Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 1 Files

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 2 Files To use files, use File class File class can’t read/write To read/write files, use Reader/Writer classes File input can be text-based (what we’ll do), stream-based

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 3 Readers, writers, streams Readers and writers deal with textual input. –Based around the char type. Streams deal with binary data. –Based around the byte type.

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 4 Text output Use the FileWriter class. –Tie File object to diskfile name –Open a FileWriter; tie to File. –Write to the FileWriter. –Close the FileWriter. Failure at any point results in an IOException.

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 5 Text output try { FileWriter writer = new FileWriter("name of file"); while(there is more text to write) {... writer.write(next piece of text);... } writer.close(); } catch(IOException e) { something went wrong with accessing the file }

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 6 // file output. public void writeto(String fileout, int nbrlines) throws IOException // have throws or throw { File outfile = new File(fileout); FileWriter fw = new FileWriter(outfile); fw.write("There were " + nbrlines + “ lines in the input file."); fw.close(); }

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 7 import java.io.*; public class countlines { // all methods that do something inserted here public static void main(String[] args) { if (args.length != 2) { System.out.println("to run, type: 'countlines filein fileout'"); System.exit(1); } // create a new countline object with first arg as input file, second as output countlines cl = new countlines(args[0], args[1]); }

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 8 Write a program to count number of lines in a file and write the number to an output file java countlines myinfile.txt myoutfile.txt would return 2 if file is: This is a file with two lines. myinfile.txt

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 9 // constructor public countlines(String filein, String fileout) { try { int nbrLines = readfrom(filein); writeto(fileout, nbrLines); } catch (IOException e) { // will catch IO exception not caught by readfrom and writeto System.out.println("Problems with IO. " + " Program aborted without successful write."); }

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 10 Scanner Class: for reading files public void readfrom(String filein) { Scanner scanner; try { scanner = new Scanner(new File(filein)); String line = scanner.nextLine( ); // also next( ), nextInt( ), nextDouble( ) while (line != null) { System.out.println(line); // for debugging only; countlines++; } catch (FileNotFoundException ex) { System.out.println("File not found...aborting program."); System.exit(1); } }

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 11 Console input Scanner class: Java 5 import java.util.*; class MyScanner { public MyScanner ( ) { Scanner scanner = new Scanner(System.in); System.out.println("Enter a number: "); int x = scanner.nextInt(); int y = scanner.nextInt(); System.out.println("The sum of " + x + " + " + y + " is " + (x+y)); }

Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level 12 File input using Scanner import java.util.*; import java.io.*; class MyScanner { public MyScanner ( ) { Scanner scanner; try { scanner = new Scanner(new File("infile.java")); int x = scanner.nextInt( ); int y = scanner.nextInt( ); System.out.println("The sum of " + x + " + " + y + " is " + (x+y)); } catch (FileNotFoundException ex) { System.out.println("File not found...aborting program."); System.exit(1); } }