1. Lecture 18
File I/O
CSE 1322
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2. File Input / Output
Consider that a program may want to make its data persistent (or not rely upon the user for input)
Reading from and writing to a file is conceptually the same as reading/writing to the console
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3. Streams Streams hold data
Input streams provide data to the program (keyboard and ReadLine, System.in and Scanner)
The program places data onto output streams (console and WriteLine, System.out and println)
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4. Process: Open the file stream
(many "modes" - read, write, append, create, etc.)
Do what you need to do (read/write)
Close the file stream
(freeing it for use by other programs)
You can imagine that there is an "active cursor" that is moved around the file as you read/write
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5. File reading Must know the structure of the file
Is it one element per line or multiple elements per line or is it a combination of the two
If it is multiple elements, how are the fields divided?
Is there a delimiter such as a space or ;
Are the fields off defined size such as first name in the first 10, last name in the next 20, age in the next 3, etc.
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6. File Structure
An airline company could store data in a file where each line represents a flight segment containing the following data:
flight number
origin airport
destination airport
number of passengers
average ticket price 
Such a file could contain the following data:
AA123,BWI,SFO,235,239.5
AA200,BOS,JFK,150,89.3
In this case, the delimiter is a comma.
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7. Opening / Closing Files
Opening the file
StreamReader sr = new StreamReader("data.txt");
StreamWriter sw = new StreamWriter("output.txt");
Closing the file
sr.Close();
sw.Close();
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8. Opening / Closing Files
Opening the input file
Scanner file = new Scanner( new File( “filename.txt” ) ); or
File datafile= new File("phoneList.txt"); Scanner in = new Scanner(datafile);
Opening the output file
FileOutputStream fos= new FileOutputStream ("phoneList.txt", false); PrintWriter pw= new PrintWriter(fos)
Closing the file
pw.close();
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9. Methods of the PrintWriter Class
Return value
Method name and argument list
void
Void
print( int i )
print( double d )
print( String s ) …
println( int i )
println( double d )
println( String s )
writes the argument to the text file
close( )
releases resources allocated to the PrintWriter object.

10. Java SOFTWARE ENGINEERING TIP
Calling the close method is optional. When the program finishes executing, all the resources of any unclosed files are released. It is good practice to call the close method, however, especially if you will be opening a number of files (or opening the same file multiple times.) Do not close the standard input, output, or error devices, however. They are intended to remain open.
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11. Reading / Writing data Reading data from the file
int x = Int32.Parse(sr.ReadLine()); or
while (!sr.EndOfStream) {
Console.WriteLine(sr.ReadLine()); }
Writing data to the file
sw.WriteLine(42);
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12. Reading / Writing data using Scanner in = new Scanner(datafile);
Reading data from the file
String x = in.next();
int y = in.nextInt(); or
while(in.hasNext()) { String line= in.nextLine(); String [] data= line.split(" "); }
Writing data to the file
pw.println(myData);
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13. loading a "game world" information:
private void LoadWorld() {
StreamReader sr = new StreamReader("world.txt");
world_width = Int32.Parse(sr.ReadLine());
world_height = Int32.Parse(sr.ReadLine());
show_width = Int32.Parse(sr.ReadLine());
show_height = Int32.Parse(sr.ReadLine());
world_background= new int[world_width, world_height]; }
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14. loading airline information
private void LoadData() {
StreamReader sr= new StreamReader(“data.txt”);
char[] delims = { ',' };
string info = sr.ReadLine();
while( info!= null)
string [] tokens = info.Split(delims);
//create/assign appropriate objects, etc.
info= sr.ReadLine(); }
sr.Close();
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15. loading airline information
private void LoadData() {
Scanner read= new Scanner(new File( “data.txt”));
char[] delims = { ',' };
while( read.hasNext())
String info= read.nextLine();
String [] tokens = info.split(delims);
//create/assign appropriate objects, etc. }
read.close();
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