Lecture 20: Java File I/o

1. CSC 213 – Large Scale Programming Lecture 20: Java File I/o

2. Project #1 Recap

3. Today's Goals

4. Today's Goals • Discuss reasons why files & file I/O important • When & where used and what real value does it offer? • Show how to read & write text to files in Java • Classes & methods needed to perform these actions • How these methods move through file as they work • Limits of these actions & why we might want more • Discuss another approach: RandomAccessFiles • Benefits of using this for reading & writing data • How this also has additional ways to access data

5. Image To Sharpen • I have a (fuzzy) 1024 x 768 picture to sharpen • Only 786,432 numbers to type into photo application • After analysis, must click & update each pixel

6. Image To Sharpen • I have a (fuzzy) 1024 x 768 picture to sharpen • Only 786,432 numbers to type into photo application • After analysis, must click & update each pixel

7. More Data Entry Positions • Testing improved jet designs for oeingB-ay • Using program to simulate designs' lift & drag • 5 possible designs (each 150MB) to test this iteration • Once results available, will tweak & retest designs • Need room of touch typists for all this data entry

8. This Is (Semi-Real) Problem • Large hadron collider about to come on-line • No black hole when smashing particles at high speeds • Creates 28.5 GB/minfor nerds seeking truth & beauty

9. This Is (Semi-Real) Problem • Large hadron collider about to come on-line • No black hole when smashing particles at high speeds • Creates 28.5 GB/minfor nerds seeking truth & beauty

10. This Is (Semi-Real) Problem • Large hadron collider about to come on-line • No black hole when smashing particles at high speeds • Creates 28.5 GB/minfor nerds seeking truth & beauty • Hired trained monkeys to type data into programs

11. This Is (Semi-Real) Problem • Large hadron collider about to come on-line • No black hole when smashing particles at high speeds • Creates 28.5 GB/minfor nerds seeking truth & beauty • Hired trained monkeys to type data into programs college students

12. This Is (Semi-Real) Problem • Large hadron collider about to come on-line • No black hole when smashing particles at high speeds • Creates 28.5 GB/minfor nerds seeking truth & beauty • Hired trained monkeys to type data into programs college students

13. Yeah, Right

14. Yeah, Right • Real world demands we use files for most I/O • Data files used to start and/or end most projects • May contain: game levels, analysis results, CCD pics • Way to read & write files needed to be useful

15. Reading a Text File • Must first instantiate java.io.File object • Pass String filename to the File constructor • Throws a (checked) exception if file does not exist • Another IOException possible for other odd errors • Once created, use File to create Scanner • Reads file's data rather than typing into keyboard • At the same time, works like any other Scanner

16. Reading a Text File try {File readFile = new File("bob.dat");Scanner scan = new Scanner(readFile);while (scan.hasNext()) { String line = scan.nextLine();System.out.println(line);}scan.close(); } catch (FileNotFoundExceptionfnfe) {System.err.println("Make the file, moron!"); } catch (IOExceptionioe) {ioe.printStackTrace(); }

17. Typical File I/O • Ordinarily we read files sequentially Scannerscan ;// Instantiate a Scannerscanfor the “file” belowchar c = ‘’;while (c != ‘s’) { c = scan.nextChar();} Are 10^15 Files Just a Peta-File? scan

18. Typical File I/O • Ordinarily we read files sequentially Scannerscan ;// Instantiate a Scannerscanfor the “file” belowchar c = ‘’;while (c != ‘s’) { c = scan.nextChar();} Are 10^15 Files Just a Peta-File? scan

19. Typical File I/O • Ordinarily we read files sequentially Scannerscan ;// Instantiate a Scannerscanfor the “file” belowchar c = ‘’;while (c != ‘s’) { c = scan.nextChar();} Are 10^15 Files Just a Peta-File? scan

20. Typical File I/O • Ordinarily we read files sequentially Scannerscan ;// Instantiate a Scannerscanfor the “file” belowchar c = ‘’;while (c != ‘s’) { c = scan.nextChar();} Are 10^15 Files Just a Peta-File? scan

21. Typical File I/O • Ordinarily we read files sequentially Scannerscan ;// Instantiate a Scannerscanfor the “file” belowchar c = ‘’;while (c != ‘s’) { c = scan.nextChar();} Are 10^15 Files Just a Peta-File? scan

22. Writing a Text File • Writing a text file only slightly more complicated • Console is file in Unix, so can guess where this goes • Need to first decide what should happen to file • Easy if file does not exist create file & write to it • Else what should happen to file's current contents? • Mode used at opening determines file's contents • If opening file in write mode, erases file at the start • Starts at end of file in append mode, saving the data

23. Opening File For Writing • Create instance of java.io.FileWriter • Must specify mode to open file at this time • Be very careful with this – there is no undo here! • If file is impossible and so cannot be written to • Cannot be done, so system throws IOException • Not told if file existed before this command FileWriternuked=new FileWriter("boom.t", false); FileWritersaved = new FileWriter("ScoreOnRebound",true);

24. Second Step To Writing Files • FileWriter helps, but slow and hard to use • Faster, simpler approach would be much nicer • Using FileWritercreate BufferedWriter • Cannot change mode; must take care initially • Two methods used to write out data to file • Both methods will expand file & advance pointer • Start writing new line – newLine() • write(String s)– writes sto file • End writing & save results with close()

25. Writing a Text File try {FileWriterfw = new FileWriter(“b.t”, true);BufferedWriterbw = new BufferedWriter(fw);for (inti = 10; i > 0; i--) {bw.write(“T minus ”);bw.write(i + “”);bw.newLine();}bw.write(“Blast off!”); bw.close(); }catch (IOExceptionioe) {ioe.printStackTrace(); }

26. Its Not All Text • We often want to store more than just text • Translate numbers into binary to be used in program • Storing as text wastes time converting back & forth • (Often) Space also wasted for larger numbers • Could instead store numbers in binary format • Optimized for machine, as not easily human-readable • But how often do we look at numbers in image file? • Easy to determine sizes; each type has specific length • To enable binary formats, use different File class

27. RandomAccessFile • Built into Java's standard set of classes • Found in the java.io package • New or existing files can be accessed with it RandomAccessFileraf = new RandomAccessFile("f.txt","rw"); • First argument ("f.txt") is name of file used • Access to file specified ("rw") in second parameter • Using write access ("w") erases any data in the file • Read & write anywhere in file using instance

28. Reading RandomAccessFile • Defines methods to read most primitive types:booleanreadBoolean()intreadInt() double readDouble() • Reads & returns value read from file • Binary encoding used automatically • File will store 32-bit int, not "125" • Not human readable, but not really needed • Can shrink files; always makes sizes predictable

29. Reading RandomAccessFile • Reading Strings takes a little extra workString readUTF() • Requires that String was recorded in UTF format • Not totally readable, but makes sense to machines • Or use readChar()to read in String… • …but need null character ('\0') at end • End of String not easy to find without some hint • Also remember that Java’s char not always readable • readByte()is readable, but needs typecast

30. Writing RandomAccessFile • Also defines methods to write to a file: void writeInt(inti) void writeDouble(double d)void writeUTF(String s) • Writes value at location in the file we are currently at • As it is needed, methods extend file also • When writing data, erases anything there previously

31. RandomAccessFile I/O • Unless specified stillread &write sequentially RandomAccessFileraf = new …;char c = ‘’;while (c != ‘s’) { c = (char)raf.readByte();raf.writeByte((byte)c);} Could I rename machine "PetaHertz"

32. Skipping Around The File • RandomAccessFile allows moving in the file • Skip past sections using intskipBytes(int n) • void seek(long pos)moves to position in file • Positions specified as bytes from beginning of file

33. RandomAccessFile I/O • Sequential access is no longer required RandomAccessFileraf = new …;char c;raf.skipBytes(raf.length()-1);c = (char)raf.readByte();raf.seek(0);raf.writeByte((byte)c); Could I rename machine "PetaHertz"

34. RandomAccessFile I/O • Sequential access is no longer required RandomAccessFileraf = new …;char c;raf.skipBytes(raf.length()-1);c = (char)raf.readByte();raf.seek(0);raf.writeByte((byte)c); Could I rename machine "PetaHertz"

35. RandomAccessFile I/O • Sequential access is no longer required RandomAccessFileraf = new …;char c;raf.skipBytes(raf.length()-1);c = (char)raf.readByte();raf.seek(0);raf.writeByte((byte)c); Could I rename machine "PetaHertz"

36. RandomAccessFile I/O • Sequential access is no longer required RandomAccessFileraf = new …;char c;raf.skipBytes(raf.length()-1);c = (char)raf.readByte();raf.seek(0);raf.writeByte((byte)c); Could I rename machine "PetaHertz"

37. RandomAccessFile I/O • Sequential access is no longer required RandomAccessFileraf = new …;char c;raf.skipBytes(raf.length()-1);c = (char)raf.readByte();raf.seek(0);raf.writeByte((byte)c); "ould I rename machine "PetaHertz"

38. For Next Lecture

39. For Next Lecture • Week #7 assignmentavailable on Angel • Still have time to talk to me; due Tuesday at 5PM • Will talk about indexed files on Wednesday • Why does Oracle love them & heavily rely on them? • Why does CS department require you to learn them? • Why do you feel like they are key idea for project #2? • Midterm #1 in class on Friday • Open-book, open-note exam (as usual) • Use any lecture’s slides IF you have notes on them • Ask me questions ASAP; I leave for conference Wed.