Java Programming: Guided Learning with Early Objects

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

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

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

5
Javas 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

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

7
try/catch/finally Block (continued)

• Syntax
• try
• //statements
• catch (ExceptionClassName1 objRef1)
• // exception handler code
• catch (ExceptionClassNameN objRefN)
• // exception handler code
• finally // statements

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

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

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

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

12
Table 10-1 Commonly Used Constructors and Methods
of the class Throwable
13
Figure 10-2 The class Exception and some of its
subclasses from the package java.lang
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Figure 10-3 The class Exception and some of its
subclasses from the package java.util
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Figure 10-4 The class IOException and some of its
subclasses from the package java.io
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Javas 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

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Table 10-2 class Exception and its Constructors
18
Table 10-3 Commonly Used Exception Classes
19
Table 10-3 Commonly Used Exception Classes
(continued)
20
Table 10-4 Exceptions Thrown by the Method nextInt
Table 10-5 Exceptions Thrown by the Method
nextDouble
21
Table 10-6 class InputMistmatchException and its
Constructors
22
Table 10-7 Exceptions Thrown by the Method next
Table 10-8 Exceptions Thrown by the Method
nextLine
23
Table 10-9 Exceptions Thrown by the Method hasNext
Table 10-10 Exceptions Thrown by the Methods of
the class Integer
24
Table 10-11 Exceptions Thrown by the Methods of
the class Double
25
Table 10-12 Exceptions Thrown by the Methods of
the class String
26
Checked and Unchecked Exceptions

• Checked 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

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

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

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

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)

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

32
Exception-Handling Techniques

• When exception occurs, programmer has three
  choices
• Terminate the program
• Fix the error and continue
• Log the error and continue

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

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

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

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

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

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

39
Table 10-14 Events Generated by key Components
40
Mouse Events

• Mouse generates seven events
• mouseClicked
• mouseEntered
• mouseExited
• mousePressed
• mouseReleased
• mouseDragged
• mouseMoved
• Events handled by MouseListener and
  MouseMotionListener

41
Table 10-15 Events Generated by mouse Components
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

43
Table 10-16 The Constructor and Commonly Used
Methods of class Timer
44
Table 10-16 The Constructor and Commonly Used
Methods of class Timer (continued)
45
Figure 10-10 An interface for showing the
position of a ball
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

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

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

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

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