1. Java Programming: From Problem Analysis to Program Design, 4e Chapter 11 Handling Exceptions and Events

2. Java Programming: From Problem Analysis to Program Design, 4e2 Chapter 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

3. Java Programming: From Problem Analysis to Program Design, 4e3 Chapter Objectives (continued) Learn how to handle exceptions within a program Discover how to throw and rethrow an exception Learn how to handle events in a program

4. Java Programming: From Problem Analysis to Program Design, 4e4 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 –An array index that goes out of bounds

5. Java Programming: From Problem Analysis to Program Design, 4e5 Handling Exception 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 if statements may not be the most effective way to handle the exception

6. Java Programming: From Problem Analysis to Program Design, 4e6 Java’s Mechanism of Exception Handling When an exception occurs, an object of a particular exception class is created Java provides a number of exception classes to effectively handle certain common exceptions such as division by zero, invalid input, and file not found Division by zero is an arithmetic error and is handled by the class ArithmeticException

7. Java Programming: From Problem Analysis to Program Design, 4e7 Java’s Mechanism of Exception Handling (continued) When a division by zero exception occurs, the program creates an object of the class ArithmeticException When a Scanner object is used to input data into a program, any invalid input errors are handled using the class InputMismatchException The class Exception (directly or indirectly) is the superclass of all the exception classes in Java

8. Java Programming: From Problem Analysis to Program Design, 4e8 try / catch / finally Block Statements that might generate an exception are placed in a try block The try block might also contain statements that should not be executed if an exception occurs The try block is followed by zero or more catch blocks A catch block specifies the type of exception it can catch and contains an exception handler

9. Java Programming: From Problem Analysis to Program Design, 4e9 try / catch / finally Block (continued) The last catch block may or may not be followed by a finally block Any code contained in a finally block always executes, regardless of whether an exception occurs, except when the program exits early from a try block by calling the method System.exit If a try block has no catch block, then it must have the finally block

10. Java Programming: From Problem Analysis to Program Design, 4e10 try / catch / finally Block (continued)

11. Java Programming: From Problem Analysis to Program Design, 4e11 try / catch / finally Block (continued) If no exception is thrown in a try block, all catch blocks associated with the try block are ignored and program execution resumes after the last catch block If an exception is thrown in a try block, the remaining statements in the try block are ignored - The program searches the catch blocks in the order in which they appear after the try block and looks for an appropriate exception handler

12. Java Programming: From Problem Analysis to Program Design, 4e12 try / catch / finally Block (continued) If the type of the thrown exception matches the parameter type in one of the catch blocks, the code of that catch block executes and the remaining catch blocks after this catch block are ignored If there is a finally block after the last catch block, the finally block executes regardless of whether an exception occurs

13. Java Programming: From Problem Analysis to Program Design, 4e13 Order of catch Blocks The heading of a catch block specifies the type of exception it handles A catch block 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

14. Java Programming: From Problem Analysis to Program Design, 4e14 Order of catch Blocks (continued) If in the heading of a catch block you declare an exception using the class Exception, then that catch block can catch all types of exceptions because the class Exception is the superclass of all exception classes In a sequence of catch blocks following a try block, a catch block declaring an exception of a subclass type should be placed before catch blocks declaring exceptions of a superclass type

15. Java Programming: From Problem Analysis to Program Design, 4e15 Java Exception Hierarchy

16. Java Programming: From Problem Analysis to Program Design, 4e16 Java Exception Hierarchy (continued)

17. Java Programming: From Problem Analysis to Program Design, 4e17 Java Exception Hierarchy (continued)

18. Java Programming: From Problem Analysis to Program Design, 4e18 Java Exception Hierarchy (continued)

19. Java Programming: From Problem Analysis to Program Design, 4e19 Java Exception Hierarchy (continued)

20. Java Programming: From Problem Analysis to Program Design, 4e20 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 exceptions Various exceptions categorized into separate classes and contained in various packages

21. Java Programming: From Problem Analysis to Program Design, 4e21 The class Exception and its Constructors

22. Java Programming: From Problem Analysis to Program Design, 4e22 Java Exception Classes

23. Java Programming: From Problem Analysis to Program Design, 4e23 Java Exception Classes (continued)

24. Java Programming: From Problem Analysis to Program Design, 4e24 Java Exception Classes (continued)

25. Java Programming: From Problem Analysis to Program Design, 4e25 Java Exception Classes (continued)

26. Java Programming: From Problem Analysis to Program Design, 4e26 Java Exception Classes (continued)

27. Java Programming: From Problem Analysis to Program Design, 4e27 Java Exception Classes (continued)

28. Java Programming: From Problem Analysis to Program Design, 4e28 Java Exception Classes (continued)

29. Java Programming: From Problem Analysis to Program Design, 4e29 Java Exception Classes (continued)

30. Java Programming: From Problem Analysis to Program Design, 4e30 Checked Exceptions Definition: any exception that can be recognized by the compiler Examples –FileNotFoundException s

31. Java Programming: From Problem Analysis to Program Design, 4e31 Unchecked Exceptions Definition: exceptions that cannot be recognized when the program compiles (must be checked for by programmer) Examples –Division by zero –Array index out of bounds Syntax ExceptionType1, ExceptionType2, and so on are names of exception classes

32. Java Programming: From Problem Analysis to Program Design, 4e32 Exceptions Example Code public static void exceptionMethod() throws InputMismatchException, FileNotFoundException { //statements } The method exceptionMethod throws exceptions of the type InputMismatchException and FileNotFoundException

33. Java Programming: From Problem Analysis to Program Design, 4e33 The class Exception 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

34. Java Programming: From Problem Analysis to Program Design, 4e34 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()); } The class Exception and the Operator instanceof

35. Java Programming: From Problem Analysis to Program Design, 4e35 Rethrowing and Throwing an Exception When an exception occurs in a try block, control immediately passes to one of the catch blocks; typically, a catch block does one of the following: –Completely handles the exception –Partially processes the exception; in this case, the catch block 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

36. Java Programming: From Problem Analysis to Program Design, 4e36 Rethrowing and Throwing an Exception (continued) 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;

37. Java Programming: From Problem Analysis to Program Design, 4e37 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()); } Rethrowing and Throwing an Exception (continued)

38. Java Programming: From Problem Analysis to Program Design, 4e38 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; } Rethrowing and Throwing an Exception (continued)

39. Java Programming: From Problem Analysis to Program Design, 4e39 The Method printStackTrace Used to determine the order in which the methods were called and where the exception was handled

40. Java Programming: From Problem Analysis to Program Design, 4e40 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(); } The Method printStackTrace (continued)

41. Java Programming: From Problem Analysis to Program Design, 4e41 public static void methodA() throws Exception { methodB(); } public static void methodB() throws Exception { methodC(); } public static void methodC() throws Exception { throw new Exception("Exception generated " + " in method C"); } The Method printStackTrace (continued)

42. Java Programming: From Problem Analysis to Program Design, 4e42 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:30) at PrintStackTraceExample1.methodB (PrintStackTraceExample1.java:25) at PrintStackTraceExample1.methodA (PrintStackTraceExample1.java:20) at PrintStackTraceExample1.main (PrintStackTraceExample1.java:9) The Method printStackTrace (continued)

43. Java Programming: From Problem Analysis to Program Design, 4e43 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

44. Java Programming: From Problem Analysis to Program Design, 4e44 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);

45. Java Programming: From Problem Analysis to Program Design, 4e45 Creating Your Own Exception Classes (continued) public class MyDivisionByZeroException extends Exception { public MyDivisionByZeroException() { super("Cannot divide by zero"); } public MyDivisionByZeroException(String strMessage) { super(strMessage); }

46. Java Programming: From Problem Analysis to Program Design, 4e46 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

47. Java Programming: From Problem Analysis to Program Design, 4e47 Event Handling (continued) class WindowAdapter –Implements interface WindowListener with empty bodies to methods class MouseAdapter –Implements interface MouseListener with empty bodies to methods

48. Java Programming: From Problem Analysis to Program Design, 4e48 Registering Listeners Registering window listener object to GUI component –Use method addWindowListener –Window listener object being registered is passed as parameter to method addWindowListener Registering mouse listener object to GUI component –Use method addMouseListener –Mouse listener object being registered is passed as parameter to method addMouseListener

49. Java Programming: From Problem Analysis to Program Design, 4e49 Event Handling (continued)

50. Java Programming: From Problem Analysis to Program Design, 4e50 Event Handling (continued)

51. Java Programming: From Problem Analysis to Program Design, 4e51 Event Handling (continued)

52. Java Programming: From Problem Analysis to Program Design, 4e52 Programming Example: Calculator

53. Java Programming: From Problem Analysis to Program Design, 4e53 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 class Exception and the Operator instanceof –Rethrowing and throwing an exception Exception –Hierarchy –Classes

54. Java Programming: From Problem Analysis to Program Design, 4e54 Chapter Summary (continued) Checked and unchecked exceptions The method printStackTrace Exception-handling techniques –Terminate program –Fix error and continue –Log error and continue Creating your own exception classes Event handling