Purposes of this assignment:

• To give you some experience in program design
• To give you more experience with JUnit

General idea of the assignment:

A number of ions (charged atoms) are placed in a two-dimensional rectangular box. The ions repel each other with a force inversely proportional to the square of their distance. That is, if they are one unit apart, they repel each other with a force of one; if they are two units apart, with a force of 1/4, or 0.25; if five units, with a force of 1/25, or 0.04; and so on. Every ion is affected by every other ion.

Ions that are close together will move apart, as each ion tries to get as far away from its neighbors as possible. After a while, the ions should settle down into a more-or-less "stable" position, and not move very much more.

In addition, some ions may be "glued in place" and unable to move.

Details:

Create an Ion class and an IonBox class. Ions come in two flavors, "free" (able to move about) and "fixed" (glued in place).

The Ion class will have:

• A constructor, Ion(double x, double y).
  
  
• Getters and setters:
  • double getX()
  • void setX(double x)
  • double getY()
  • void setY(double y)
  • void setXandY(double x, double y)

The IonBox class will have:

• A constructor, IonBox(double width, double height). Legal coordinates for x will be between 0.0 and width; legal coordinates for y will be between 0.0 and height.
  
  
• A method void putIon(Ion ion) to put an ion in a particular starting location. When this is called, all the ions will move around to reach a new stable position.
  
  
• A method void putFixedIon(Ion ion) to put an ion in a particular location, and "glue" it there. When this is called, all the other ions will move around to reach a new stable position.
  
  
• A method void removeIon(Ion ion) to remove a particular ion. Again, this will cause the other ions to move around until they reach a new, stable position.
  
  
• A main method to allow the user to create an IonBox, place and remove Ions in it, and find out where they end up. After each change, it should also display the minimum, maximum, and average distance from each ion to its nearest neighbor.

Most importantly, write JUnit tests to convince me that your methods work, and that the ions reach a stable position as far from one another as possible. And follow all the "best practices" that we have been talking about, such as writing tests first, keeping methods short and single purpose, etc.

I'm leaving more unspecified in this assignment than in previous assignments. It's up to you to fill in the details. I do want the above methods, so that I can do a bit of my own testing.

Programming hints:

• Since you don't know how many ions there might be, keeping them in an array is not appropriate. An ArrayList<Ion> is like an array of ions, but can expand and shrink as needed. To declare one, you can use this syntax:
  
       ArrayList<Ion> myIonList = new ArrayList<Ion>();

The available ArrayList methods are described in java.util.ArrayList in the Java API.
  
  
• Also, in your JUnit testing, assertEquals(x, y) can't be used for doubles; you need to use assertEquals(x, y, delta), where delta tells how close together x and y need to be in order to be considered "approximately" equal.
  
  
• The main method should allow the user to add (or remove) ions one at a time, and see the results.
  
  
• You do not need:
  • Any kind of visual display. This is not a graphics assignment. The user should be informed where the ions end up, but you can do this by simply printing their coordinates.
  • JUnit tests for the main method. It is mostly concerned with interacting with the user--getting commands and printing results--it isn't appropriate to try to write JUnit tests for it.
  • Random numbers.
  • Fancy math.
    
    
• You can arrive at a "solution" by moving each ion a little bit in the direction it is most strongly "pushed," and doing this over and over again. You may need to experiment a bit. You may want to set a maximum distance that an ion can be pushed, and reduce this maximum distance a little bit each time. Don't be afraid to move the ions hundreds of times, if necessary.

Due date:

Thursday, November 1, before midnight. Turn in your program electronically, using Blackboard. (See my Instructions for Using Zip Files and Blackboard). Only assignments submitted via Blackboard will be accepted--do not send your program by email. The usual late penalty of 5 points per day (out of 100 points) will apply.