1. Week 14 - Wednesday
CS 121

2. Last time
What did we talk about last time?
Inheritance

3. Questions?

4. Project 5

5. Files

6. Files in Java
In this course, you have already dealt with image and audio files, but you didn't do any direct input or output to them
It is possible to read and write individual pieces of data to a file
Files are great because they exist after the program is done
Reading and writing to a file is very similar to reading and writing to the command line (using Scanner and System.out)

7. Reading Reading from a text file is almost ridiculously easy
We use Scanner, just like reading from the command line
We just have to create a new File object that gives the file we want to read from
This code will read from some file called input.txt, as if someone were typing its contents into the command line
Scanner in = new Scanner(new File("input.txt"));

8. I take exception to that…
Unfortunately, if you type that into Eclipse, you'll get a red squiggle underneath the code
The problem is this: What would happen if input.txt doesn't exist?
This is an error situation, and Java uses something called exceptions to deal with errors
You can catch an exception and do something to recover from the situation

9. Checked exceptions
However, the error if the file isn't there is called a FileNotFoundException, and it's a checked exception
If there is the possibility of throwing a checked exception, your code has to deal with it or else your program will not compile
Well, that's annoying: Now we have to learn how to deal with catching exceptions

10. Unchecked exceptions You've seen exceptions before:
NullPointerException
ArrayIndexOutOfBoundsException
etc.
These are called unchecked exceptions, because you don't have to deal with them
You usually can't deal with them: They mean that you're program is messed up

11. Throw them 'bows!
The alternative to catching an exception is throwing it up to the next level, making it someone else's problem
Sure, your program will crash if no one deals with it, but at least your code will compile
We do this by putting a throws FileNotFoundException on the declaration of main() (or whatever method we're in)
public static void main(String[] args)
throws FileNotFoundException {
Scanner in = new Scanner(new File("input.txt"));

12. Writing Java loves objects
If you want to write to a file, you've got to create a PrintWriter object, based on a FileOutputStream object (which takes the file name as a parameter)
Once you've got a PrintWriter, you can use it just like System.out
PrintWriter out = new PrintWriter(new FileOutputStream ("output.txt"));

13. More exceptions!
Just like making a Scanner from a File, making a PrintWriter from a FileOutputStream will potentially throw a FileNotFoundException
Weird, isn't it? I mean, you don't expect to find a file when you're about to write one
Sometimes Java doesn't make sense
Anyway, adding the throws FileNotFoundException to the method declaration will still solve the problem

14. Shut 'em down!
Unlike the command line, you should really close files when you're done reading from them
If you forget to close a file you’re writing to, everything you write might not show up
But, for situations where you're accessing multiple files, it may be important to close them
Scanner in = new Scanner(new File("input.txt"));
PrintWriter out = new PrintWriter(new FileOutputStream ("output.txt"));
//do stuff
in.close();
out.close();

15. Reading and writing example
Let's write a program that prompts the user for 1o int values and then writes them to a file called numbers.txt
Then, let's write another program that opens numbers.txt, reads all 10 numbers, sorts them, and prints them out in order

16. More file practice
Let's write a program that prints the first million prime numbers to a file

17. Quiz

18. Upcoming

19. Next time…
Finish file I/O
Lab 14

20. Reminders
Keep working on Project 5