Lecture 20: Java File I/o

LECTURE 20: JAVA FILE I/O

CSC 213 – Large Scale Programming

Project #1 Recap

Today's Goals

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

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

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

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 Creates 28.5 GB/min for nerds seeking

truth & beauty




truth & beauty




truth & beauty Hired trained monkeys to type data into

programs





programs college students





programs college students

Yeah, Right

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 sequentiallyScanner scan ;// Instantiate a Scanner scan for the “file” belowchar c = ‘’;while (c != ‘s’) {

c = scan.nextChar(); }

scan

Are 10^15 Files Just a Peta-File?

Typical File I/O



scan


Typical File I/O



scan


Typical File I/O



scan


Typical File I/O



scan


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 itRandomAccessFile 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 Strings 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 sequentiallyRandomAccessFile 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


Sequential access is no longer requiredRandomAccessFile 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"












"ould I rename machine "PetaHertz"

For Next Lecture

For Next Lecture

Week #7 assignment available 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.

Documents

Lecture 20: Java File I/o