Lab 3: Using Objects and Methods

Lab Notes and Pre-Lab Assignment

In this week's lab you will write Java programs that use the classes String, Random, and Math defined in the Java Standard Class Library in addition to the Keyboard class defined by the authors of the textbook. At the end of the lab, you will write an applet that uses the Graphics and Color classes which are also part of the Java library. The main concepts we will focus on are in the text in sections 2.6 - 2.9 (for objects and methods) and 1.5, 2.10 and 2.11 (for applets and graphics). The goals of lab are for you to gain experience with the following concepts:

Exercise #1: Fill in the blanks in the program below as follows: (Section 2.6, especially the example on page 90, should be helpful):
(a) declare the variable town as a reference to a String object and initialize it to "Salem, VA".
(b) write an assignment statement that invokes the length method of the String class to find the length of the college String object and assigns the result to the stringLength variable
(c) complete the assignment statement so that change1 contains the same characters as college but all in upper case
(d) complete the assignment statement so that change2 is the same as change1 except all capital O's are replaced with the asterisk (*) character.
(e) complete the assignment statement so that change3 is the concatenation of college and town (use the concat method of the String class rather than the + operator)

// **************************************************
// FILE:  StringPlay.java
// Author: J. Ingram
// Purpose: Play with String objects
// **************************************************
public class StringPlay
{
   public static void main (String[] args)
   {
      String college = new String ("Roanoke College");

      ________________________________________________________; // part (a)

      int stringLength;
      String change1, change2, change3; 

      ________________________________________________________; // part (b)

      System.out.println (college + " contains " + stringLength + " characters.");

      change1 = ______________________________________________; // part (c)

      change2 = ______________________________________________; // part (d)

      change3 = ______________________________________________; // part (e)

      System.out.println ("The final string is " + change3);
    }
}

Exercise #2: The following program should read in the lengths of two sides of a right triangle and compute the length of the hypotenuse (recall that the length of the hypotenuse is the square root of the sum of side 1 squared plus side 2 squared). Complete it by adding statements to read the input from the keyboard and to compute the length of the hypotenuse (you need to use a Math class method for that).

// *******************************************************************
// FILE: RightTriangle.java
// Purpose: Compute the length of the hypotenuse of a right triangle
//          given the lengths of the sides     
// *******************************************************************
import cs1.Keyboard;

public class RightTriangle
{
   public static void main (String[] args)
   {
      double side1, side2;  // lengths of the sides of a right triangle
      double hypotenuse;    // length of the hypotenuse

      // Get the lengths of the sides as input
      System.out.print ("Please enter the lengths of the two sides of " +
                          "a right triangle (separate by a blank space): ");
      
      _____________________________________________________________;

      _____________________________________________________________;

      // Compute the length of the hypotenuse

      _____________________________________________________________;

      // Print the result
      System.out.println ("Length of the hypotenuse: " + hypotenuse);
    }
{

Exercise #3: In many situations a program needs to generate a random number in a certain range. The Java Random class lets the programmer create objects of type Random and use them to generate a stream of random numbers (one at a time). The following declares the variable generator to be an object of type Random and instantiates it with the new operator.

     Random generator = new Random();
The generator object can be used to generate either integer or floating point random numbers using either the nextInt() or nextDouble() methods, respectively. The integer returned by nextInt could be any valid integer (positive or negative) whereas the number returned by nextDouble is between 0 and 1 (up to but not including the 1). Most often the goal of a program is to generate a random integer in some particular range, say 30 to 99 (inclusive). There are three ways to do this.

Fill in the blanks in the following program to generate the random numbers as described in the documentation. NOTE that that java.util.Random must be imported to use the Random class.


// **************************************************
// FILE: LuckyNumbers.java
// Purpose: To generate three random "lucky" numbers 
// **************************************************
import java.util.Random;

public class LuckyNumbers
{
   public static void main (String[] args)
   {
      Random generator = new Random();
      int lucky1, lucky2, lucky3;   

      // Generate lucky1 (a random integer in the range 50 to 79) using 
      // the nextInt method with no argument

      lucky1 = ______________________________________________________;
 
      // Generate lucky2 (a random integer in the range 11 to 30) using 
      // nextInt with an argument

      lucky2 = ______________________________________________________;

      // Generate lucky3 (a random integer in the range 20 to 40) using nextDouble

      lucky3 = ______________________________________________________;

      System.out.println ("Your lucky numbers are " + lucky1 + ", " + lucky2 +
           ", and " + lucky3);
    }
}

Exercise #4: In lab you will write an applet that does some simple graphics (these concepts are introduced in Sections 1.5, 2.10, and 2.11 of the text). What is the difference between an applet and an application?