edsger dijkstra
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Edsger Dijkstra
[M]ost important, but also most elusive, aspect of any tool is its influence on the
habits of those who train themselves in its use.
LECTURE 10:FILE I/O IN JAVA
CSC 213 – Large Scale Programming
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
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 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 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 Text File
Surprise! Already know how to do this work In fact, this is part of Java been using for
long time Based upon Unix arcana that is suddenly
relevant Unix (& Java) treat everything as file
Keyboards, files, network connections, mice, etc.
java.util.Scanner works with all files Change constructor call & this will be good
to go
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
This is an example file we access
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
This is an example file we access
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
This is an example file we access
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
This is an example file we access
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
This is an example file we access
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 fw=new FileWriter("b.txt", false);FileWriter app=new FileWriter("bob.java",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();
}
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 needed for these to work 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);
}This is an example file we access
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 requiredRandomAccessFile raf = new …; char c;raf.skipBytes(raf.length()-1);c = (char)raf.readByte();raf.seek(0);raf.writeByte((byte)c);
This is an example file we access
RandomAccessFile I/O
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);
shis is an example file we access
For Next Lecture
Week #4 assignment available on Angel Continues to be due Wednesday at 5PM Ask me questions, if you have trouble on a
problem
Will be talking about indexed files on Friday What are they and why should Steve be
excited? Why does CS department require you to
learn them? Why does it seem like they are part of
project #1?
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