1. Working with files

2. Motivation
All our programs so far have only worked with data stored in primary storage (RAM)
Data is lost when program is terminated
Almost all ”real” programs allow us to save the state – by some definition – of the program to permanent storage (e.g disk)
Such permanent storage is often a file
RHS – SWC

3. What is a file?
A collection of bytes, stored on a perma-nent storage media
Bytes can be interpreted as text, numerics, picture, etc.
Files have a name, and a position (path)
Myschedule.doc (name)
C:\documents\per\rhs (path)
RHS – SWC

4. Text files
A text file just contains characters (bytes inter-preted as characters)
Usually named .txt
Created with simple text editor like Notepad, or NetBeans…
RHS – SWC

5. Text files and Java
It is rather easy to read a text file in Java – we use the FileReader class and the Scanner class
FileReader reader = new FileReader(”input.txt”);
Scanner in = new Scanner(reader);
String line = in.nextLine();
RHS – SWC

6. Text files and Java FileReader reader = new FileReader(”input.txt”);
Constructor takes file name as a String (remember ””)
FileReader reader = new FileReader(”input.txt”);
Scanner in = new Scanner(reader);
String line = in.nextLine();
Several next… methods available
next()
nextInt()
nextDouble() …
FileReader itself cannot be used for reading – need a Scanner object
RHS – SWC

7. Text files and Java FileReader reader = new FileReader(”input.txt”);
Scanner in = new Scanner(reader);
int lineCount = 0;
while (in.hasNextLine()) {
String line = in.nextLine();
lineCount++; }
in.close();
System.out.println(”Lines in file: ” + lineCount);
RHS – SWC

8. Text files and Java
When providing a file path as a constant string, the string mus look like this:
”c:\\documents\\psl\\work.txt”;
Notice the use of double backslash!
Recall that \ is used for escape characters!
We do not need double backslash when e.g the user types the path in a dialog
RHS – SWC

9. Text files and Java
It is also easy to write to a file in Java – we use the PrintWriter class
PrintWriter out = new PrintWriter(”output.txt”);
That’s it – we can now use the well-known print(…) and println(…) methods
RHS – SWC

10. Text files and Java PrintWriter out = new PrintWriter(”output.txt”);
for (int i = 0; i < 10; i++) {
out.println(”Writing to a file is easy!”); }
out.close();
System.out.println(”Done writing to file”);
RHS – SWC

11. Text files and Exceptions
Working with files is easy, but…
…errors beyond the control of the pro-grammer can happen!
File was not found
File was not a text file
File was different format than expected
…and so on
RHS – SWC

12. Text files and Exceptions
Code for handling files should include proper handling of exceptions!
IOException class, and subclass thereof
We can define more specific classes as well
RHS – SWC