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

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

3. 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
Java Programming: From Problem Analysis to Program Design, 4e

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

5. 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
Java Programming: From Problem Analysis to Program Design, 4e

6. 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
Java Programming: From Problem Analysis to Program Design, 4e

7. 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
Java Programming: From Problem Analysis to Program Design, 4e

8. 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
Java Programming: From Problem Analysis to Program Design, 4e

9. 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
Java Programming: From Problem Analysis to Program Design, 4e

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

11. 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
Java Programming: From Problem Analysis to Program Design, 4e

12. 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
Java Programming: From Problem Analysis to Program Design, 4e

13. 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
Java Programming: From Problem Analysis to Program Design, 4e

14. 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
Java Programming: From Problem Analysis to Program Design, 4e

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

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

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

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

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

20. 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
Java Programming: From Problem Analysis to Program Design, 4e

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

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

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

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

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

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

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

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

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

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

31. 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
Java Programming: From Problem Analysis to Program Design, 4e

32. 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, 4e

33. 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
Java Programming: From Problem Analysis to Program Design, 4e

34. The class Exception 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();
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 "
Java Programming: From Problem Analysis to Program Design, 4e

35. 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
Java Programming: From Problem Analysis to Program Design, 4e

36. 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;
Java Programming: From Problem Analysis to Program Design, 4e

37. Rethrowing and Throwing an Exception (continued)
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, 4e

38. Rethrowing and Throwing an Exception (continued)
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, 4e

39. 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, 4e

40. The Method printStackTrace (continued)
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, 4e

41. The Method printStackTrace (continued)
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");
Java Programming: From Problem Analysis to Program Design, 4e

42. The Method printStackTrace (continued)
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)
Java Programming: From Problem Analysis to Program Design, 4e

43. 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, 4e

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

45. Creating Your Own Exception Classes (continued)
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, 4e

46. Event Handling Action events Window events Mouse events Key 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, 4e

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

48. 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
Java Programming: From Problem Analysis to Program Design, 4e

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

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

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

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

53. Chapter Summary Exception Handling exceptions within a program
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
Hierarchy
Classes
Java Programming: From Problem Analysis to Program Design, 4e

54. 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
Java Programming: From Problem Analysis to Program Design, 4e