1. CSS161: Fundamentals of Computing
File I/O
This slide set was compiled from the Absolute Java textbook slides (Walter Savitch) and the professor’s own class materials.
CSS161: Fundamentals of Computing

2. CSS161: Fundamentals of Computing
Streams
Stream: an object that enables the flow of datat between a program and external peripherals.
Input stream: data flows into a program
System.in
FileInputStream
Socket.getInputStream( )
Output stream: data flows out of a program
System.out
FileOutputStream
Socket.getOutputStream( )
program
Eat my shorts.
program
The quick brown fox….
CSS161: Fundamentals of Computing

3. Text Files and Binary Files
ASCII files that encode data into man-readable character set.
Binary Files
Binary digits that are machine code but not man-readable.
More efficient to process than text files
Smaller space
No ASCII to primary data conversion
CSS161: Fundamentals of Computing

4. CSS161: Fundamentals of Computing
Writing to a Text File
Open a file
FileOutputStream outFile = new FileOutputStream( “filename” );
Filter an output stream
PrintWriter outputStream = new PrintWriter( outFile );
Write data through the stream
outputStream.println( “This is a text to write” );
Close the file
outputStream.close( );
CSS161: Fundamentals of Computing

5. Opening a File for Writing
new FileOutputStream( “filename” )
Let the underlying Operating System create a file if necessary and mark the file in writing.
Set a position to write texts from the top of the file
Need to incorporate system-provided classes:
import java.io.FileOutputStream;
import java.io.FileNotFoundException;
Get prepared to deal with an exception “file not found” in case if filename is unknown.
try {
// open a file
} catch( FileNotFoundException e ) {
// handle an error }
CSS161: Fundamentals of Computing

6. Creating a Filter for Output Stream
Syntax
FileOutputStream outFile = new FileOutputStream( “filename” );
PrintWriter outputStream = new PrintWriter( outFile );
Need to incorporate a system-provided classe:
import java.io.PrintWriter;
Filter a file output stream
import java.io.FileOutputStream;
import java.io.FileNotFoundException;
// the below is within main( )
PrintWriter outputStream = null;
try {
outputStream =
new PrintWriter( new FileOutputStream( “data.txt” ) ); }
catch ( FileNotFoundException e )
System.out.println( “Error opening the file data.txt” );
System.exit( 0 );
Can be replaced with just
import java.io.*;
Why can’t we declare it within try?
Where is outFile?
CSS161: Fundamentals of Computing

7. Writing Texts to Output Stream
println( argument )
writes to a given file an argument that can be a string, character, integer, floating-point nmber, boolean, or any combination of these, connected with + signs. A new line is added.
Example:
outputStream.println( “result = ” + result );
print( argument )
Is the same as println( ) except no new line added.
outputStream.print( “sum = ” + (a + b) );
printf( format, arguments )
Is the same as System.out.printf( ) except that arguments are written to a file.
outputStream.printf( “value = %f%n” + value );
CSS161: Fundamentals of Computing

8. CSS161: Fundamentals of Computing
Closing a File
close( );
The underlying Operating System will:
Flush out all on-going writes to a given file.
Put an EOF (end of file) mark at the end of the file.
Reset the file from in-writing to unused.
CSS161: Fundamentals of Computing

9. Example - Writing to a Text File
import java.io.PrintWriter;
import java.io.FileOutputStream;
import java.io.FileNotFoundException;
public class TextFileOutputDemo{
public static void main(String[] args) {
PrintWriter outputStream = null;
try
outputStream =
new PrintWriter(new FileOutputStream("stuff.txt")); }
catch(FileNotFoundException e)
System.out.println("Error opening the file stuff.txt.");
System.exit(0);
System.out.println("Writing to file.");
outputStream.println("The quick brown fox");
outputStream.println("jumped over the lazy dog.");
outputStream.close( );
System.out.println("End of program.");
Open a file
Create a fileter
Write texts
Close the file
CSS161: Fundamentals of Computing

10. PITFALL: Overwriting an Output File
To open a file for writing:
outputStream =
new PrintWriter( new FileOutputStream( “data.txt” ) );
Create a file if data.txt does not exist.
Delete old contents if data.txt exists.
Thus, always start with an empty file
To append data to an existing file:
new PrintWriter( new FileOutputStream( “data.txt” ), true );
Previous contents
Newly add contents
CSS161: Fundamentals of Computing

11. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on Textbook p573’s exercises 3 ~ 7
CSS161: Fundamentals of Computing

12. Reading from a Text File
Open a file
FileInputStream inFile = New FileInputStream( “filename” );
Filter an input stream
Scanner inputStream = new Scanner( inFile );
Write data through the stream
outputStream.nextInt( );
outputStream.nextLong( );
outputStream.next( );
outputSstream.nextLine( ); etc.
Close the file
inputStream.close( );
CSS161: Fundamentals of Computing

13. Opening a File for Reading
new FileInputStream( “filename” )
Let the underlying Operating System mark a file in read.
This means other programs can no longer modify the same file concurrently but still can read the file.
Set a reading position to the top of the given file.
Need to incorporate system-provided classes:
import java.io.FileInputStream;
import java.io.FileNotFoundException;
Get prepared to deal with an exception “file not found” in case if filename is unknown.
try {
// open a file
} catch( FileNotFoundException e ) {
// handle an error }
CSS161: Fundamentals of Computing

14. Creating a Filter for Input Stream
Syntax
FileInputStream inFile = new FileInputStream( “filename” );
Scanner inputStream = new Scanner( inFile );
Need to incorporate a system-provided classe:
import java.io.Scanner;
Filter a file input stream
import java.io.FileInputStream;
import java.io.FileNotFoundException;
// the below is within main( )
Scanner inputStream = null;
try {
inputStream =
new Scanner( new FileInputStream( “data.txt” ) ); }
catch ( FileNotFoundException e )
System.out.println( “The file data.txt was not found.” );
System.exit( 0 );
Can be replaced with just
import java.io.*;
Why can’t we declare it within try?
Where is inFile?
Unlike FileOutputStream, FileInputStream
needs an existing file.
CSS161: Fundamentals of Computing

15. Reading Data from a Text File
Exception
public int nextInt( );
Returns the next token as int.
public long nextLong( );
Returns the next token as long.
public byte nextByte( );
Returns the next token as byte.
public short nextShort( );
Returns the next token as short.
public double nextDouble( );
Returns the next token as double.
public float nextFloat( );
Returns the next token as float.
public boolean nextBoolean( );
Returns the next token as boolean.
public String next( );
Returns the next token.
public String nextLine( );
Returns the rest of the current input line. The line terminater ‘\n’ is read and discarded.
public Scanner useDelimiter( String newDelimiter );
Changes the delimiter for input .
readable
true hello
4567 false
readable
Exception
readable
Exception
readable
CSS161: Fundamentals of Computing

16. Checking More Data to Read
To avoid NoSuchElementException
Call hasNext( ) that returns false if there are no more tokens.
To avoid InputMismatchException
Call hasNext***( ) before calling has***( ) where *** is Int, Long, Byte, Short, Double, Float and Boolean
To avoid IllegalStateException
No more calls of any functions of scanner after closing scanner, (I.e., calling close( )).
CSS161: Fundamentals of Computing

17. CSS161: Fundamentals of Computing
Closing a File
close( );
The underlying Operating System will:
Reset the file status from in-read to unused.
This means other programs can now modify the same file.
CSS161: Fundamentals of Computing

18. Example 1 - Reading from a Text File (1 of 2)
CSS161: Fundamentals of Computing

19. Example 1 - Reading from a Text File (2 of 2)
CSS161: Fundamentals of Computing

20. Example 2 - Checking for EOF (1 of 2)
CSS161: Fundamentals of Computing

21. Example 2 - Checking for EOF (1 of 2)
CSS161: Fundamentals of Computing

22. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on Textbook p583’s exercises 8 ~ 11.
CSS161: Fundamentals of Computing