Lab 3 In-Class: Using Objects and Methods

Getting Started

Using the String Class - Exercise #1

1. Study the program to see what is already there.
2. Follow the comments to add statements to complete the program (the comments indicating things you are to do start with *** - there are 8 of them).
3. Compile and run the program. Test it on several ending phrases (for example the ending phrase in your book's program is except from vending machines).

Using the String Class - Exercise #2

       double price = 34.56;

       Variable Type:       double
       Variable Name:       price
       Initial Value:       34.56

        
       int  indexOf(String str)
 
       Returns the index within this string of the first occurrence of the 
       specified substring (str).

       String title = "Java Software Solutions";

We can use the indexOf method to find the locations of the characters that separate the parts of the declaration/initialization. For example, if we know the index of the first blank space we can extract the substring for the type (that would tell us the index of the blank space that marks the end of the type and we know the index of the beginning of the type, right?).

Download the file DeclarationParse.java, open it in emacs, and complete the program using the comments as a guide. Be sure to test the program on several different declaration/initializations.

Using the Math Class

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

To be fair, she wants the "WRKE Super Prize Birthday" to be selected at random. Write a complete Java program that will randomly generate a month (1-12) and day (1-31) for her to announce on the air. Your program should print out a message that includes some text (e.g. "The WRKE Super Prize Birthday is: ") as well as the numeric representation of the date (e.g. "2/28"). Your program must include comments to explain why you will occasionally produce an invalid date.

Applets and Graphics

// ****************************************************************
// FILE:  SimpleShape.java
// Author:
//
// Purpose: The program will draw a simple shape on the screen
// ****************************************************************

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

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

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

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

	// Assign the corner point and width and height
	x = 200;
	y = 100;
	width = 200;
	height = 200;
	page.drawOval(x, y, width, height);
    }
}

• The program imports javax.swing.JApplet because this is an applet (actually it is a JApplet defined in the Java swing package) and it imports java.awt.* because it uses graphics.
• There is no main method -- instead there is a paint method. The paint method is automatically invoked when an applet is executed, just as the main method is automatically invoked when an application is executed.
• Most of the methods that draw shapes (see the list on page 98), including the drawOval method used in this example, require parameters that specify the upper left-hand corner of the shape (using the coordinate system described in section 2.7) and the width and the height of the shape.
• This applet will be drawn assuming the window for drawing (the Graphics object - named page here) is 600 pixels wide and 400 pixels high. These numbers are defined in constants at the beginning of the program (and currently have no use but you will use them later). The width and height of the applet are actually specified in the HTML file that instructs the Web browser to run the applet (remember applets are executed by Web browsers and Web browsers get their instructions from HTML documents -- note that the code executed by the browser is the bytecode for the program, the RandomShapes.class file). The code in the HTML document is as follows:

  <html>
  <applet code="SimpleShape.class" width=600 height=400>
  </applet>
  </html> 
  

Save the files SimpleShape.java and SimpleShape.html to your lab3 directory. (Warning: When you click on the link to SimpleShape.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 SimpleShape.java. Normally an applet can be run through a browser such as Mozilla Firefox or IE. However, occasionally a browser will not run Java 1.5 applets (remember the browser needs a bytecode interpreter in order to run Java bytecode - some browsers have not been updated to the latest version of Java). For program development, there is a special program, called the appletviewer, that will run an applet. Run the program through the appletviewer by typing the command

              appletviewer SimpleShape.html
      

      at the shell prompt in your xterm window. You should see a new window open displaying a blue circle.
   2. Close the applet window by clicking on Applet, then either close or quit, or just clicking the X.
   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 circle?
   4. Now change the width to 200 and the height to 300. Save, recompile and run to see how this affects the circle.
   5. Change x to 400, y to 40, width to 50 and height to 200. Test the program to see the effect.

2. Copy the program to a new file named Darts.java. We are going to expand this applet to draw a dartboard and throw some darts. To do this we will need to follow several steps
   1. Open the Darts.java file and make the necessary changes to make it compile.

   2. Remember that applets are embedded in HTML documents - so copy the SimpleShape.html to Darts.html and make the necessary changes to that file.

   3. Use the drawOval method to draw 4 concentric circles that are located in the center of the viewing window. Start with a big one:
      • We know that the the maximum height is 400 (because that is the size of the window), so

        height = PAGE_HEIGHT;

      • Since we want a circle, the width and the height are the same:

         width = height;

      • The center of the screen is at (PAGE_WIDTH/2, PAGE_HEIGHT/2). But we need the upper left corner, which must factor in the height and width of the circle. so

             x = PAGE_WIDTH/2 - width/2;
             y = PAGE_HEIGHT/2 - height/2;
        

      • Test the first circle to make sure that it works

      • To draw additional circles, work through this same process. Consider what changes need to be made if you want to draw the second circle with a height that is 100 units smaller.

   4. Once you have the dartboard drawn,lets add some darts. To represent a dart, we are going to use the drawLine method, which works as follows:

          void drawLine (int x1, int y1, int x2, int y2)
                // draws a line from the point (x1,y1) to the point (x2,y2) 
      

      Use two drawline methods to draw a dart shaped like an 'X' that is 9 pixels wide by 9 pixels high, centered on the page.

   5. Now lets use the Random class to simulate throwing 3 darts at the board. To do this, you will need to
      • Add the command to import the java.util.Random class.
      • Declare and instantiate a Random object named generator
      • Declare two variables to hold the center coordinates of a dart.
      • Use generator to get random values that can be assigned to the coordinates of the dart (Be mindful of the boundaries for your random numbers) and then draw the dart on the screen.
      • Compile and view the applet to make sure all is well.
      • Repeat the last step to handle multiple darts.

   6. Go back into your code and change all the drawOval commands to fillOval commands. Save, recompile and execute. What happened to all your circles?
      Since your circles overlap, you will need to change the color for each circle to make them distinct. You can do this by using the setColor method.

          void setColor(Color color)
      

      Notice that this method takes a color object as a parameter. A list of predefined colors objects can be found on page 93 of your textbook. Adjust your code to use the setColor method to make each circle a different color.

3. 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 using the fillRect method (see the example on page 100 and the description on page 98). Position the rectangle so that it is in the lower right corner of the screen. Make the rectangle at least 100 pixels wide and 50 high.
   2. Use the drawString method to draw your name (approximately) 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.

4. Using Color objects You will now learn how to create your own colors for the shapes that you have been drawing. 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 Color.red, 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. Immediately before the statement that draws the rectangle add two statements:

              Color myColor = new Color (200, 100, 255);
              page.setColor(myColor);
      

      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. Choose a set of colors you like (not the pre-defined colors) to use for the rectangle.
   4. 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!