Transcript CS 177
Week 14 - Monday
What did we talk about last time?
Image manipulation
Inheritance
The Prey class now extends the Hunter
class, getting everything that it had before
Then we just needed to change the
update() method to run from Hunters
instead of chasing Prey objects
To make the Hunter and Prey classes
work, we now need to know if a specific
object belongs to a given type
Every object has a getClass() method
We can test to see if an object’s
getClass() method gives back
Hunter.class or Prey.class
By doing so, we can tell the class of a given
object
What if we create a super hunter?
Super hunters are larger and faster than
hunters, and try to eat everything, including
each other
We make SuperHunter extend Hunter
We call the Hunter constructor with the
right speed, size, and color
Then, we change the update() method to
reflect the right behavior
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
Scanner in = new Scanner(new
File("input.txt"));
This code will read from some file called
input.txt, as if someone were typing its
contents into the command line
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"));
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)
PrintWriter out = new PrintWriter(new
FileOutputStream ("output.txt"));
Once you've got a PrintWriter, you can
use it just like System.out
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();
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
Finish file I/O
Lab 14
Keep working on Project 5