The Scenario

We have implemented a program that simulates two monopoles in Lab 3. In this lab, we are going to further extend that program to simulate two magnets. Each magnet has a north pole labeled by "N" and a south pole labeled by "S". Your program should allow the person using it to move either magnet around the screen by dragging it with the mouse. So, if one magnet is dragged to a position where one or both of its poles is close to the similar poles of the other magnet, the other magnet should move away as if repelled by magnetic forces. If, on the other hand, opposite poles come close to one another, the free magnet should move closer and become stuck to the magnet being dragged. The magnets will not be allowed to rotate. They only slide up, down and across the screen.

Similar to monopoles, a magnet can be flipped from end to end (swapping the poles) by clicking on the magnet without moving the mouse. This will provide a way to separate the two magnets if they get stuck together (since as soon as one of them is reversed it will repel the other).

Here is a demo of what we have in mind:

Design of the program

Your program consists of three classes:

• A MagnetGame class that extends FrameWindowController. It will be very similar to the PoleController class from Lab 2. In fact you would not need to add any line of code.
• A Magnet class that model the behavior of a magnet. This is the main class that you need to design and write.
• The exact Monopole class from Lab 2. You would not need to make any change. If you think that you need to modify the Monopole class, talk to me first.

As mentioned earlier, before writing code, you should write a design. The design should show how you plan to organize your classes to accomplish the actions required. In particular, you need to describe what methods are needed and the behavior that they should provide. Inside each method, you should write (in English comments, not Java) your plan for how the method will provide the necessary behavior. You should also describe (in English) what variables you think are necessary for each class. This level of preparation will allow you to progress much more quickly in lab so that you can take better advantage of your time.

Before implementing your design, review your design: Does it make sense to you? Do you understand what instance variables are needed? When you're satisfied with it, translate it into Java code. Don't erase your comments - they'll help you keep track of what you're trying to do.

We will describe below a few details that you need to pay attention to:

• The width of a magnet is exactly three times the width of a pole and the height of a magnet is the same as the height of a pole. For example, if a pole has size 50 x 50, then a magnet has size 150 x 50, with a north pole at one end and a south pole at the other end. There is a gap of size 50 x 50 in the middle.
  
  
• The interact range of the pole should not exceed twice the size of the pole. For example, if the size of a pole is 50 x 50, then the range at which two poles starts to repel or attract each other should not be more than 100. The purpose of this limit is to simplify the program such that a pole would not be simultaneously repelled and attracted by the two poles of another magnet.
  
  
• Your Magnet class should provide an interact method. In this method, you would first let the two poles of the current magnet interact with one of the other magnet's poles. In this process, that pole of the other magnet poles may be moved (by being repelled or attracted). So you would need to move the other magnet along with its moving pole (this needs to be accomplished in another method). Repeat this process with the other magnet's second pole, and this should effect the interaction between two magnets. A proper implementation of this method should take no more than 6 lines of code in its body.

Adapted from materials developed at the Department of Computer Science, Williams College