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CIS 201 – Computer Science I
Laboratory Assignment 2
Learning objectives
- Creating a class in emacs
- Using the keyboard in FANG
- Using variables
- Writing JavaDoc formatted comments
- What to comment
- Identifying what code does what
- Looking up features in documentation
Create a new game called Catnip
All Java programs are built using Java classes , and all Java classes have names. We normally name a class so that the name generally describes the purpose of the class. Each Java class defines a new Java type. We will describe the terms class and type in more detail as our course progreses. The name of a Java class must begin with an uppercase letter and must consist only of letters, digits, and underscores.
A Java program is simply a text file (that you can create this in emacs). The text file has a specific
layout that defines the objects and behaviors necessary to carry out some specific defined activity
- one that you define or one that someone else has defined and that you are implementing. We call such a file a “source” file, since it is the beginning step (the “source”) necessary to create a running program.
To define a Java class, you need to create a file with the same name as the class but ending with
the .java. The .java part of the name is called a filename extension. Other extensions you will
likely see in this class and elsewhere are .class, .txt, .pdf, and so forth.
Be sure you are logged in and that you have started a terminal window (shell). Then cd into your
CS1 directory. (You should create the CS1 directory if you haven’t done so before.) Then create a
new directory with name Labs and cd into it. Finally, create a new directory with name Lab02 and
cd into it. These are the commands you should use:
mkdir Labs
cd Labs
mkdir Lab
cd Lab
Next copy the file Catnip.java from the directory /home/student/Classes/201/Labs/Lab02/Catnip.java
into your current directory with the following command:
cp /home/student/Classes/201/Labs/Lab02/Catnip.java.
Again, don’t forget the dot!
Now start emacs with the command
emacs Catnip.java
When emacs opens, it is about 35 lines high. This value was set as part of the initialization of emacs
so that the windows fit on the monitors in the classroom. You might be more productive if you’re able
to fill your screen vertically with as much of the emacs window that will fit. If you want to change the
emacs window size, move your mouse pointer to the bottom border of the emacs window, click on it
with the first mouse button, and pull the window downward or upward to change the vertical size.
You should see the contents of the file Catnip.java displayed in your editor window.
Programs should always contain comments. Comments are used to help others who read your code – or you, if you haven’t looked at the code for a long time – to understand what the program is about and how it is supposed to work. Comments don’t affect the way the program actually works, since comments are essentially ignored by the program compiler.
Java has two ways to include comments: multi-line comments, and end-of-line comments.
Java multi-line comments start with /* and end with */. This means that the stuff on lines 3 through
14 of your program are comments. End-of-line comments start with // and continue to the end of
that line. End-of-line comments are typically much shorter than multi-line comments and normally appear along with program code instead of at the beginning of your program.
You will need to add import statements for the types of these variables, So, at the top of your
program , after the fang.core.Game line, add the following lines:
import fang.sprites.RectangleSprite;
import fang.sprites.OvalSprite;
Now compile your code and notice that these errors have been fixed.
Checkpoint 1
Show us that your code compiles correctly at this point.
1 Examine the setup method
The setup method is part of the FANG library. Go to your moodle account and bring up the
FANG documentation. (You can also get to this documentation from our department homepage,
cs.potsdam.edu, under Documentation.)
The documentation comes up in a window that has a big list of lots of things. We are interested in
the class fang.core.Game so click on one of the fang.core links to limit the view to that package.
Click on it. This provides the documentation generated from comments in that class – you might notice that the FANG documentation is pretty sparse, but this doesn’t mean that your documentation should be as bad.
If you scroll down the page, you will find that setup returns the void type and has an empty
parameter list. We have to remember that the method must be public for the game to call it. So
our method signature is for setup looks like this:
public void setup()
Looking at the fang.sprites page of the documentation, the RectangleSprite constructor takes
a width and a height for the sprite. Notice that our RectangleSprite object has been created –
using the new operator – with width 0.10 and height 0.10. The game’s drawing canvas is always
assumed to be width 1.0 and height 1.0. The top-left corner of the canvas has coordinates
(0,0), so that the horizontal ( x ) coordinates go from left to right, and the vertical ( y ) coordi-
nates go from top to bottom.
The setup code also sets the speeds of the sprites. These speeds are measured in units of
screens per second****.
The advance method is what makes the compontents move: both the cat and the cat sitter move
vertically (only their y -coordinates change) and bounce off the bottom and top of the screen. The
catnip moves from left to right across the screen. If it is “caught” by the cat, it’s a hit. If not, it’s a miss.
Checkpoint 2
Show us your running game at this point.
Changing the speed parameters
You can change the speeds of the cat, the cat sitter, and the catnip. Experiment with these values and see what happerns. What if you set the cat sitter speed to zero? What if you set the catnip speed to zero?
Checkpoint 3
Show us some of your changes.
Keeping track of the score
Declare a score variable along with your other variables like cat. What should be the visibility of
this variable? What should be its type?
In your setup method, initialze the score to zero, similar to setting the catnipSpeed to zero.
The difference is that the score’s initial value should be an integer, not a decimal number. Add
the appropriate line to do so. Also, un-comment the line in the advance method that updates
the score. Finally, un-comment the line in the updateScore method that displays the score using
System.out.println.
Checkpoint 4
Show us your changes, and show us where the score is being displayed!
