I drew a smiley on the board and provided the skeleton for a new class
that drew only the circle that represented the face. I asked the
students to get it to display a smiley according to the drawing. While
they did that, I downloaded my source code for Smiley.java and
the modified classes from the BlueJ sample
project (added move() to Triangle and Circle) and showed them my
implementation. I wanted them to define a class that supported many of
the operations the basic shapes did. I started with a single Circle
attribute representing the face, a constructor that created the
circle, and a makeVisible() method that made the face visible.
The major conceptual point I wanted to make was:
You can define your own class that contains objects. You can call
methods on those objects and you don’t have to worry about the
implementation details. You can support the other methods by passing
the method calls to the different objects.
A number of students started by looking at Circle.java and trying to
model their work on it, but I suggested working from the very simple
skeleton we had on the board. This greatly helped the students who
were getting lost in the source code.
I also pointed out that they could use BlueJ to determine the
coordinates and sizes experimentally. Instead of edit-compile-test,
they could interactively create an object, manipulate it, and then
inspect it to find the necessary values.
This exercise was open-ended. After students got the smiley to
display, they added support for moveUp(), moveDown(), moveLeft(), and
moveRight(). moveHorizontal(int distance) and moveVertical(int
distance) followed shortly after. move(int x, int y) required a bit
more thought because they couldn’t just pass the request on. I also
had a simple animate() method. I suggested making it possible for the
user to change the smiley’s colors (face, eyes, and mouth – separate
methods, or one method with three parameters) or size (bit more
I think this exercise worked very well. I did not have to tell the
students exactly what to do, although I gave them a number of hints. I
exposed my planning process – first, get the face to display, then add
the eyes, then the mouth, then add other methods one at a time.
Students learned how to plan implementation in small steps. I did not
tell them what they had to do; I just drew a figure on the board and
had a demo up on the projector. On their own, they figured out that
they needed to specify negative height in order to get an inverted
triangle and add offsets during the move.