In the programs that you have written so far, you have used the
Canvas class to draw pictures. While you undoubtedly had a lot of fun
coming up with different drawings, the problem is that you have to
"hard-code" whatever you want to draw. In other words, every time you
want to change your drawing, you have to change the code and recompile
it. Perhaps you find yourself drawing the same things over and over
again, and you'd like to specify a way to just draw, say, a car,
simply by calling a method that you've written to do so. This can be
done by defining a Car class, and then creating multiple
objects from that class.
In this lab, you will take a very small class called Car
and gradually add new capabilities to it. Once you have a more
sophisticated Car, you'll move on to starting a new class
(Tree) from scratch. By the end of the lab, you'll have
a picture of a bunch of cars on a tree-lined road, plus a bit more
knowledge about the structure of Java classes.
Car and Road classesCreate a directory named car, and copy
Car.java, Road.java, and
Canvas.java into it from
/Accounts/courses/cs117/ondich/car.
Take a look first at Road.java. What do you expect to
see when you run java Road? Notice that it uses objects
from the class Car that we are creating. Take a look at
Car.java to see how to set up a class that we can create
objects from.
Compile Car.java, Road.java, and
Canvas.java and run java Road. Does the
picture look like you expected it to? If not, take a closer look at
the code.
Car of a different color You may have noticed a tragic uniformity in the color of the cars. Let's fix that.
Color object variable (call it
color) to the Car class. variable
should be declared near the top of the class, right along with the
declarations for the other object variables..setColor method to the Car
class. This method should begin with "public void setColor(
Color newColor )", and it should simply assign a new value
to the color instance variable.Car constructor to set the
color variable to the default value (red).draw method to use color
instead of red.main in Road to change
the color of one of the cars to white or yellow or something.Now that you can control the color of your Car, let's
try changing the size of the car. You can start by following
instructions like those in the previous section: add a
width instance variable to Car, add a
setWidth method, initialize width in the
constructor, and change main in Road.
The tricky part will be changing draw. In the
original version of draw, you'll see numbers that depend
on the width of the car being 100. But now you'll need to change
those numbers to expressions that vary depending on the value of
width.
You'll know you have gotten it right if you can modify the widths of your cars and get different-colored cars of different sizes on your road.
Let's add a little foliage. Using the ideas from Car,
start from scratch and create a Tree class that allows
you to control the height of the tree. Use the Car class
as an example. Don't get too fancy with your drawing--a brown
rectangle plus a green circle make a lovely tree at this stage.
Again, you'll know that you have succeeded when your landscape has
several trees of different sizes, all created as separate instances of
a single Tree class.
You do not need to hand anything in for this lab, but please make sure you complete the lab and ask me any questions you have about it.
Have fun.