file io includes exceptions. topics general concepts exception handling creating a file in...
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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 Create your own
Exception (NegativeBalanceException)
Be prepared for two quick exercises – start Eclipse now! Create a project named ExceptionsDemo
Handling Errors
try – prepare to execute statement that might yield an error
success – continue on
error – exception is thrown
You need to think about:• what statements might cause errors?• how to handle an error that occurs
error – exception is caught
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.
Scanner in = new Scanner(reader);
“in.txt” FileReader
FileReader
Scanner Stringintdoubleetc.
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 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 Read an input file Write the lines, with line numbers, to an
output file
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 linespublic 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);
lineNumber++;
}
out.close();
}FileReader line “opens” the fileWhat if files is not there? EXCEPTION! For now, we “acknowledge” with a throws clause
Option 1: hardcode file names In mainLineNumberer lines1 = new LineNumberer();
lines1.setInputName("srcCode.txt");
lines1.setOutputName("srcCode1.out");
lines1.numberLines();
This is clearly not the most flexible option! Use sparingly!
Option 2: prompt for namespublic void promptForInputFile() {
Scanner scan = new Scanner(System.in);
System.out.print("Enter the input file name: ");
inputName = scan.next();
}
In mainLineNumberer lines2 = new LineNumberer();
lines2.setOutputName("srcCode2.out");
lines2.promptForInputFile();
lines2.numberLines();
Option 3: File Chooser• Can use javax.swing.JFileChooser. public void chooseFileName() {
JFileChooser chooser = new JFileChooser();
if (chooser.showOpenDialog(null) ==
JFileChooser.APPROVE_OPTION) {
// static int
File selectedFile = chooser.getSelectedFile();
inputName = selectedFile.getAbsolutePath();
}
} In main
LineNumberer lines3 = new LineNumberer();
lines3.setOutputName("srcCode3.out");
lines3.chooseFileName();
lines3.numberLines();
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();
lines4.setOutputName("srcCode4.out");
if (args.length > 0) {
lines4.setInputName(args[0]);
lines4.numberLines();
}
Eclipse New Run Configuration
Fill in Name,Project and Main Class asneeded
Select Run iconPress New Configuration Icon (be sure Java application is selected
Eclipse Arguments Click on Arguments tag to add Program Arguments
What did we just learn? How to open a file How to read from a file using Scanner Four ways to specify file names The throws clause
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 Structurepublic 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");
ft.sumFileValues();
ft.displaySum();
}
}
The fun partpublic void sumFileValues() {
FileReader reader = null;
Scanner in = null;
try {
reader = new
FileReader(inFile);
in = new Scanner(reader);
} catch
(FileNotFoundException e) {
System.out.println
(e.getLocalizedMessage());
}
String strNum = null;
try {
while (in.hasNext()) {
strNum = in.next();
int num =
Integer.parseInt(strNum);
sum += num;
}
} catch
(NumberFormatException e)
{
System.out.println("Error,
non-numeric value " +
strNum);
} }
What if we wanted to print error but continue summing?
Quick Exercise – 5 minutes!Goal: Try basic error handlingCreate a Java Project called ExceptionsDemo Create a class named ReadAFileCreate a method named loadFileWrite 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 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!try
{
FileReader reader = new FileReader(filename);
…
} catch (Exception e) {} // So there!
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 minutesGoal: Understand checked/unchecked exceptionsRemove 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
Throwable
Error
NullPointerException
NoSuchElementException
IndexOutOfBoundsException
IllegalStateException
ClassCastException
ArithmeticException
RuntimeException
NumberFormatException
ArrayIndexOutOfBoundsException
Exception
IOException
EOFException
FileNotFoundException
MalformedURLException
UnknownHostException
ClassNotFoundException
CloneNotSupportedException
Finally Clause• Sometimes cleanup is needed even after
an exception occurs
PrintWriter out = new PrintWriter(filename);try { writeData(out);}finally{out.close(); // always executed, including after
} // exception is handled in a catch
Throwing an Exceptionpublic class ThrowException {
public void getInput() throws Exception {
Scanner scan = new Scanner(System.in);
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
100!");
}
public static void main(String[] args) throws Exception {
ThrowException demo = new ThrowException();
try {
demo.getInput();
} catch (Exception e) {
System.out.println(e);
System.out.println(e.getMessage());
}
}
}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 Calculates the sum of values in file Values must be greater than 0
NOTE: In reality, we might handle this with a simple if/elseBut this is a simple exercise to see how custom exceptions workException Best Practices are discussed in Programming Languages
RestrictedFileTotal - Program Structurepublic 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 public class NegativeNumberException extends Exception {
private int number;
public NegativeNumberException() {}
public NegativeNumberException(String message) {
super(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 Exceptionpublic 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 = in.next();
int num = Integer.parseInt(strNum);
if (num < 0)
throw new NegativeNumberException(num);
sum += num; }
}
catch (FileNotFoundException e) {
System.out.println(e.getLocalizedMessage());
} catch (NumberFormatException e) {
System.out.println("Error, non-numeric value “
+ strNum);
}
}
Catch your Exception
public static void main(String[] args) {RestrictedFileTotal ft = new
RestrictedFileTotal("numbers.txt");try {
ft.sumFileValues();} catch (NegativeNumberException e) {
System.out.println(e);}ft.displaySum();
}