1. Introduction to Programming G50PRO University of Nottingham Unit 11 : Files Input/Ouput Paul Tennent http://paultennent.wordpress.com/G50PRO.html Paul.tennent@nottingham.ac.uk Room C7

2. 2http://www.cs.nott.ac.uk/~eoe Overview Java I/O UserInput Class Demo Exceptions Intro try, catch, and finally blocks Exceptions Example Files I/O File Examples Extra Reading

3. 3http://www.cs.nott.ac.uk/~eoe Java I/O Input/output operations direct data from the keyboard, files or programs into your program and from there to the screen, printer, files or other programs. Java uses the concept of streams to manage this flow of data. System.in and System.out objects read and write from the standard input and output streams. Java also uses file streams, data streams, pipe streams and object streams to manipulate I/O. For these streams you must access the io class library using: import java.io.*

4. 4http://www.cs.nott.ac.uk/~eoe UserInput Class Demo // readChar Method public static char readChar() { char returnValue = 'a'; DataInputStream dis = new DataInputStream(System.in); try { String userInput = new String(dis.readLine()); //return just the first character returnValue = userInput.charAt(0); } catch(Exception e) { System.out.println("Exception while reading user's input as a char"); } return returnValue; } // end readChar()

5. 5http://www.cs.nott.ac.uk/~eoe Exceptions Intro The Java programming language uses exceptions to enable the developer to handle errors or exceptional events The term exception is shorthand for the phrase "exceptional event." An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions try, catch, and finally blocks are used to write an exception handler

6. 6http://www.cs.nott.ac.uk/~eoe try, catch, and finally blocks enclose the code that might throw an exception within a try block provide one or more catch blocks directly after the try block to deal with different expected exceptions Each catch block is an exception handler and handles the type of exception indicated by its argument The finally block always executes when the try block exits. This ensures that the finally block is executed even if an unexpected exception occurs – usually used for cleanup code

7. 7http://www.cs.nott.ac.uk/~eoe Exceptions Example : public static void main (String[] args){ int[] ar = new int[10]; try { //trying to write to the 10th element of the array ar[10] = 0; } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught " + "ArrayIndexOutOfBoundsException: " + e.getMessage()); } finally { System.out.println("finally block executed"); } }//end main

8. 8http://www.cs.nott.ac.uk/~eoe Files I/O Access file properties by creating a file handle using File class File myFile = new File(“data.txt"); // name in current dir A file handle can be used d by various file class methods to access the properties of a specific file Use file streams which are primitive streams whose sources or destinations are files. Byte based streams [8-bit] FileInputStream and FileOutputStream Character based streams [16-bit] FileReader and FileWriter

9. 9http://www.cs.nott.ac.uk/~eoe Example : Read File public static void main( String[] args ) { try{ int n; FileInputStream fis = new FileInputStream("data1.csv"); while ( ( n = fis.available() ) > 0 ) { byte[] b = new byte[ n ]; int result = fis.read( b ); if ( result != -1 ){ String s = new String( b ); System.out.print( s ); } } // end while } catch ( IOException e ) { System.err.println( "Error: " + e.getMessage() ); } } // end main

10. 10http://www.cs.nott.ac.uk/~eoe Example : Create empty File public static void main( String[] args ) { Path file = “data.txt”; try { file.createFile(); //Create the empty file with default permissions, etc. } catch (FileAlreadyExists x) { System.err.println("file named already exists”); } catch (IOException x) { //Some other sort of failure, such as permissions. System.err.println(“Error:” + x.getMessage()); } } // end main

11. 11http://www.cs.nott.ac.uk/~eoe Example : Write to File public static void main( String[] args ) { FileOutputStream fos; try{ fos = new FileOutputStream( "data.txt" ); String s = "Test write to file "; byte data[] = s.getBytes(); fos.write( data ); } catch ( IOException e ) { System.err.println( e.getMessage() ); } } //end main

12. 12http://www.cs.nott.ac.uk/~eoe Extra Reading http://java.sun.com/docs/books/tutorial/essent ial/io/file.html

13. 13http://www.cs.nott.ac.uk/~eoe Summary Java I/O UserInput Class Demo Exceptions Intro try, catch, and finally blocks Exceptions Example Files I/O File Examples Extra Reading