Week 14 - Monday

 What did we talk about last time?  Inheritance

 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 )

 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"));

 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

 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

 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

 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")); public static void main(String[] args) throws FileNotFoundException { Scanner in = new Scanner(new File("input.txt"));

 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"));

 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

 Unlike the command line, you should really close files when you're done reading from them  If you forget, it's okay: Java will automatically close them when your program quits  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(); Scanner in = new Scanner(new File("input.txt")); PrintWriter out = new PrintWriter(new FileOutputStream ("output.txt")); //do stuff in.close(); out.close();

 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

 Let's write a program that prints the first million prime numbers to a file

 Finish file I/O  Lab 14

 Keep working on Project 5