Lab 10 In-Class: More Classes

In this lab you will develop a class that models a random walk and write a few client programs that use the class. A random walk is basically a sequence of steps of fixed length where the direction of each step is random. Often the walk is in some enclosed space. The walk terminates either when a maximum number of steps has been taken or a step goes outside of the boundary of the space. Random walks are used to model physical phenomena such as the motion of molecules and economic phenomena such as stock prices.

We will assume that the random walk takes place on a square grid with the point (0,0) at the upper left hand corner (as in a graphics window). By default each step will be a fixed number of units either up, down, to the left, or to the right. (No diagonal movement.) The class will need to keep track of the current position in the walk and how many steps have been taken. In addition to the usual methods such as a constructor and toString the class will need methods to take a single step and to simulate a complete walk (the sequence of steps).

The basic RandomWalk class will have the following instance data (all type int - you must use the variable names specified):

• x - the x coordinate of the current position
• y - the y coordinate of the current position
• stepSize - the length of each step (number of units on the grid)
• maxSteps - the maximum number of steps in the walk (the walk terminates when the number of steps exceeds this)
• numSteps - the number of steps taken so far in the walk
• gridSize - the width of the square grid (a positive integer -- the x and y coordinates of the position can vary between 0 and this value)

1. First declare the instance data (as described above) and add the following constructor and toString method.
   • A constructor RandomWalk that takes five parameters - the starting x and starting y, the length of each step, the maximum number of steps in the walk, and the width of the square grid - and uses them to initialize the Random Walk object.
   • String toString() - returns a String containing the number of steps taken so far and the current position -- The string should look something like: Steps: 12; Position: (30,55)

2. Make sure the class has no compilation errors (no red x's) then open the file TestWalk.java. This file will be used to test your RandomWalk methods. So far it prompts the user to enter the x and y coordinates of a starting position, the length of each step, the a maximum number of steps, and a width for the grid. Add the following:
   • Declare and instantiate a RandomWalk object using the values entered by the user.
   • Print out the object (remember you just print the object without explicitly calling toString). Note that you won't get any information about the boundary or maximum number of steps (think about what your toString method does), but that's ok.

   Run the program to make sure everything is correct so far.

3. Now let Eclipse add getter methods for all of the instance data. Recall from Lab 9, you do this as follows:
   1. From the menu, choose Source, Generate Getters and Setters.
   2. In this window click on the small arrow beside each instance variable and select the getter method.
   3. Be sure "Generate method comments" is checked (it should be). Now click okay.
   4. You should see the complete code for the method plus the javadoc.

4. Next add the following method to the RandomWalk class: void takeStep()

   This method simulates taking a single step either up, down, left, or right. To "take a step" generate a random number with 4 values (say 0, 1, 2, 3) then use a switch statement to change the position (one random value will represent going right, one left, and so on). Remember that the position will change by the step size in the given direction. Your method should also increment the number of steps taken. Use either the Random class or Math.random from the Math class to generate the random number.

5. To test the takeStep method add a for loop to TestWalk.java to have your RandomWalk object take 5 steps. Print out the object after each step so you can see what is going on. Run the program to make sure it is correct so far. The walk is random so each time you run the program the positions will change.

6. Now add to RandomWalk.java the following two methods. Each should be a single return statement (no if needed!) that returns the value of a boolean expression.
   • boolean moreSteps() - returns true if there are more steps to be taken (that is, if the number of steps taken is less than the maximum number); returns false otherwise
   • boolean inBounds() - returns true if the current position is on the square (include the boundary as part of the square); returns false otherwise.

7. Add to the RandomWalk class a method named walk that has no parameters and returns nothing. Its job is to simulate a complete random walk. That is, it should continue taking steps as long the maximum number of steps has not been taken and it is still in bounds (inside the square). This should be a very simple loop (while or do... while) --- you will need to call the methods takeStep, moreSteps, and inBounds.

8. Test the walk method by adding to TestWalk.java a statement to have the object walk using the walk method (add this AFTER the for loop that takes 5 steps). Print the object after the walk. Run the program more than once -- you should be able to tell by the value printed whether the object went out of bounds or whether it stopped because it reached the maximum number of steps.

9. Now you can see what a random walk looks like! The file AnimateWalk.java contains a program that uses your class to create a RandomWalk object and draws each step as it is taken. The code is not complete, however. The randomWalk instance variable must be initialized. Open the java file in Eclipse and add code to initialize the randomWalk object in the AnimateWalk constructor method. Make sure that the initial x and y coordinates of the walk are set to the middle of the grid and that the randomWalk is initialized to the same size as the frame. Run the program several times to make sure that it works.

10. Now write a client program to simulate a drunk staggering randomly on some sort of platform (imagine a square dock in the middle of a lake). Name the class DrunkenWalk. The goal of the program is to simulate the walk many times (because of randomness each walk is different) and count the number of times the drunk falls off the platform (goes out of bounds). Your program should do the following:
    • Read in the width of the platform, the maximum number of steps, and the number of drunks to simulate.
    • Then have a loop (a for loop would be a good idea) that on each iteration instantiates a new RandomWalk object to represent a drunk (start the drunk in the middle of the platform and use 1 for a step size), has the object walk, then determines whether or not the drunk fell off the platform (and updates a counter if it did).
    • After the loop print out the number of times the drunk fell off and the number of times it didn't.
    
    To see the "randomness" you should run your program several times with at least 1000 for the number of drunks to simulate. Try input of 10 for the width and 200 for the number of steps first (sometimes the drunk falls off, sometimes not); try 10 for the width and 500 for the steps (you should see different behavior); try 50 for the width and 200 for the steps (again different behavior).

11. Finally you will write a second client program that simulates two particles moving in space. Its goal is to determine the number of times the two particles collide (occupy exactly the same position after the same number of steps -- the steps could be thought of as simulating time). This means your program needs a way to determine whether or not the particles (the RandomWalk objects) are in the same position. For this we will add a method to the RandomWalk class. So, do the following:
    1. First, add the following method to the RandomWalk class.
       • public static boolean samePosition (RandomWalk p1, RandomWalk p2) - returns true if RandomWalk objects p1 and p2 have the same x coordinates and the same y coordinates.

       Make sure there are no syntax errors before proceding.

    2. Now write a client program to simulate two particles moving in space. The two particles will start in the center of a grid with width 600, each a fixed offset from the center but in opposite directions along the horizontal (for example, if the offset is 9, then one particle will start at point (300-9, 300) and the other at (300+9, 300)). Use 100,000 for the maximum number of steps. The file Collisions.java contains the skeleton of the program for you to complete. Note that the program has constants for the width of the grid and the maximum number of steps and also has code to read in the offset from the center for the particles to start. You need to instantiate two RandomWalk objects to simulate each particle. Use the appropriate constants and variables and use 1 for the step size. Your program should contain a loop that has each particle take a step (not a complete walk!) as long as the particles have not exceeded the maximum number of steps (it is ok to let them go out of bounds). The program should determine how often the particles have collided. The program should call the static method samePosition you just added to the RandomWalk class to determine if the particles are in the same position after taking each step (have "collided"). After the loop, print the number of collisions (appropriately labeled.

       Compile and run your program to make sure it works. As before run it several times.

12. Before printing anything be sure that all your files have appropriate documentation (including your name) and are properly formatted.

What to Turn In