CSC 213 – Large Scale Programming

Project #1 Recap

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: RandomAccessFile s  Benefits of using this for reading & writing data  How this also has additional ways to access data

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

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

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

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

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

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

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

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

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

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

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 (FileNotFoundException fnfe) { System.err.println("Make the file, moron!"); } catch (IOException ioe) { ioe.printStackTrace(); }

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

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

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

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

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

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

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 FileWriter nuked=new FileWriter("boom.t", false); FileWriter saved = new FileWriter("ScoreOnRebound",true);

Second Step To Writing Files  FileWriter helps, but slow and hard to use  Faster, simpler approach would be much nicer  Using FileWriter create 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 s to file  End writing & save results with close()

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

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

RandomAccessFile  Built into Java's standard set of classes  Found in the java.io package  New or existing files can be accessed with it RandomAccessFile raf = 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

Reading RandomAccessFile  Defines methods to read most primitive types: boolean readBoolean() int readInt() 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

Reading RandomAccessFile  Reading String s takes a little extra work String 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

Writing RandomAccessFile  Also defines methods to write to a file: void writeInt(int i) 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

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

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

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

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

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

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

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

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