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File IO. Includes Exceptions. Topics. General concepts Exception handling Creating a file in Eclipse Use of Scanner class Select a file (LineNumberer) Hard-code name Accept from keyboard File Chooser Command Line Exceptions (FileTotal) throws clause try/catch. Throw an exception

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File IO

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File io

File IO

Includes Exceptions



  • General concepts

    • Exception handling

    • Creating a file in Eclipse

    • Use of Scanner class

  • Select a file (LineNumberer)

    • Hard-code name

    • Accept from keyboard

    • File Chooser

    • Command Line

  • Exceptions (FileTotal)

    • throws clause

    • try/catch

  • Throw an exception

  • Create your own Exception (NegativeBalanceException)

Be prepared for two quick exercises – start Eclipse now! Create a project named ExceptionsDemo

Handling errors

Handling Errors

try – prepare to execute statement that might yield an error

error – exception is thrown

error – exception is caught

success – continue on

You need to think about:

  • what statements might cause errors?

  • how to handle an error that occurs

Prepare to read

Prepare to read

  • FileReader is used to read data from file

  • Test to ensure you can find the file.

  • Wrap the FileReader in a Scanner object (pass the FileReader to the constructor)

  • Handle errors in data format.

FileReader reader = new FileReader(“in.txt”);

Scanner in = new Scanner(reader);









Prepare for output

Prepare for output

  • Use PrintWriter. (from documentation):

    • Print formatted representations of objects to a text-output stream.

    • Implements all of the print methods found in PrintStream.

    • Does not contain methods for writing raw bytes.

    • If automatic flushing is enabled it will be done only when one of the println() methods is invoked.

    • Methods include println (variety of parameters), write, close, flush, checkError (no exceptions thrown, must check for errors).

Let s get started create a file

Let’s get started – create a file

  • We’ll create a simple text file within Eclipse.

  • Choose File->New->Untitled Text (or just file)

  • Be sure to save in the project – not inside a package

Line numberer

Line Numberer

  • Read an input file

  • Write the lines, with line numbers, to an output file

Linenumberer basic structure

LineNumberer – basic structure

  • We will need names for both the input and output files

  • We’ll see 4 different ways to do this. For now, set up variables.

    public class LineNumberer {

    private String inputName;

    private String outputName;

    public void setOutputName(String outputName) {

    this.outputName = outputName;


    public void setInputName(String inputName) {

    this.inputName = inputName;


    // rest of class


Number the lines

Number the lines

public void numberLines() throws FileNotFoundException {

FileReader reader = new FileReader(inputName);

Scanner in = new Scanner(reader);

PrintWriter out = new PrintWriter(outputName);

int lineNumber = 1;

while (in.hasNextLine()) {

String line = in.nextLine();

out.println("/* " + lineNumber + " */ " + line);





FileReader line “opens” the file

What if files is not there? EXCEPTION!

For now, we “acknowledge” with a throws clause

Option 1 hardcode file names

Option 1: hardcode file names

  • In main

    LineNumberer lines1 = new LineNumberer();




This is clearly not the most flexible option! Use sparingly!

Option 2 prompt for names

Option 2: prompt for names

public void promptForInputFile() {

Scanner scan = new Scanner(;

System.out.print("Enter the input file name: ");

inputName =;


  • In main

    LineNumberer lines2 = new LineNumberer();




Option 3 file chooser

Option 3: File Chooser

  • Can use javax.swing.JFileChooser.

    public void chooseFileName() {

    JFileChooserchooser = new JFileChooser();

    if (chooser.showOpenDialog(null) ==

    JFileChooser.APPROVE_OPTION) {

    // static int

    File selectedFile = chooser.getSelectedFile();

    inputName= selectedFile.getAbsolutePath();



  • In main

    LineNumberer lines3 = new LineNumberer();




Option 4 command line

Option 4: Command line

  • Can accept filenames on the command line. May be easier to automate (e.g., put in a script)

    LineNumberer lines4 = new LineNumberer();


    if (args.length > 0) {




Eclipse new run configuration

Eclipse New Run Configuration

Select Run icon

Press New Configuration Icon (be sure Java application is selected

Fill in Name,

Project and

Main Class as


Eclipse arguments

Eclipse Arguments

  • Click on Arguments tag to add Program Arguments

File total

File Total

  • Program to calculate the sum of the values in a file

  • Handles situation if file not found

  • Handles situation if non-numeric value found

Filetotal program structure

FileTotal - Program Structure

public class FileTotal {

private int sum;

private String inFile;

public FileTotal(String inFile) {

this.inFile = inFile;


public void displaySum() {

System.out.println("Sum is " + sum);


public static void main(String[] args) {

FileTotal ft = new FileTotal("numbers.txt");





The fun part

The fun part

public void sumFileValues() {

FileReader reader = null;

Scanner in = null;


reader = new


in = new Scanner(reader);

} catch

(FileNotFoundException e) {




String strNum = null;

try {

while (in.hasNext()) {

strNum =;

int num =


sum += num;


} catch

(NumberFormatException e)



non-numeric value " +




Quick exercise 5 minutes

Quick Exercise – 5 minutes!

  • Create a Java Project called ExceptionsDemo

  • Create a class named ReadAFile

  • Create a method named loadFile

  • Write lines of code in loadFile to:

    • open a file named “numbers.txt”

    • read the first number

    • display on console

    • put the code in a try/catch block

    • display an error if file not found

NOTE: names are just suggestions, this will not be turned in

Software engineering quality tip

Software Engineering Quality Tip

  • Throw Early, Catch Late. Better to throw an exception than come up with an imperfect fix. What should program do if a file is not found? Is it always an error?

  • Do not squelch exceptions!



    FileReader reader = new FileReader(filename);

    } catch (Exception e) {} // So there!

Checked and unchecked

Checked and Unchecked

  • Checked exceptions – can’t ignore, compiler requires that you handle or propagate

  • Unchecked exceptions – can be ignored. Generally subclass of RuntimeException, like NumberFormatException or NullPointerException. Too expensive to check all of these, often due to programmer error (checked exceptions often user error, like InputMismatchException)

  • If you aren’t going to handle a checked exception, put throws clause on function header:

    public void read(String filename) throws FileNotFoundException

    { . . .}

Quick exercise 5 minutes1

Quick Exercise – 5 minutes

  • Remove the try/catch from your previous exercise. What error message do you see?

  • Put a “throws clause” on your loadFile method. What error do you see?

  • Put a “throws clause” on main.

  • Run the program. What happens?

  • Put the lines: Point p; system.out.println(p.getX()); in main. Does the compiler show an error? Would there be an error when you run the program?

Exception hierarchy

Exception Hierarchy























Finally clause

Finally Clause

  • Sometimes cleanup is needed even after an exception occurs

    PrintWriter out = new PrintWriter(filename);

    try {





    out.close(); // always executed, including after

    } // exception is handled in a catch

Throwing an exception

Throwing an Exception

public class ThrowException {

public void getInput() throws Exception {

Scanner scan = new Scanner(;

System.out.print("Enter a number from 1 - 100: ");

int num = scan.nextInt();

if (num < 1 || num > 100)

throw new Exception("The value " +

num + " is not between 1 and



public static void main(String[] args) throws Exception {

ThrowException demo = new ThrowException();

try {


} catch (Exception e) {






Is this a good use of exceptions? Probably not… if you can handle immediately with if/else logic, that’s better!

But this example at least shows the mechanics of throwing an exception.

Restricted file total

Restricted File Total

  • Calculates the sum of values in file

  • Values must be greater than 0

NOTE: In reality, we might handle this with a simple if/else

But this is a simple exercise to see how custom exceptions work

Exception Best Practices are discussed in Programming Languages

Restrictedfiletotal program structure

RestrictedFileTotal - Program Structure

public class RestrictedFileTotal {

private int sum;

private String inFile;

public RestrictedFileTotal(String inFile) {

this.inFile = inFile;


public void displaySum() {

System.out.println("Sum is " + sum);



Designing your own exception

Designing your own Exception

public class NegativeNumberException extends Exception {

private int number;

public NegativeNumberException() {}

public NegativeNumberException(String message) {



public NegativeNumberException(int number) {

super("Error: value can't be < 0, input = " + number);

this.number = number;


public String toString() {

return "Error: negative number encountered " + number;



Could extend RuntimeException, if programmer can prevent

Do we really need number? Probably not! Again, this is just to show the mechanics in a simple example.

Throw your exception

Throw your Exception

public void sumFileValues() throws

NegativeNumberException {

FileReader reader = null;

Scanner in = null;

String strNum = null;

try {

reader = new FileReader(inFile);

in = new Scanner(reader);

while (in.hasNext()) {

strNum =;

int num = Integer.parseInt(strNum);

if (num < 0)

throw new NegativeNumberException(num);

sum += num; }


catch (FileNotFoundException e) {


} catch (NumberFormatException e) {

System.out.println("Error, non-numeric value “

+ strNum);



Catch your exception

Catch your Exception

public static void main(String[] args) {

RestrictedFileTotalft = new


try {


} catch (NegativeNumberException e) {





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