Exception Handling (Chapter 8), Lecture notes of Accounting

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Exception Handling (Chapter 8) CS 180 Recitation - February 29, 2008Department of Computer SciencePurdue University

Announcements •^ Projects are examined by software thatdetects potential cheating •^ Project 4 handed back •^ Project 4 test cases are posted. Beforeregrade request, test your program withthem. For regrade, contact with

Jalaja

Padma

Introduction •^ To handle exceptions, Java’s exceptionhandling facilities should be used. •^ 2 sections of a program:

•^ Code segments that handle normal flow ofcontrol. •^ Code segments that handle the exceptionalcases that might occur through out theexecution.

Test Class Consider this simple program:

import java.util.InputMismatchException;import java.util.Scanner;public class Test{

public static void main(String[] args){

Scanner keyboard = new Scanner(System.in);System.

out .println("Please enter the number of eggs:");

int numberOfEggs = keyboard.nextInt();System.out.println(numberOfEggs);

Test Class (Right Way) import java.util.Scanner;public class Test{ public static int DEFAULT_EGGS = 5;public static void main(String[] args){

Scanner keyboard = new Scanner(System.in);System.out.println("Please enter the number of eggs:");int numberOfEggs;try{

numberOfEggs = keyboard.nextInt(); } catch(Exception e){

numberOfEggs = DEFAULT_EGGS;System.out.println("Bad input!!!\nSetting the default value, "

• numberOfEggs);

} System.out.println(numberOfEggs); } }

Test Class (Handling multiple exceptions) import java.util.Scanner;public class Test{ public static int DEFAULT_EGGS = 5;public static void main(String[] args){

Scanner keyboard = new Scanner(System.in);System.out.println("Please enter the number of eggs:");int numberOfEggs = 0;int numberOfBasket = 0;int eggsPerBasket = 0;try{

numberOfEggs = keyboard.nextInt();System.out.println("Please enter the number of baskets:");numberOfBasket = keyboard.nextInt();eggsPerBasket = numberOfEggs/numberOfBasket; } catch(Exception e){

//Opps, now what to catch? Bad user input or division by zero? } System.out.println(numberOfEggs); } }

Handling multiple exceptions •^ Although it catches multiple exceptions, thispiece of code doesn’t execute all the statementsif an exception occurs at the first line of try block. •^ If there is a way to handle exception without atry-catch block, use it. (i.e. instead of using^ ArithmeticException

an if statement that checks if

numberOfBasket<

would suffice)

-^ For each statement, form a different try-catchblock and catch every possible exception.

Handling multiple exceptions •^ While catching exceptions, it is crucial that order goesfrom specific exception (sub-class) to general exception(parent class) since the first matching catch is executed.

Scanner keyboard = new Scanner(System.in);try{

int tmp = 10/keyboard.nextInt() - keyboard.nextInt();

} catch (

Exception

e){

e.printStackTrace();

} catch (

InputMismatchException

e){

System.out.println(e.getMessage());

CompileError

Parent Class

Sub-class of Exception

The flow of control will never go into the last catchblock since all of the exception will be caught by thefirst catch block. This will cause a compile error.So, these 2 catch blocks should be swapped.

Accounting for Exceptions (

st^ option)

-^ Catching an exception inside of a method^ import java.util.InputMismatchException;import java.util.Scanner;public class Test{

public static void main(String[] args){

int numberOfEggs = getInput("Please enter the number of eggs:", 1);int numberOfBasket = getInput("Please enter the number of baskets:", 1);if (numberOfBasket < 1)

numberOfBasket = 1; int eggsPerBasket = numberOfEggs/numberOfBasket;System.out.println(numberOfEggs+"-"+numberOfBasket+"-"+eggsPerBasket); } public static int getInput(String prompt, int numOfTry){

Scanner keyboard = new Scanner(System.in);System.out.println(prompt);int k = 0;try{

k = keyboard.nextInt(); } catch(InputMismatchException e){

System.out.println("Bad input!!!(try number: "+numOfTry+")");k = getInput(prompt, numOfTry+1); } return k; } }

Accounting for Exceptions (

nd^

option)

-^ Catching an exception where the method is called^ public

static

void main(String[]

args)

int numberOfEggs

try{

numberOfEggs = getInput("Please

enter

the^ number

of^ eggs:");

}^ catch

(InputMismatchException

e){

numberOfEggs = 5; } int numberOfBasket =

try{

numberOfBasket =

getInput("Please

enter

the^ number

of^ baskets:");

}catch

(InputMismatchException

e){

numberOfEggs = -1;numberOfBasket =

… } public

static

int^ getInput(String

prompt)

throws

InputMismatchException{

Scanner

keyboard

= new Scanner(System.in);

System.out.println(prompt);int k =

keyboard.nextInt(); return

k;

}

Keep It Simple •^ If the way the exception is handleddepends on the calling method, let thecalling method handle the exception (i.e.^2

nd^ option, in the second try-catch the firstvariable is changed used in the first try-catch).

•^1

st^ option is more specific and is less likelyto be used through out the program, so itis a good practice to implement 2

nd^ option.

Implementing New ExceptionClasses •^ Don’t forget catch handlers won’t be partof user-defined exception classes •^ In other words you should explicitlyindicate in your program when to throwyour own exception •^ Test class example continued:

throws

Clause Not Allowed in

actionPerformed •^ A throws clause cannot be added to methodactionPerformed in any action listener class. •^ Any exception thrown in methodactionPerformed must be caught in methodactionPerformed. •^ Similarly, if method windowClosing is redefinedin a window listener class, you may not add athrows clause to method windowClosing.

Quiz Scanner

keyboard

=^ new

Scanner(System.in);

int^ tmp

=^ 0;

try{tmp

=^ 10/keyboard.nextInt()

• keyboard.nextInt();

}^ catch

(InputMismatchException

e){

System.out.println(“InputMismatchException”);tmp^ =

}^ catch

(Exception

e){

System.out.println(“Exception”);tmp^ =

} Assume a user tries to enter 0 first, then 5. What is the value of tmp at the end?