowing and Throwing an Exception public static int getNumber() throws InputMismatchException { int num; try { System.out.print("Line 11: Enter an “ + "integer: "); num = console.nextInt(); System.out.println(); return num; } catch (InputMismatchException imeRef) { throw imeRef; } } } Java Programming: From Problem Analysis to Program Design, Second Edition

36. The Method printStackTrace • Used to determine the order in which the methods were called and where the exception was handled. Java Programming: From Problem Analysis to Program Design, Second Edition

37. The Method printStackTrace import java.io.*; public class PrintStackTraceExample1 { public static void main(String[] args) { try { methodA(); } catch (Exception e) { System.out.println(e.toString() + " caught in main"); e.printStackTrace(); } } Java Programming: From Problem Analysis to Program Design, Second Edition

38. The Method printStackTrace public static void methodA() throws Exception { methodB(); } public static void methodB() throws Exception { methodC(); } public static void methodC() throws Exception { thrownew Exception("Exception generated " + "in method C"); } } Java Programming: From Problem Analysis to Program Design, Second Edition

39. The Method printStackTrace Sample Run: java.lang.Exception: Exception generated in method C caught in main java.lang.Exception: Exception generated in method C at PrintStackTraceExample1.methodC (PrintStackTraceExample1.java:31) at PrintStackTraceExample1.methodB (PrintStackTraceExample1.java:26) at PrintStackTraceExample1.methodA (PrintStackTraceExample1.java:22) at PrintStackTraceExample1.main (PrintStackTraceExample1.java:11) Java Programming: From Problem Analysis to Program Design, Second Edition

40. 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. Java Programming: From Problem Analysis to Program Design, Second Edition

41. Creating Your Own Exception Classes • Exception class you define extends classException or one of its subclasses. • Syntax to throw your own exception object: throw new ExceptionClassName(messageString); Java Programming: From Problem Analysis to Program Design, Second Edition

42. Creating Your Own Exception Classes public class MyDivisionByZeroException extends Exception { public MyDivisionByZeroException() { super("Cannot divide by zero"); } public MyDivisionByZeroException(String strMessage) { super(strMessage); } } Java Programming: From Problem Analysis to Program Design, Second Edition

43. Event Handling • Action events: • Handled by implementing interface ActionListener. • Window events: • Handled by implementing interface WindowListener. • Mouse events: • Handled by implementing interface MouseListener. • Key events: • Handled by implementing interface KeyListener. Java Programming: From Problem Analysis to Program Design, Second Edition

44. Event Handling • class WindowAdapter: • Implements interfaceWindowListener with empty bodies to methods. • classMouseAdapter: • Implements interface MouseListener with empty bodies to methods. Java Programming: From Problem Analysis to Program Design, Second Edition

45. Registering Listeners • To register window listener object to GUI component: • Use method addWindowListener. • Window listener object being registered is passed as parameter to method addWindowListener. • To register mouse listener object to GUI component: • Use method addMouseListener. • Mouse listener object being registered is passed as parameter to method addMouseListener. Java Programming: From Problem Analysis to Program Design, Second Edition

46. Registering Listeners Java Programming: From Problem Analysis to Program Design, Second Edition

47. Registering Listeners Java Programming: From Problem Analysis to Program Design, Second Edition

48. Registering Listeners Java Programming: From Problem Analysis to Program Design, Second Edition

49. Programming Example: Calculator Java Programming: From Problem Analysis to Program Design, Second Edition

50. Chapter Summary • Exception definition • Handling exceptions within a program: • try/catch/finally block. • Order of catch blocks. • Using try/catch blocks in a program. • The classException and the Operator instanceof. • Rethrowing and throwing an exception. Java Programming: From Problem Analysis to Program Design, Second Edition