6.030 Fall 1999
Introduction to Interactive Programming

Laboratory 6: Communicating Applications

Handout 1


There are two parts to this laboratory assignment. In this part, you will build a simple graphical user interface and some additional infrastructure. In the second half of this lab, you will augment that work. By the end of this project, you will have built a very simple two-player video game. The first week's lab focuses on the graphical user interface (GUI).

This assignment must be donewith a partner. Part of the assignment is to be done jointly with your partner. Other parts of the assignment are to be done individually. If you do not have a partner to work with, please let the course staff know immediately.

This lab focuses on the following ideas:

You should read through this entire assignment and complete the Lab preparation section before you come to lab. Some portions of the Post Lab write-up also require your thinking about them before and during the laboratory portion of the assignment. It is important that you plan your complete design before coming to lab.

Please include the names of anyone with whom you collaborate, in any way, on this assignment. As always, you are welcome to discuss the assignment with other students in the class as well. You should also indicate the nature of any collaborations. [Failure to include this information is a violation of the collaboration policy.]

This assignment is a two-week assignment, but the current handout covers work to be done during the first week's lab. You will need to have this labwork complete in order to be checked in to next week's lab. This handout covers work to be done during the week of 1 November, 1999.



The thing that you will be creating in this first half of the lab is called a position panel. This is a Java component that keeps track of where the mouse is and paints some sort of object (e.g., a circle) in the appropriate place. It is important that you build a transaction-safe version of this component, i.e., that it not be possible for the position panel to paint the mark in a place where the mouse has never been.


Finger Exercises

In answering these questions, you may find it useful to refer to the Java Applications Programmer Interface (API) documentation, especially portions of the java.awt and java.awt.event packages.

1. Design a data repository class that keeps track of an x and a y coordinate and has an additional field of type int. (The additonal field might, for example, be used to hold a symbolic constant.) Make sure that your class provides transaction-safe access to its fields. It should be possible to change the coordinates (both together), but not the additional type field. It should also be possible to hand off (return, give another object) the x and y coordinates safely. (Hint: think about the SafeCoordinate class that we built in lecture.)

2. Design a class that extends java.awt.Panel and keeps track of a location in screen coordinates. (It can, for example, just have two ints as fields.) The paint method of this class should draw a filled oval centered at the specified point.

3. Design a class that extends java.awt.Panel and keeps track of the mouse position. That is, it should have a getMouse() method that can be called by other objects and returns an object corresponding to the mouse's current location. You may, of course, define auxilliary objects to help handle the relevant events.

Questions to think about:

4. Now design a class that combines all of these behaviors. Your new class should extend java.awt.Panel and keep track of (hint: field) the last location where the mouse was depressed, i.e., the last place where a mousePressed occurred. The paint method of this class should draw a filled oval centered at the specified point.

  1. (Bonus) Modify your class so that it keeps track of the current mouse position instead.
  2. (Extra Bonus) Add a getMouse() method to your class. This method should be callable by other objects and should return an object corresponding to the mouse's current location.

    Questions to think about:

    • What events should you handle to provide this information?
    • The mouse's location has two parts, a horizontal coordinate and a vertical coordinate. How do you return both safely?
    • How many threads can be running in this Panel extension at once? Do they share state?  How can you ensure that they are transaction-safe?

B. Additional Lab Preparation

In addition to working on the finger exercises, for this week you should think through the assignment below and come to lab with a plan that includes a solution to the problem set and a development plan for how you intend to implement and test one stage at a time of your solution.

Setting Up a Panel

In this lab, you will be building a standalone application version of your mouse-tracking position panel. In particular, you will need:

Development Plan

The final stage of pre-lab preparation is to write up a step-by-step description of your incremental development plan. In particular, think about the following:

  1. What is the minimum self-contained (i.e., testable) functionality that you can implement?
  2. How will you test it?
  3. What can you add to make a slightly more complex and still fully testable version?
  4. How would you test this?

Continue listing added features and tests until you scale up to your complete design. This technique, incremental refinement, is an extremely useful and practical way to write code. You may want to read through the suggestions in the In the Lab section, below.


Using our DefaultFrame

To test your code, you may want to use our cs101.awt.DefaultFrame.java class. DefaultFrame's constructor takes a Component as an argument and displays that Component inside itself. (Your extended Panel should do just fine.) DefaultFrame's init() method is responsible for adding the Component, sizing, and showing it.

You will also need to write a main file that does the following:

  1. create a new instance of your panel.
  2. create a new instance of cs101.awt.DefaultFrame, passing your panel as an argument to DefaultFrame's constructor.
  3. calls the DefaultFrame's init() method.

Note: You may want to include the following code to your extended panel class:

 public Dimension getMinimumSize() {
    return new Dimension(200,200);
 public Dimension getPreferredSize() {
    return new Dimension(400,400);

This defines a size for the Panel; otherwise it will not report any size and may not be displayed.


What to Bring to Lab

You should bring your answers to the finger exercises as well as your design and implementation plan for this lab. In addition, you should have thought out answers to all of the questions in the laboratory preparation section and should be prepared to begin writing code immediately upon entering lab. When you arrive at lab, you should have answers to the questions in the lab preparation a copy of your code design and development plan (on-line or on paper) ready so that we can check you into lab.

Getting Started

In lab, you should write and test the code that you designed.

As always, you should implement your code in simple, testable stages, building on your code only as each stage works robustly. In this case, it should be as simple as following your development plan.

You will, of course, want to make use of your answers to the finger exercises.

Before you leave

Before you leave lab, you will need to have your work checked off by a course staff member.

At a minimum, you should expect to demonstrate an application that is responsive to your mouse.

If your code does not behave as expected, you should be prepared to explain why or to describe what you have done to try to figure out why.

If your code has more than this minimal functionality, you should be able to describe and demonstrate what it does as well as how you were able to achieve this behavior.

It is always more important that you write clean, modular, well-documented and easy-to-understand code than that you go on to advanced features.

Post-Lab, AKA What To Turn In

There is nothing to turn in for this week's assignment, as the assignment continues into next week. However, the work that you have done this week will be a part of the turn-in for next week's assignment. Your completed assignment should include:

This course is a part of Lynn Andrea Stein's Rethinking CS101 project at the MIT AI Lab and the Department of Electrical Engineering and Computer Science at the Massachusetts Institute of Technology.

Questions or comments: