Working with files

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 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