1. Java Programming: Guided Learning with Early Objects
Chapter 10
Exception Handling and Events

2. Objectives Learn what an exception is
Learn how to use a try/catch block to handle exceptions
Become acquainted with the hierarchy of exception classes
Learn about checked and unchecked exceptions
Learn how to handle exceptions within a program
Java Programming: Guided Learning with Early Objects

3. Objectives (continued)
Discover how to rethrow and throw an exception
Become familiar with the adapter classes
Learn how to handle key and mouse events
Learn how to use the Timer class
Java Programming: Guided Learning with Early Objects

4. Handling Exceptions within a Program
Recall when you try to input incompatible data into a variable, the program terminates
Generates exception
Example:
Divide by zero generates ArithmeticException: / by zero
Exception could be prevented by checking that divisor is not zero before dividing
Add an if statement
Java Programming: Guided Learning with Early Objects

5. Java’s Mechanism of Exception Handling
Using if statements may not be appropriate or convenient to handle exceptions
Put exception code in one place
Easier to modify exception-handling code
When exception occurs, object of exception class created
Java provides several exception classes
class Exception is the superclass to all exception classes in Java
Java Programming: Guided Learning with Early Objects

6. try/catch/finally Block
try block:
Contains statements that might generate exception
Contains statements that should not be executed if exception occurs
Followed by zero or more catch blocks
catch block:
Specifies type of exception to catch
Contains an exception handler
finally block: code always executes
Java Programming: Guided Learning with Early Objects

7. try/catch/finally Block (continued)
Syntax:
try {
//statements }
catch (ExceptionClassName1 objRef1) {
// exception handler code …
catch (ExceptionClassNameN objRefN) {
finally { // statements }
Java Programming: Guided Learning with Early Objects

8. try/catch/finally Block (continued)
If no exception thrown in try block, all catch blocks ignored
Execution resumes after last catch block
If exception thrown in try block, remaining statements ignored
Program searches catch blocks in order, looking for a matching catch block
finally block executes regardless of exception
finally block is optional
Java Programming: Guided Learning with Early Objects

9. Order of catch Blocks
Can catch all exceptions of a specific type or all types of exceptions
Heading of catch block specifies types of exceptions it handles
If heading declares exception using class Exception, catch block catches all types
When exception caught by catch block, remaining catch blocks ignored
Order is important
Java Programming: Guided Learning with Early Objects

10. Using try/catch Blocks in a Program
Common error while inputting numeric data is typing nonnumeric character
Methods nextInt and nextDouble throw InputMismatchException
Division by zero with integer values generates ArithmeticException
When exception occurs, program throws an object of a specific exception class
Java Programming: Guided Learning with Early Objects

11. Java Exception Hierarchy
Every class can potentially cause exceptions
class Throwable:
Derived from class Object
Superclass of class Exception
Methods getMessage, printStackTrace, and toString are public
Inherited by subclasses of class Throwable
Java Programming: Guided Learning with Early Objects

12. Table 10-1 Commonly Used Constructors and Methods of the class Throwable
Java Programming: Guided Learning with Early Objects

13. Figure 10-2 The class Exception and some of its subclasses from the package java.lang
Java Programming: Guided Learning with Early Objects

14. Figure 10-3 The class Exception and some of its subclasses from the package java.util
Java Programming: Guided Learning with Early Objects

15. Figure 10-4 The class IOException and some of its subclasses from the package java.io
Java Programming: Guided Learning with Early Objects

16. Java’s Exception Classes
class Exception is the superclass of Java exception-handling classes
Java categorizes exceptions into separate classes
Exception classes placed in the package that contains the methods that throw them
Method getMessage returns string containing detailed message stored in exception object
Java Programming: Guided Learning with Early Objects

17. Table 10-2 class Exception and its Constructors
Java Programming: Guided Learning with Early Objects

18. Table 10-3 Commonly Used Exception Classes
Java Programming: Guided Learning with Early Objects

19. Table 10-3 Commonly Used Exception Classes (continued)
Java Programming: Guided Learning with Early Objects

20. Table 10-4 Exceptions Thrown by the Method nextInt
Table 10-5 Exceptions Thrown by the Method nextDouble
Java Programming: Guided Learning with Early Objects

21. Table 10-6 class InputMistmatchException and its Constructors
Java Programming: Guided Learning with Early Objects

22. Table 10-7 Exceptions Thrown by the Method next
Table 10-8 Exceptions Thrown by the Method nextLine
Java Programming: Guided Learning with Early Objects

23. Table 10-9 Exceptions Thrown by the Method hasNext
Table Exceptions Thrown by the Methods of the class Integer
Java Programming: Guided Learning with Early Objects

24. Table 10-11 Exceptions Thrown by the Methods of the class Double
Java Programming: Guided Learning with Early Objects

25. Table 10-12 Exceptions Thrown by the Methods of the class String
Java Programming: Guided Learning with Early Objects

26. Checked and Unchecked Exceptions
Any exception the compiler can recognize
Example: FileNotFoundException
Compiler encounters statements to open a file
Reduces number of statements not properly handled
Unchecked exceptions:
Any exception the compiler cannot recognize
Examples: division by zero, index out of bounds
Programmer checks for these exceptions
Java Programming: Guided Learning with Early Objects

27. More Examples of Exception Handling
Java accepts only strings as input in dialog boxes and text fields
Methods parseInt, parseFloat, and parseDouble may terminate with number format error
Throw NumberFormatException if string does not contain a number
Java Programming: Guided Learning with Early Objects

28. class Exception and the Operator instanceof
Recall that reference variable of superclass type can point to objects of its subclasses
Operator instanceof determines if reference variable points to object of particular class
Used to combine two catch blocks
Java Programming: Guided Learning with Early Objects

29. Rethrowing or Throwing an Exception
When exception occurs in try block, control passes to first matching catch block
catch block may:
Completely handle the exception
Partially process the exception
Rethrows same exception
Throws another exception for calling environment to handle the exception
Rethrow same exception for calling environment to handle
Java Programming: Guided Learning with Early Objects

30. Rethrowing or Throwing an Exception (continued)
Rethrowing an exception or throwing an exception handled by throw statement
throw statement throws either checked or unchecked exception
Exceptions are objects of a specific type
Syntax to rethrow an exception:
throw exceptionReference
General throw syntax:
throw new ExceptionClassName(messageStr);
Java Programming: Guided Learning with Early Objects

31. Method printStackTrace
Java keeps track of method call sequence
class Exception subclass of class Throwable
class Throwable contains public method printStackTrace
Use method printStackTrace to determine the order in which methods were called
Determine where exception handled
Java Programming: Guided Learning with Early Objects

32. Exception-Handling Techniques
When exception occurs, programmer has three choices:
Terminate the program
Fix the error and continue
Log the error and continue
Java Programming: Guided Learning with Early Objects

33. Terminate the Program
In some cases, best to terminate when exception occurs
Example:
Input data from file
File not found
No point in continuing
Java Programming: Guided Learning with Early Objects

34. Fix the Error and Continue
Handle the exception, let the program continue
Example:
Prompt the user for a file name
User misspells file name
File not found
Reprompt the user
Java Programming: Guided Learning with Early Objects

35. Log the Error and Continue
Sometimes terminating the program is unsafe or unnecessary
Prefer to record the exception and continue
Example:
Analyzing airline-ticketing transactions
Large volume of transactions each day
Analysis program would take too long if it fixed every error
Program records exceptions and continues processing transactions
Java Programming: Guided Learning with Early Objects

36. Creating Your Own Exception Classes
Java does not provide exception-handling classes for every possible circumstance
Must throw programmer-defined exceptions using throw statement
Define classes by extending the class Exception or a subclass
Typically, constructors are the only methods included when defining exception class
Programmer-defined classes inherit members of superclasses
Java Programming: Guided Learning with Early Objects

37. GUI Adaptor Classes, Events, and the Timer Class (Optional)
Java provides interfaces to handle events
Cannot instantiate object of an interface
Create an object to handle an event, and create a class that implements appropriate interface
ActionListener handles action events
WindowListener handles window events
Class may implement more than one interface
Java Programming: Guided Learning with Early Objects

38. Key Events Three types of key events:
keyPressed
keyReleased
keyTyped
keyTyped: generated when alphanumeric key pressed
keyPressed: generated when meta key pressed
keyReleased: generated when any key released
Java Programming: Guided Learning with Early Objects

39. Table 10-14 Events Generated by key Components
Java Programming: Guided Learning with Early Objects

40. Mouse Events Mouse generates seven events:
mouseClicked
mouseEntered
mouseExited
mousePressed
mouseReleased
mouseDragged
mouseMoved
Events handled by MouseListener and MouseMotionListener
Java Programming: Guided Learning with Early Objects

41. Table 10-15 Events Generated by mouse Components
Java Programming: Guided Learning with Early Objects

42. Timer Class Control activities over time No visual representation
Cannot be displayed on screen
Generate action events at specific time intervals
Constructor sets delay time between events
Second parameter specifies action event to be generated at time interval
Java Programming: Guided Learning with Early Objects

43. Table 10-16 The Constructor and Commonly Used Methods of class Timer
Java Programming: Guided Learning with Early Objects

44. Table 10-16 The Constructor and Commonly Used Methods of class Timer (continued)
Java Programming: Guided Learning with Early Objects

45. Figure 10-10 An interface for showing the position of a ball
Java Programming: Guided Learning with Early Objects

46. Summary Exception is an object of the exception class
Java provides exception classes
Programmer creates own exception classes
try/catch/finally block handles exceptions in a program
Statements that may generate exception placed in try block
try block also contains statements that should not be executed in presence of exception
Java Programming: Guided Learning with Early Objects

47. Summary (continued) try block followed by zero or more catch blocks
catch block specifies type of exception caught and contains exception handler
Last catch block may be followed by finally block
finally block contains code that executes regardless of exception
If try block not followed by catch block, must have finally block
Java Programming: Guided Learning with Early Objects

48. Summary (continued)
catch block catches all exceptions or specific type of exceptions
Heading of catch block specifies the type of exception it handles
class Exception is superclass of all exception-handling classes
Contained in package java.lang
Exception classes placed in package containing the methods that throw them
Java Programming: Guided Learning with Early Objects

49. Summary (continued)
Checked exception: any exception recognized by the compiler
Unchecked exception: any exception not recognized by the compiler
Programmer-defined exception classes extend class Exception
class WindowAdapter implements interface WindowListener
Provides empty bodies to methods
Java Programming: Guided Learning with Early Objects

50. Summary (continued)
Register a window listener using method addWindowListener
A WindowListener object is passed as a parameter to addWindowListener
Timer class objects control activities over time
No visual representation
Cannot be displayed on screen
Generate action events at specific time intervals
Java Programming: Guided Learning with Early Objects