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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)

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Presentation on theme: "File IO Includes Exceptions. Topics  General concepts  Exception handling  Creating a file in Eclipse  Use of Scanner class  Select a file (LineNumberer)"— Presentation transcript:

1 File IO Includes Exceptions

2 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

3 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

4 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 Scanner String int double etc. FileReader reader = new FileReader(“in.txt”);

5 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).

6 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

7 Line Numberer  Read an input file  Write the lines, with line numbers, to an output file

8 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 }

9 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); lineNumber++; } out.close(); } FileReader line “opens” the file What if files is not there? EXCEPTION! For now, we “acknowledge” with a throws clause

10 Option 1: hardcode file names  In main LineNumberer lines1 = new LineNumberer(); lines1.setInputName("srcCode.txt"); lines1.setOutputName("srcCode1.out"); lines1.numberLines(); This is clearly not the most flexible option! Use sparingly!

11 Option 2: prompt for names public void promptForInputFile() { Scanner scan = new Scanner(System.in); System.out.print("Enter the input file name: "); inputName = scan.next(); }  In main LineNumberer lines2 = new LineNumberer(); lines2.setOutputName("srcCode2.out"); lines2.promptForInputFile(); lines2.numberLines();

12 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();

13 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(); }

14 Eclipse New Run Configuration Fill in Name, Project and Main Class as needed Select Run icon Press New Configuration Icon (be sure Java application is selected

15 Eclipse Arguments  Click on Arguments tag to add Program Arguments

16 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

17 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

18 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"); ft.sumFileValues(); ft.displaySum(); }

19 The fun part public 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?

20 Quick Exercise – 5 minutes! Goal: Try basic error handling  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

21 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!

22 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 {...}

23 Quick Exercise – 5 minutes Goal: Understand checked/unchecked exceptions  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?

24 Exception Hierarchy Throwable Error NullPointerException NoSuchElementException IndexOutOfBoundsException IllegalStateException ClassCastException ArithmeticException RuntimeException NumberFormatException ArrayIndexOutOfBoundsException Exception IOException EOFException FileNotFoundException MalformedURLException UnknownHostException ClassNot FoundException CloneNot Supported Exception

25 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

26 Throwing an Exception public class ThrowException { public void getInput() throws Exception { Scanner scan = new Scanner(System.in); System.out.print("Enter a number from : "); int num = scan.nextInt(); if (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.

27 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

28 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); }

29 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.

30 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 = 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); }

31 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(); }


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