Lab 3 In-Class: Using Objects and Methods

Getting Started

Log onto the Linux system, open emacs, Mozilla, and an xterm window. In Mozilla, go to the home page for this class and open up this lab. In an xterm, go to your labs directory for this course and create a lab3 subdirectory for today's work. Change into that subdirectory.

Using the String Class

The following program illustrates the use of some of the methods in the String class. Study the program to see what it is doing.
// ***************************************************************
//  FILE:
//  Purpose: Test several methods for manipulating String objects
// ***************************************************************

import java.util.Scanner;

public class StringManips
    public static void main (String[] args)
	String phrase = new String ("This is a String test.");
	int phraseLength;   // number of characters in the phrase String
	int middleIndex;    // index of the middle character in the String
	String firstHalf;   // first half of the phrase String
	String secondHalf;  // second half of the phrase String
	String switchedPhrase; // a new phrase with original halves switched

        Scanner scan = new Scanner (;

	// compute the length and middle index of the phrase
	phraseLength = phrase.length();
	middleIndex = phraseLength / 2;

	// get the substring for each half of the phrase
	firstHalf = phrase.substring(0,middleIndex);
	secondHalf = phrase.substring(middleIndex, phraseLength);

	// concatenate the firstHalf at the end of the secondHalf
	switchedPhrase = secondHalf.concat(firstHalf);

	// print information about the phrase
	System.out.println ("Original phrase: " + phrase);
	System.out.println ("Length of the phrase: " + phraseLength +
			    " characters");
	System.out.println ("Index of the middle: " + middleIndex);
	System.out.println ("Character at the middle index: " + 
	System.out.println ("Switched phrase: " + switchedPhrase);


The file contains this program. Save the file to your lab3 directory (from Mozilla), compile and run it. Study the output and make sure you understand the relationship between the code and what is printed. Now open the file in emacs and make the following modifications.
  1. Declare a variable of type String named middle3 (put your declaration with the other declarations near the top of the program) and use an assignment statement and the substring method to assign middle3 the substring consisting of the middle three characters of phrase (the character at the middle index together with the character to the left of that and the one to the right.) Write your assignment so it would work for any string, not just the one instantiated in this example. To do that use the middleIndex variable rather than integer literals in your arguments for the substring method. Be sure to think about where the statement should be placed in the program. Add a println statement to print out the result, appropriately labeled. Save, compile, and run to test what you have done so far.

  2. Add an assignment statement to replace all blank characters in switchedPhrase with an asterisk (*). The result should be stored back in switchedPhrase (so switchedPhrase is actually changed). (Do not add another print -- place your statement in the program so that this new value of switchedPhrase will be the one printed in the current println statement.) Save, compile, and run your program.

  3. Currently the program works for just the one phrase that is instantiated when the phrase object is declared. Change the program to get the phrase as input from the user as follows:
    1. Change the line that declares and instantiates phrase to just a declaration.
    2. Add a prompt asking the user to enter a phrase and then add a line to read the phrase in. The Scanner method to read in a string is nextLine() which reads in everything entered on one line. Be sure to correctly place these statements in the program.
    Compile and run the program. Test it on strings of different lengths and make sure it is printing the correct middle 3 characters.

  4. Declare two new variables city and state of type String. Add statements to the program to prompt the user to enter their hometown -- the city and the state. Read in the results using the nextLine method. (NOTE: The city and state must be entered on different lines to read them into separate variables.) Then, using String class methods, create and print a new string that consists of the state name (all in uppercase letters) followed by the city name (all in lowercase letters) followed again by the state name (uppercase). So, if the user enters Lilesville for the city and North Carolina for the state, the program should create and print the string
  5. Print the final version of the program. Remember you can use the print command you created last lab: ~/print

Using the Math Class

In this exercise you will complete a program that computes two different distances. The first is the distance between two points in an ordinary coordinate system; the second is the horizontal distance that a projectile (such as a ball) will go when launched (thrown) at a given angle with a given initial velocity. The file contains an incomplete program. Complete it as follows.
  1. First add code to compute the distance between two points. Recall that the distance between the two points (x1, y1) and (x2, y2) is computed by taking the square root of the quantity (x1 - x2)2 + (y1 - y2)2. The program already has code to get the two points as input. You need to add an assignment statement to compute the distance and then a print statement that prints out the points and the distance. Test your program using the following data: The distance between the points (3, 17) and (8, 10) is 8.6023... (lots more digits printed); the distance between (-33, 49) and (-9, -15) is 68.352....

  2. Now add the code to compute the second distance - the distance a projectile will travel given the initial velocity and the angle. The variables and constants are already declared. The formula for the distance traveled (range) is
               range = sin(2*angle) * velocity2 / g
    where g is the gravitational constant (which is about 32.174 feet/sec/sec in English units) and the angle is measured in radians. You need to:
    1. Add code to prompt the user to enter the initial velocity (in feet/sec) and the angle (in degrees) the projectile will be thrown, then read in these values. (NOTE: Use the variables already declared.)
    2. Convert the number of degrees to radians. The formula is: the number of radians equals the number of degrees times PI divided by 180. Use the constant Math.PI from the Math library for the value of PI (this is much more accurate than defining your own constant).
    3. Now compute the range using the above formula. Use both the sin and the pow functions from the Math library.
    4. Finally, add a statement to print out the answer, appropriately labeled.
    5. Test your calculations: If the projectile is thrown with an initial velocity of 55 feet/sec and an angle of 35 degrees, its range would be about 88.3499... feet; if the initial velocity is 40 feet/sec and the angle is 50 degrees, the range is about 48.97409... feet.
    6. Print the completed program.

Using the Random Class

Write a complete Java program that simulates the rolling of a pair of dice. For each die in the pair, the program should generate a random number between 1 and 6 (inclusive). It should print out the result of the roll for each die and the total roll (the sum of the two dice), all appropriately labeled. You must use the Random class. The example from the Pre-Lab (which you can look at by bringing it up in Mozilla) or the example on page 126 of the text will help. Print your completed program.

Applets and Graphics

The following is a simple applet that draws a blue rectangle on a yellow background.

// ****************************************************************
// FILE:
// Author:
// Purpose: The program will draw two filled rectangles and a
//          filled oval positioned randomly on the screen.
// ****************************************************************

import javax.swing.JApplet;
import java.awt.*;

public class RandomShapes extends JApplet
    public void paint (Graphics page)
	// Declare size constants
	final int MAX_SIZE = 300;
	final int PAGE_WIDTH = 600;
	final int PAGE_HEIGHT = 400;

	// Declare variables
	int x, y;    // x and y coordinates of upper left-corner of each shape
	int width, height; // width and height of each shape 

	// Set the background color
	setBackground (Color.yellow);

	// Set the color for the next shape to be drawn
	page.setColor (;

	// Assign the corner point and width and height
	x = 200;
	y = 150;
	width = 100;
	height = 70;

	// Draw the rectangle
	page.fillRect(x, y, width, height);


Study the code noting the following:

Save the files and RandomShapes.html to your lab3 directory. (Warning: When you click on the link to RandomShapes.html the applet will start running. Using the File, Save As ... option will save the HTML code above.) Now do the following:

  1. Experiment with the coordinate system:
    1. Compile Normally an applet can be run through a browser such as Mozilla or IE. However, neither browser seems to run Java 1.5 applets (remember the browser needs a bytecode interpreter in order to run Java bytecode - it seems that the browsers have not been updated to the latest version of Java). However, there is a special program, called the appletviewer, that will run an applet. Run the program through the appletviewer by typing the command
              appletviewer RandomShapes.html
      at the shell prompt in your xterm window. You should see a new window open displaying a blue rectangle on a yellow background (you may need to click on Applet, then restart to get the background to show up).
    2. Close the applet window by clicking on Applet, then either close or quit. The control character CTRL-C will also close the applet window.
    3. Now open the program in emacs and change the x and y variables both to 0. Save and recompile the program, then view it in the Applet Viewer. What happened to the rectangle?
    4. Now change the width to 200 and the height to 300. Save, recompile and run to see how this affects the rectangle.
    5. Change x to 400, y to 40, width to 50 and height to 200. Test the program to see the effect.

  2. Modify the program so the position and size of the rectangle is random. To do this you need to:
    1. Add the command to import the java.util.Random class.
    2. Declare and instantiate a Random object named generator
    3. Modify the assignment statements to assign x a random value between 0 and PAGE_WIDTH and y a random value between 0 and PAGE_HEIGHT (use these contant identifiers). (Your random values should go up to but not include these maximums.)
    4. Modify the assignment statements to assign width and height random values between 50 and MAX_SIZE + 50 (up to but not including this value).
    Save, recompile, and run the program to test the changes.

  3. Now add two more random rectangles -- this only requires duplicating the code you already have so copying and pasting come in handy. Highlight the code beginning with the "Set the color for the next shape..." comment through the command to draw the rectangle. You may copy this using keystrokes (ESC-w copies, then CTRL-y pastes so you would do ESC-w once and CTRL-y two times to get the three rectangles). Test the changes.

  4. Change the colors for at least two of the shapes so the three shapes are different colors (a list of colors is on page 95) AND change one of the fillRect methods to fillOval so the final program draws two randomly positioned and sized rectangles and one oval.
  5. Test your program in the Applet Viewer. Each time you restart the appletviewer (either through Applet, restart from the menu or re-executing the appletviewer command) the program executes again, computing new random values for the positions and sizes of the shapes.

  6. Add a rectangle with your name in it at the bottom of the applet window as follows:
    1. Draw a rectangle at the bottom of the applet window centered from left to right. Make the rectangle at least 100 pixels wide and 50 high.
    2. Use the drawString method (see the example on page 97 and the description on page 100) to draw your name centered in the rectangle you just added. Note that for your name to show up, it must be in a different color than the rectangle. So, after drawing the rectangle, set the color to something different, then draw the string.
    3. Test your program - be sure the rectangle and string are placed correctly.

  7. Using Color objects You will now create your own colors for the rectangle and string you just added. The basic scheme for representing a picture in a computer is to break the picture down into small elements called pixels and then represent the color of each pixel by a numeric code (this idea is discussed in section 2.7 of the text). In most computer languages, including Java, the color is specified by three numbers -- one representing the amount of red in the color, another the amount of green, and the third the amount of blue. These numbers are referred to as the RGB value of the color. In Java, each of the three primary colors is represented by an 8-bit code. Hence, the possible base 10 values for each have a range of 0-255. Zero means none of that color while 255 means the maximum amount of the color. Pure red is represented by 255 for red, 0 for green, and 0 for blue, while magenta is a mix of red and blue (255 for red, 0 for green, and 255 for blue). In Java you can create your own colors rather then using the pre-defined colors, such as, from the Color class. One way to create a Color object is to declare a variable of type Color and instantiate it using the constructor that requires three integer parameters -- the first representing the amount of red, the second the amount of green, and the third the amount of blue in the color. The signature for the constructor is
               Color (int r, int g, int b)
    where r is the integer code for red, g the code for green, and b the code for blue. In a program we could declare the Color object myColor and instantiate it to a color with code 255 for red, 0 for green, and 255 for blue. Then the statement page.setColor(myColor) will set the foreground color for the page to be the color defined by the myColor object.

    Do the following:

    1. Before the statement that draws the bottom rectangle add two statements:
              Color myColor = new Color (200, 100, 255);
      The first declares myColor to be a Color object and instantiates it to be the color with color code 200 for red, 100 for green, and 255 for blue - this gives a shade of purple. The second sets the foreground color to be that color (so anything drawn afterwards will be that color). Compile and run the applet using the appletviewer.

    2. Change the instantiation (constructor) so the color code is (0,0,0) --- absence of color. What color should this be? Run the program to check.
    3. Try a few other combinations of color codes to see what you get. Page 95 of the text shows you the codes for the pre-defined colors in the Color class.
    4. Choose a color you like (not one of the pre-defined colors) to be the rectangle color.
    5. Now replace the statement that sets the color of your name with two statements - one that creates a new color for the string (use a color object) and the other that sets the foreground color to the color you created. Make sure the color you choose shows up so your name is visible!

  8. Print the final version of your program.