Chapter 6: Object-Oriented Design (2)

Lab Exercises

 

Topics                                     Lab Exercises

Overall class design                            Random Walks

 

GUI Layouts                                         Telephone Keypad

 

                                                               

                                                               


Random Walks

 

In this lab you will develop a class that models a random walk and write two client programs that use the class. A random walk is basically a sequence of steps in some enclosed space where the direction of each step is random. 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 center. The boundary of the square will be a single integer that represents the maximum x and y coordinate for the current position on the square (so for a boundary value of 10, both the x and y coordinates can vary from -10 to 10, inclusive). Each step will be one unit up, one unit down, one unit to the left, or one unit to the right. (No diagonal movement.)

The RandomWalk class will have the following instance data (all type int):

Create a new file RandomWalk.java. You'll define the RandomWalk class incrementally testing each part as you go.

  1. First declare the instance data (as described above) and add the following two constructors and toString method.
  2. Compile what you have so far then open the file TestWalk.java. This file will be used to test your RandomWalk methods. So far it prompts the user to enter a boundary, a maximum number of steps, and the x and y coordinates of a position. Add the following:

Compile and run the program to make sure everything is correct so far.

  1. 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). Your method should also increment the number of steps taken.

  2. Add a for loop to TestWalk.java to have each of your RandomWalk objects take 5 steps. Print out each object after each step so you can see what is going on. Compile and run the program to make sure it is correct so far.

  3. Now add to RandomWalk.java the following two methods. Each should be a single return statement that returns the value of a boolean expression.
  4. 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 generate a sequence of 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.

  5. Add to TestWalk.java a statement to instantiate a RandomWalk object with a boundary of 10 and 200 as the maximum number of steps. (You may want to comment out most of the code currently in TestWalk -- especially the user input and the loop that takes five steps -- as the walk method will be easier to test on its own. The /* ... */ style of comment is useful for this. But don't delete that other code, as you'll need it later in the lab.) Then add a statement to have the object walk. Print the object after the walk. Compile and run the program. Run it 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.

  6. Now write a client program in a file named DrunkenWalk.java. The program should simulate a drunk staggering randomly on some sort of platform (imagine a square dock in the middle of a lake). The goal of the program is to have the program 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 read in the boundary, the maximum number of steps, and the number of drunks to simulate. It should 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, 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.  Compile and run your program. To see the "randomness" you should run it several times. Try input of 10 for the boundary and 200 for the number of steps first (sometimes the drunk falls off, sometimes not); try 10 for the boundary and 500 for the steps (you should see different behavior); try 50 for the boundary and 200 for the steps (again different behavior).

  7. Now write a second client program in a file named Collisions.java. This program should simulate 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). We'll assume the particles are in a very large space so use a large number for the boundary (such as 2,000,000). Use 100,000 for the maximum number of steps. (Don't enter the commas.) Start one particle at (-3, 0) and the other at (3, 0). (You can hardcode these values into the program; no need to enter them.) Your program should contain a loop that has each particle take a step as long as the particles have not exceeded the maximum number of steps. The program then determines how often the particles have collided. Note that in order for your program to know whether or not the two different RandomWalk objects are in the same place it needs to be able to find out the position. Hence, you need to add the following two methods to the RandomWalk class.

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

  1. In your Collisions.java program the condition to determine if the points are at the same position is a bit cumbersome. This is something that would be best put in a separate method. Add a static method to Collisions.java (after the main method) with signature

                    public static boolean samePosition (RandomWalk p1, RandomWalk p2)

The method should return true if p1 and p2 are at the same position and return false otherwise. Modify your main method so it calls samePosition rather than directly testing to see if the objects are at the same position. Test the program.

  1. In using random walks to simulate behavior it is often of interest to know how far away from the origin the object gets as it moves.

a.        Add an instance variable maxDistance (type int) to the RandomWalk class. This should be set to 0 in each constructor.

b.       Now the takeStep method needs to update this maximum when a step is taken. We'll add a support method to the class to do this. Add a private method named max that takes two integer parameters (say num1 and num2) and returns the largest of the two.

c.        Add code to takeStep to update maxDistance. This can be done in a single statement using the max method -- the new value of maxDistance should be the maximum of 1) the old value of maxDistance, and 2) the current distance to the origin. Note that if the current point is (-3, 15) the distance to the origin is 15; if the current point is (-10, 7) the distance to the origin is 10. Remember that Math.abs returns the absolute value of a number.

d.       Finally add an accessor method to return that distance so a client program can access it:

       public int getMaxDistance()

e.        Test the maximum by adding statements in TestWalk.java to get and print the maximum distance for each of the objects after the loop that had them take and print out 5 steps (this way you can see if the maximum is correct – each step is printed).

 
 
// ***********************************************************
//  TestWalk.java
//
//  Program to test methods in the RandomWalk class.
// ***********************************************************
 
import java.util.Scanner;
 
public class TestWalk
{
    public static void main (String[] args)
    {
        int maxSteps;   // maximum number of steps in a walk
        int maxCoord;   // the maximum x and y coordinate
        int x, y;       // starting x and y coordinates for a walk
 
        Scanner scan = new Scanner(System.in);
 
        System.out.println ("\nRandom Walk Test Program");
        System.out.println ();
 
        System.out.print ("Enter the boundary for the square: "); 
        maxCoord = scan.nextInt();
 
        System.out.print ("Enter the maximum number of steps: ");
        maxSteps = scan.nextInt();
 
        System.out.print ("Enter the starting x and y coordinates with “ +
                          “a space between: ");
        x = scan.nextInt();
        y = scan.nextInt();
 
    }
}

 

Telephone Keypad

 

Files Telephone.java and TelephonePanel.java contain the skeleton for a program to lay out a GUI that looks like telephone keypad with a title that says “Your Telephone!!”.  Save these files to your directory. Telephone.java is complete, but TelephonePanel.java is not.

 

1.        Using the comments as a guide, add code to TelephonePanel.java to create the GUI.  Some things to consider:

a.        TelephonePanel (the current object, which is a JPanel) should get a BorderLayout to make it easy to separate the title from the keypad.  The title will go in the north area and the keypad will go in the center area.  The other areas will be unused.

b.       You can create a JLabel containing the title and add it directly to the north section of the TelephonePanel.  However, to put the keypad in the center you will first need to create a new JPanel and add the keys (each a button) to it, then add it to the center of the TelephonePanel.  This new panel should have a 4x3 GridLayout.

c.        Your keypad should hold buttons containing 1 2 3, 4 5 6, 7 8 9, * 0 # in the four rows respectively.  So you’ll create a total of 12 buttons.

 

2.        Compile and run Telephone.java.  You should get a small keypad and title.  Grow the window (just drag the corner) and see how the GUI changes – everything grows proportionately. 

 

3.        Note that the title is not centered, but it would look nicer if it were.  One way to do this is to create a new JPanel , add the title label to it, then add the new JPanel to the north area of the TelephonePanel (instead of adding the label directly).  This works because the default layout for a JPanel is a centered FlowLayout, and the JPanel itself will expand to fill the whole north area.  Modify your program in this way so that the title is centered.

 

 

//******************************************************
// Telephone.java
//
// Uses the TelephonePanel class to create a (functionless) GUI 
// like a telephone keypad with a title.
// Illustrates use of BorderLayout and GridLayout.
//******************************************************
import javax.swing.*;
public class Telephone
{
    public static void main(String[] args)
    {
        JFrame frame = new JFrame("Telephone");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(new TelephonePanel());
        frame.pack();
        frame.setVisible(true);
    }
}

 

 

//******************************************************
// TelephonePanel.java
//
// Lays out a (functionless) GUI like a telephone keypad with a title.
// Illustrates use of BorderLayout and GridLayout.
//******************************************************
import java.awt.*;
import javax.swing.*;
 
public class TelephonePanel extends JPanel
{
    public TelephonePanel()
    {
        //set BorderLayout for this panel
        
        //create a JLabel with "Your Telephone" title 
 
        //add title label to north of this panel
 
        //create panel to hold keypad and give it a 4x3 GridLayout
 
        //add buttons representing keys to key panel
 
        //add key panel to center of this panel
    }
}