Chapter 12 Exception Handling & Text...
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Chapter 12 Exception Handling & Text I/O DR. JONES KENNESAW STATE UNIVERSITY CS 2302 SUMMER 2015 slides created by Rashaad Jones
Transcript of Chapter 12 Exception Handling & Text...
Chapter 12
Exception Handling
& Text I/O DR. JONES
KENNESAW STATE UNIVERSITY
CS 2302
SUMMER 2015
slides created by Rashaad Jones
Contents
Overview
Exception
Exception handling
What they are
Error types
Try-Throw-Catch Exceptions
Getting Information from
Exceptions
finally
General Guidelines
File
File Class
PrintWriter
Scanner
Exception Handling
Exceptions are thrown from a method
Directly by using a throw statement in a try block
Invoking a method that may throw an exception
Exceptions have messages
Help users understand what happened
Help users understand how to resolve
Help programmers and troubleshooters understand how to diagnose
Specificity is important
Try block -- Example
Code snippet
System.out.println("Enter a double:");
double a = scan.nextDouble();
Output if number is not entered
ÏException in thread "main" java.util.InputMismatchException
Code snippet
try
{
System.out.println("Enter a double:");
double a = scan.nextDouble();
}
catch (Exception e)
{
System.out.println("You did not enter a number");
}
Output if number is not entered
You did not enter a number
WITHOUT TRY WITH TRY
throw example
Code snippet
public static void method(double a, double b)
{
if(b != 0)
{
System.out.println(a/b);
}
else
System.out.println(“Divisor is not a number”);
}
Output if b is not a number
Divisor is not a number
Code snippet
public static void method(double a, double b) throws Exception
{
if(b != 0)
{
System.out.println(a/b);
}
else
throw new Exception(“Divisor is not a number”);
}
Output if b is not a number
You did not enter a number
WITHOUT THROW WITH THROW
Throw -- Example
Catching errors -- Example
public static void methodwithException(double
a, double b) throws Exception
{
if(b != 0)
{
System.out.println(a/b);
}
else
throw new Exception(“Divisor is not a
number”);
}
public static void methodCaller()
{
java.util.Scanner scan = new Scanner(System.in);
double a = scan.nextDouble();
double b = scan.nextDouble();
try
{
methodwithException(a, b);
}
catch (Exception e)
{
System.out.println(“ERROR” + e.getMessage());
}
}
METHOD WITH
EXCEPTION
METHOD THAT
CATCHES EXCEPTION
Getting information from errors -- Example
public static void methodwithException(double
a, double b) throws Exception
{
if(b != 0)
{
System.out.println(a/b);
}
else
throw new Exception(“Divisor is not a
number”);
}
public static void methodCaller()
{
java.util.Scanner scan = new Scanner(System.in);
double a = scan.nextDouble();
double b = scan.nextDouble();
try
{
methodwithException(a, b);
}
catch (Exception e)
{
System.out.println(“ERROR” + e.getMessage());
}
}
METHOD WITH
EXCEPTION
METHOD THAT
CATCHES EXCEPTION
Error Types
System errors
• Provide user message prior to closing
• Represented by the Error class
Run-time exceptions
• Programming errors
• Represented by the RuntimeException class
Exception errors
• Provide mechanisms to continue running program
• Represented by the Exception class
FATAL
NON-
FATAL
public void openFile()
{
FileReader reader = null;
try
{
reader = new FileReader("someFile");
int i=0;
while(i != -1)
{
i = reader.read();
System.out.println((char) i );
}
}
catch (IOException ioe)
{
System.out.println(“ERROR:” + ioe.getMessage());
}
finally
{
if(reader != null)
{
try
{
reader.close();
}
catch (IOException e)
{
//nothing needs to be done
//because reader is null
}
}
}
General Guidelines
main method catch all exceptions
try-catch-finally block
all other methods throws exceptions
use Custom Exceptions
Class that extends a specific Exception or Error class
Specific message that helps to diagnose and/or resolve
Bad error message: “Error encountered in code”
Good error message: “Code: 6215. Error detected in line 592 of code. Cannot parse the file, StudentNames.txt because information is missing in row 12 of file. Program will not be able to continue analysis. Please contact support and provide this message.”
File Class
Contains behaviors and properties for obtaining of a file/directory
and for renaming and deleting a file/directory
Examples
Directory: File f = new File(“book”);
File: File f = new File(“book.dat”);
Reminders
Do not use absolute file names in your program
PrintWriter Class
java.io.PrinterWriter output = null;
try
{
output = new java.io.PrintWriter(new
File(“scores.txt”);
output.print(“John Doe”);
output.println(90);
output.print(“Jane Doe”);
output.println(85);
}
catch (Exception e)
{
System.out.println(“Error with writing
to file”);
}
finally
{
//close the file (MUST BE DONE)
output.close();
}
Scanner Class
Scanner input = new Scanner(new File(“scores.txt”));
try
{
while(input.hasNext())
{
String firstName = input.next();
String lastName = input.next();
int score = input.nextInt();
System.out.println(firstName + “ “ + lastName + “ “ + score);
}
}
catch (Exception e)
{
System.out.println(“Error “ + e.getMessage();
}
finally
{
//close the file
input.close();
}
