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

Java Programming: Guided Learning with Early Objects

Chapter 10Exception Handling and Events

Java Programming: Guided Learning with Early Objects 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


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


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’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


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


try/catch/finally Block (continued)

• Syntax:try {

//statements} catch (ExceptionClassName1 objRef1) {

// exception handler code} …catch (ExceptionClassNameN objRefN) {

// exception handler code}finally { // statements }


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


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


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 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


Table 10-1 Commonly Used Constructors and Methods of the class Throwable


Figure 10-2 The class Exception and some of its subclasses from the package java.lang


Figure 10-3 The class Exception and some of its subclasses from the package java.util


Figure 10-4 The class IOException and some of its subclasses from the package java.io


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


Table 10-2 class Exception and its Constructors


Table 10-3 Commonly Used Exception Classes


Table 10-3 Commonly Used Exception Classes (continued)


Table 10-4 Exceptions Thrown by the Method nextInt

Table 10-5 Exceptions Thrown by the Method nextDouble


Table 10-6 class InputMistmatchException and its Constructors


Table 10-8 Exceptions Thrown by the Method nextLine

Table 10-7 Exceptions Thrown by the Method next


Table 10-10 Exceptions Thrown by the Methods of the class Integer

Table 10-9 Exceptions Thrown by the Method hasNext


Table 10-11 Exceptions Thrown by the Methods of the class Double


Table 10-12 Exceptions Thrown by the Methods of the class String


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


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


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


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


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);


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


Exception-Handling Techniques

• When exception occurs, programmer has three choices:– Terminate the program

– Fix the error and continue

– Log the error and continue


Terminate the Program

• In some cases, best to terminate when exception occurs

• Example:– Input data from file

– File not found

– No point in continuing


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


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


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


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


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


Table 10-14 Events Generated by key Components


Mouse Events

• Mouse generates seven events:– mouseClicked– mouseEntered– mouseExited– mousePressed– mouseReleased– mouseDragged– mouseMoved

• Events handled by MouseListener and MouseMotionListener


Table 10-15 Events Generated by mouse Components


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


Table 10-16 The Constructor and Commonly Used Methods of class Timer


Table 10-16 The Constructor and Commonly Used Methods of class Timer (continued)


Figure 10-10 An interface for showing the position of a ball


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


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


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


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


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 Chapter 10 Exception Handling and Events.

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