Lab 3 In-Class: Using Objects and Methods

Getting Started

Using the String Class

// ***************************************************************
// FILE:  StringManips.java
// Author:
//
// Purpose: Test several methods for manipulating String objects
// ***************************************************************

import cs1.Keyboard;

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

	// 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();
	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: " + 
			    phrase.charAt(middleIndex));
	System.out.println ("Switched phrase: " + switchedPhrase);

	System.out.println();
    }
}

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). Add a println statement to print out the result. 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. 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 appropriate Keyboard class method. 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

          NORTH CAROLINAlilesvilleNORTH CAROLINA
   

4. Print the final version of the program. Remember you can use the print command you created last lab: ~/print   StringManips.java

Using the Math Class

Using the Random Class

Applets and Graphics

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

import java.applet.Applet;
import java.awt.*;

public class RandomShapes extends Applet
{
    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);

	// Draw a rectangle in the background color to cover the whole window
	page.setColor (Color.yellow);
	page.fillRect (0, 0, PAGE_WIDTH, PAGE_HEIGHT);

	// 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 = 150;
	width = 100;
	height = 70;

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

    }
}

• The program imports java.applet.Applet because this is an applet 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 112), including the fillRect method used in this example (which draws a rectangle filled with the current foreground color), require parameters that specify the upper left-hand corner of the rectangle (using the coordinate system described in section 1.5) and the width and the height of the rectangle.
• 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="RandomShapes.class" width=600 height=400>
  </applet>
  </html> 
  

Save the files RandomShapes.java 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. Compile RandomShapes.java (but don't run -- this is an applet so it is run through a browser or a special program called the Applet Viewer).
2. Run the program through the Mozilla browser by choosing File, Open Page from the menu bar. Click on Choose File, then browse for the file (which should be in ~/cpsc120/labs/lab3/RandomShapes.html), then click on Open. You should see a blue rectangle on a yellow background.
3. Now run the program through the Applet Viewer by typing the command

           appletviewer RandomShapes.html
   

   at the shell prompt in your xterm window. You should see a new window open displaying the rectangle.
4. 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 (this is generally less trouble when making lots of changes than using the browser). What happened to the rectangle?
5. Now change the width to 200 and the height to 300. Save, recompile and run to see how this affects the rectangle.
6. Change x to 400, y to 40, width to 50 and height to 200. Test the program to see the effect.
7. Modify the program so the position and size of the rectangle is random. To do this you need to
   • Add the command to import the java.util.Random class.
   • Declare and instantiate a Random object named generator
   • 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.)
   • Modify the assignment statements to assign width and height random values between 50 and MAX_SIZE + 50.