Copyright Chris Johnson, 1998.

Human Computer Interface Design Using Java


Exercises 5: Starting on Your Own

A. Introduction

Up to this point, you have been editing applets that I have already implemented. This exercise introduces an applet that you should try to implement yourself before you look at the code that I developed.

B. The Traffic Light Scenario

The following applet is intended to mimic a traffic light with red, amber and green lamps. Unlike a traffic light, there are specific buttons to change the colour of each bulb.

The design begins with a control panel. Three buttons are declared and are added to that panel, in exactly the same way that exercise 3 used left, right, up and down buttons. Then there is a canvas that is added to the applet; the three lights will be drawn onto this canvas in the same way that the maze was drawn onto a canvas in exercise 3.

B. Your Tasks

Task 1: Build your own version of the Traffic_Light applet

Have a go at trying to implement the traffic light applet, without looking at my version. A good approach would be to borrow parts of exercise3. For example, declares the control panel and a canvas for drawing into. will have to be radically pruned and simplified so that it just draws lights rather than all of the walls in the maze.

Hint: the following draws a filled circle into the graphic component g:

	g.fillOval(X, Y, width, height); // set width and height both to 50?
Similarly, the following draws an empty circle (ie a line) into g:
	g.drawOval(X, Y, width, height);

Task 2: Compare it with my Traffic_Light applet

My version of the program is based around three classes:

C. Summary

This exercise has formed a bridge between the previous examples, in which you simply edited an existing applet, to the more open design tasks that characterise the real-world practice of interface development. It can be extremely hard to get an applet working in the early stages - please do not be dispirited if it does not work first, second or even tenth time. Read the notes on debugging and good luck!

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