Copyright © 2014 by John Wiley & Sons. All rights reserved.1 Input/Output and Exception Handling

Copyright © 2014 by John Wiley & Sons. All rights reserved. 1

Input/Output and Exception Handling


Goals

To read and write text files To process command line arguments To throw and catch exceptions To implement programs that propagate checked exceptions


Read Text Files

Read Text Files:

• Use Scanner class for reading text files

• To read from a disk file:

1. Construct a File object representing the input file

File inputFile = new File("input.txt");

2. Use this File object to construct a Scanner object:

Scanner in = new Scanner(reader);

3. Use the Scanner methods to read data from file

next(), nextInt(), and nextDouble()


Read Text Files

Process the text in file:

• A loop to process numbers in the input file:

while (in.hasNextDouble()){

double value = in.nextDouble();

//Process value.

}


Write to a file

Write to a file:

1. Construct a PrintWriter object:

PrintWriter out = new PrintWriter("output.txt");

• If file already exists, it is emptied before the new data are written into it.

• If file doesn't exist, an empty file is created.

2. Use print and println to write into a PrintWriter:

out.println("Hello, World!");

out.printf("Total: %8.2f\n", total);


Write to a file

3. You must close a file when you are done processing it:

in.close();

out.close();

Otherwise, not all of the output may be written to the disk file.


FileNotFoundException When the input or output file doesn't exist, a FileNotFoundException can

occur.


FileNotFoundException When the input or output file doesn't exist, a FileNotFoundException can occur.

To handle the exception, label the main method like this:public static void main(String[] args) throws FileNotFoundException{

PrintWriter out = new PrintWriter("output.txt"); …}


Programming Question Read a file input.txt containing numbers

Write the numbers in a column followed by their total in a file output.txt:

• Hint: use printf statements to write to file.

32 54 67.5 29 35 80115 44.5 100 65

input.txt

Program template in next slide

32.00 54.00 67.50 29.00 35.00 80.00 115.00 44.50 100.00 65.00Total: 622.00


public class Total{ public static void main(String[] args) throws FileNotFoundException { //TODO: Construct the Scanner object to read from input.txt file //TODO: Construct the PrintWriter object for writing in file output.txt //TODO: Read the input and write the output //TODO: Close reader and writer }}


Answerimport java.io.File;import java.io.FileNotFoundException;import java.io.PrintWriter;import java.util.Scanner;

public class Total{ public static void main(String[] args) throws FileNotFoundException { // Construct the Scanner object to read from input.txt file File inputFile = new File("input.txt"); Scanner in = new Scanner(inputFile); // Construct the PrintWriter object for writing in file output.txt PrintWriter out = new PrintWriter("output.txt");

// Read the input and write the output double total = 0; while (in.hasNextDouble()) { double value = in.nextDouble(); out.printf("%15.2f\n", value); total = total + value; } out.printf("Total: %8.2f\n", total); //close reader and writer in.close(); out.close(); }}

Total.java


Question

What happens when you supply the name of a nonexistent input file to the Total program? Try it out if you are not sure.


Answer

Answer: The program throws a FileNotFoundException and terminates.


Question

What exception do you get when input is not a properly formatted number when calling parseInt() method?

String population = line.substring(i);

int populationVal = Integer.parseInt(population);

• Look up in Java API

• Try some code with improperly formatted numbers


Answer

If the input is not a properly formatted number when calling parseInt or parseDouble method NumberFormatException occurs


NumberFormatException


Programming Question

To see a NumberFormatException , try the following code:

public class ExceptionDemo{ public static void main(String args[]) { String strA = "3rd"; int a = Integer.parseInt(strA); }}



InputMismatchException

Thrown by Scanner• If the input is not a properly formatted number when calling nextInt or nextDouble method




Try following to see a InputMismatchException :

import java.util.Scanner;

public class ExceptionDemo2{ public static void main(String args[]) { Scanner sc = new Scanner(System.in); sc.nextInt(); //supply anything other than a number e.g. q }}



NoSuchElelmentException

If there is no input at all when you call nextInt or nextDouble, a “no such element exception” occurs.

To avoid exceptions, use the hasNextInt methodif (in.hasNextInt()) { int value = in.nextInt();

}



Try following to see a NoSuchElementException:


public class ExceptionDemo3{ public static void main(String args[]) { Scanner sc = new Scanner("2 3"); //string with two ints for(int i=1;i<=3;i++) //try to read 3 ints { sc.nextInt(); } }}



NullPointerException

Thrown when an application attempts to use null in a case where an object is required.

These include:• Calling the instance method of a null object.• Accessing or modifying the field of a null object.• Taking the length of null as if it were an array.• Accessing or modifying the slots of null as if it were an array.



Try following to see a NullPointerException:


public class ExceptionDemo4{ public static void main(String args[]) { Scanner sc = new Scanner("abc"); sc=null; sc.next(); }}



Project Phase II Discussion


Hierarchy of Exception Classes

System errors are thrown by JVM and represented in the Error class.

The Error class describes internal system errors.

-LinkageError

-VirtualMachineError

…

Such errors rarely occur.

If one does, there is little you can do beyond notifying the user and trying to terminate the program gracefully. (Fatal)

Exception describes errors caused by your program and external circumstances.

These errors can be caught and handled by your program.(Non-Fatal)

RuntimeException is caused by programming errors, such as bad casting, accessing an out-of-bounds array, and numeric errors.

(from bad programming)

•Used to indicate that exceptional situations have occurred


Checked Exceptions vs. Unchecked Exceptions

Unchecked Exceptions:• RuntimeException, Error and their subclasses• your fault. • The compiler does not check whether you handle an

unchecked exception.• typically can be prevented by proper coding

Checked Exceptions:• All other exceptions• occurdue to external circumstances that the programmer

cannot prevent• The compiler checks that your program handles these

exceptions.

31


Exception Handling - Throwing Exceptions

How to handle error after detection?

• Method1: Throw Exception

• Method 2: Catch Exception using Try Catch block


Throwing an Exception

Method1: Throw Exception Throw an exception object to signal an exceptional condition E.g. someone tries to withdraw too much money from a bank

account • Throw an IllegalArgumentException• in withdraw method of BankAccount class:



Modify BankAccount class withdraw method so that it throws an IllegalArgumentException if amount requested is greater than balance.

Find program template in next slide


public class BankAccount{ private double balance;

public BankAccount(double initialBalance) { balance = initialBalance; }

public void deposit(double amount) { balance = balance + amount; }

public void withdraw(double amount) { balance = balance - amount;

}

public double getBalance() { return balance; }

public static void main(String args[]) { BankAccount account = new BankAccount(1000); account.withdraw(2000); System.out.println("new balance="+account.getBalance()); } }


Answerpublic class BankAccount{ private double balance;



public void withdraw(double amount) { if(amount>balance) throw new IllegalArgumentException("Amount exceeds Balance!"); balance = balance - amount; }



BankAccount.java



Question

Suppose balance is 100 and amount is 200. What is the value of balance after these statements?

if (amount > balance){ throw new IllegalArgumentException("Amount exceeds balance");}balance = balance – amount;


Answer

Answer: It is still 100. The last statement was not executed because the exception was thrown.


Catching Exceptions Method 2: Catch Exception using Try Catch block

• handled exception in your program• Place the statements that can cause an exception inside a try block• Place handler inside a catch clause.• Each catch clause contains a handler.

try{ String filename ="abc.txt"; Scanner in = new Scanner(new File(filename)); String input = in.next(); int value = Integer.parseInt(input); . . .}catch (IOException exception){ exception.printStackTrace();}catch (NumberFormatException exception){ System.out.println(exception.getMessage());}



Modify Total class to use try/catch statements to handle possible exceptions.• List all thrown Exceptions by checking all method and

constructor calls against JavaDoc API• All these exceptions must be caught and handled

After try/catch blocks, write a print statement to print “Test code execution after try-catch block” • This code would be executed even when an exception was

caught


public class Total{ public static void main(String[] args) {

double total =0; Scanner console = new Scanner(System.in); System.out.print("Input file: "); String inputFileName = console.next(); System.out.print("Output file: "); String outputFileName = console.next(); File inputFile = new File(inputFileName); Scanner in = new Scanner(inputFile); PrintWriter out = new PrintWriter(outputFileName); while (in.hasNextDouble()) { double value = in.nextDouble(); out.printf("%15.2f\n", value); total = total + value; } out.printf("Total: %8.2f\n", total); in.close(); out.close(); }}

What errors can be thrown?


Answerpublic class Total{ public static void main(String[] args) {

double total =0; Scanner console = new Scanner(System.in); System.out.print("Input file: "); String inputFileName = console.next(); System.out.print("Output file: "); String outputFileName = console.next(); File inputFile = new File(inputFileName); Scanner in = new Scanner(inputFile); PrintWriter out = new PrintWriter(outputFileName); while (in.hasNextDouble()) { double value = in.nextDouble(); out.printf("%15.2f\n", value); total = total + value; } out.printf("Total: %8.2f\n", total); in.close(); out.close(); }}

Total.java


Checked/Unchecked? NoSuchElementException - RuntimeException IllegalStateException - RuntimeException NullPointerException - RuntimeException FileNotFoundException – IOException (Checked Exception) InputMismatchException - RuntimeException IllegalFormatException - RuntimeException

Let us handle 2 exceptions


Answerpublic class Total{ public static void main(String[] args) { try { double total =0; Scanner console = new Scanner(System.in); System.out.print("Input file: "); String inputFileName = console.next(); System.out.print("Output file: "); String outputFileName = console.next(); File inputFile = new File(inputFileName); Scanner in = new Scanner(inputFile); PrintWriter out = new PrintWriter(outputFileName); while (in.hasNextDouble()) { double value = in.nextDouble(); out.printf("%15.2f\n", value); total = total + value; } out.printf("Total: %8.2f\n", total); in.close(); out.close(); } catch (IOException exception) { exception.printStackTrace(); } catch (NumberFormatException exception) { System.out.println(exception.getMessage()); }

System.out.println(“Test code after try catch block”); }}

Total.java

Where are we catching the FileNotFound exception?

TODO: 1. Comment catch block of

NumberFOrmatException. What happens when you compile?

2. Comment catch block of IOException. What happens when you compile?

3. Include a catch block to handle FileNotFoundException below catch block for IOException. What happens when you compile?


Total.java

TODO: 1. Comment catch block of

NumberFOrmatException. What happens when you compile?

2. Comment catch block of IOException. What happens when you compile?

3. Include a catch block to handle FileNotFoundException below catch block for IOException. What happens?

Compile successfully

Compiler error occurs

Compiler error occurs. Move the catch block above catch block for IOException



Catching subclass exceptions• Placing catch blocks for a superclass exception type before

other catch blocks that catch subclass exception types would prevent those catch blocks from executing, so a compilation error occurs.

Only the First Matching catch Executes• If there are multiple catch blocks that match a particular

exception type, only the first matching catch block executes when an exception of that type occurs.


Catching Exceptions Three exceptions may be thrown in the try block:

• The Scanner constructor can throw a FileNotFoundException.

• Scanner.next can throw a NoSuchElementException.

• Integer.parseInt can throw a NumberFormatException.

If any of these exceptions is actually thrown, then the rest of the instructions in the try block are skipped.


Syntax 11.2 Catching Exceptions


DivisionByZero – No Exception Handlingimport java.util.Scanner;

public class DivideByZeroNoExceptionHandling{ public static int quotient( int numerator, int denominator ) { return numerator / denominator; // possible division by zero }

public static void main( String[] args ) { Scanner scanner = new Scanner( System.in ); System.out.print( "Please enter an integer numerator: " ); int numerator = scanner.nextInt(); System.out.print( "Please enter an integer denominator: " ); int denominator = scanner.nextInt();

int result = quotient( numerator, denominator ); System.out.printf("\nResult: %d / %d = %d\n", numerator, denominator, result ); }}



DivisionByZero – With Exception Handlingpublic class DivideByZeroWithExceptionHandling{ public static int quotient( int numerator, int denominator ) throws ArithmeticException { return numerator / denominator; // possible division by zero } public static void main( String[] args ) { Scanner scanner = new Scanner( System.in ); // scanner for input boolean continueLoop = true; // determines if more input is needed do { try { System.out.print( "Please enter an integer numerator: " ); int numerator = scanner.nextInt(); System.out.print( "Please enter an integer denominator: " ); int denominator = scanner.nextInt(); int result = quotient( numerator, denominator ); System.out.printf( "\nResult: %d / %d = %d\n", numerator, denominator, result ); continueLoop = false; // input successful; end looping } catch ( ArithmeticException arithmeticException ) { System.err.printf( "\nException: %s\n", arithmeticException ); System.out.println("Zero is an invalid denominator. Please try again.\n" ); } } while ( continueLoop ); } }



DivisionByZero – Avoid Runtime Exception Handling by proper coding


public class DivideByZeroNoExceptionHandling2{ public static int quotient( int numerator, int denominator ) { return numerator / denominator; // possible division by zero } public static void main( String[] args ) { Scanner scanner = new Scanner( System.in ); System.out.print( "Please enter an integer numerator: " ); int numerator = scanner.nextInt(); System.out.print( "Please enter an integer denominator: " ); int denominator = scanner.nextInt(); while(denominator==0) //avoid handling divisionByZero error by proper coding { System.out.println("Zero is an invalid denominator. Please try again.\n" ); System.out.print( "Please enter an integer numerator: " ); numerator = scanner.nextInt(); System.out.print( "Please enter an integer denominator: " ); denominator = scanner.nextInt(); } int result = quotient( numerator, denominator ); System.out.printf( "\nResult: %d / %d = %d\n", numerator, denominator, result ); }}



Checked Exceptions - throws Sometimes, it's appropriate for code to catch

exceptions that can occur within it (try/catch)

In other cases, however, it's better to let a method further up the call stack handle the exception.

Add a throws clause to the method header

E.g.

public void readData(String filename) throws FileNotFoundException{ File inFile = new File(filename); Scanner in = new Scanner(inFile); . . .}


Checked Exceptions - throws The throws clause signals to the caller of your

method that it may encounter a FileNotFoundException.• The caller must decide

o To handle the exception (try/catch)o Or declare the exception may be thrown (throws)

Throw early, catch late• Throw an exception as soon as a problem is detected.• Catch it only when the problem can be handled


Syntax 11.3 throws Clause

(Unchecked) optional


Programming Question Modify the Total3 to use throws clause in writeFileTotal method declaration. Then modify main method to: • (1) throw exceptions at user • (2) handle error in main method using try/catch blocks

public class Total3{ public static void main(String[] args){ writeFileTotal(); } public static void writeFileTotal(){

Scanner console = new Scanner(System.in); System.out.print("Input file: "); String inputFileName = console.next(); System.out.print("Output file: "); String outputFileName = console.next();

File inputFile = new File(inputFileName); Scanner in = new Scanner(inputFile); PrintWriter out = new PrintWriter(outputFileName);

while (in.hasNextDouble()){ double value = in.nextDouble(); out.printf("%15.2f\n", value); total = total + value;

} out.printf("Total: %8.2f\n", total); in.close(); out.close();

}}


Option1: Calling methods also throw exception (delegate error):

import java.io.File;import java.io.FileNotFoundException;import java.io.IOException;import java.io.PrintWriter;import java.util.Scanner;

public class Total3{ public static void main(String[] args) throws IOException{ writeFileTotal(); System.out.println("end of program"); }


public static void writeFileTotal() throws FileNotFoundException{

double total = 0;

Scanner console = new Scanner(System.in); System.out.print("Input file: "); String inputFileName = console.next(); System.out.print("Output file: "); String outputFileName = console.next(); File inputFile = new File(inputFileName); Scanner in = new Scanner(inputFile); PrintWriter out = new PrintWriter(outputFileName);

while (in.hasNextDouble()){ double value = in.nextDouble(); out.printf("%15.2f\n", value); total = total + value; } out.printf("Total: %8.2f\n", total);

in.close(); out.close(); }}



Answer Option2: calling method use try catch block to

internally handle errorimport java.io.File;import java.io.FileNotFoundException;import java.io.IOException;import java.io.PrintWriter;import java.util.Scanner;

public class Total3{ public static void main(String[] args){ try{ writeFileTotal(); } catch(FileNotFoundException exp) { System.out.println("File not found!"); } System.out.println("end of program"); }


public static void writeFileTotal() throws FileNotFoundException{

double total = 0;

Scanner console = new Scanner(System.in); System.out.print("Input file: "); String inputFileName = console.next(); System.out.print("Output file: "); String outputFileName = console.next(); File inputFile = new File(inputFileName); Scanner in = new Scanner(inputFile); PrintWriter out = new PrintWriter(outputFileName);

while (in.hasNextDouble()){ double value = in.nextDouble(); out.printf("%15.2f\n", value); total = total + value; } out.printf("Total: %8.2f\n", total);

in.close(); out.close(); }}



The finally Clause A try block may optionally have a finally block

associated with it.

If provided, a finally block follows any catch blocks associated with that same try block.

try{ …}catch(FileNotFoundException e){

…}catch(IOException e){

…}finally{ …}


The finally Clause Use finally block when you do some clean up

• Example - closing files

PrintWriter out = new PrintWriter(filename);try{ writeData(out);}catch(IOException e){e.printStackTrace();

}finally{ out.close();}

Executes the close even if an exception is thrown.


The code within a finally block is guaranteed to execute no matter what happens in the try/catch code that precedes it:

• The try block executes to completion without throwing any exceptions whatsoever.

• The try block throws an exception that is handled by one of the catch blocks.

• The try block throws an exception that is not handled by any of the catch blocks


Syntax 11.4 finally Clause


Designing Your Own Exception Types

You can design your own exception types • subclasses of Exception or RuntimeException.

Throw an InsufficientFundsException when the amount to withdraw an amount from a bank account exceeds the current balance.if (amount > balance){ throw new InsufficientFundsException( "withdrawal of " + amount + " exceeds balance of " + balance);}

Make InsufficientFundsException an unchecked exception • Extend RuntimeException or one of its subclasses


Designing Your Own Exception Types

Supply two constructors for the class • A constructor with no arguments• A constructor that accepts a message string describing reason

for exception

public class InsufficientFundsException extends RuntimeException{ public InsufficientFundsException() {} public InsufficientFundsException(String message) { super(message); }

}



Implement InsufficientFundsException class

Modify BankAccount class to throw this exception when withdrawal amount exceeds balance.


Answer

public class InsufficientFundsException extends RuntimeException{ public InsufficientFundsException() {} public InsufficientFundsException(String message) { super(message); }}

InsufficientFundsException.java


Answerpublic class BankAccount{ private double balance;



public void withdraw(double amount) { if(amount>balance) throw new InsufficientFundsException("Amount exceeds Balance!"); balance = balance - amount; }




Documents

Copyright © 2014 by John Wiley & Sons. All rights reserved.1 Input/Output and Exception Handling