Laboratory 8: Prime Number Finder

March 28, 29, 2006

Bring your Java, Java, Java text to the laboratory.

Objectives

1. To give practice designing and writing a simple Java program.
2. To give practice using for and while loops.
3. To give practice using basic arithmetic and relational operators.

Problem statement

Problem Decomposition

As we have done in previous programs, the applet will prompt the user for an integer. It will then pass that integer to the PrimeFinder's getPrimesLessThanN() method, which will return a String giving all of the prime numbers less than the user's number. The applet will display these numbers in its text area. (Note: It is important to insert tabs ("\t") and new lines ("\n") into the returned string so that the output is formatted in the text area.)

Note that the PrimeFinder class contains the a helper method named isPrime(). This method can be called by getPrimesLessThanN() to test whether a given integer is or is not a prime.

Implementation

Step 1. Set Up.

• index.html
• LeapYearApplet.java is renamed PrimesApplet.java
• LeapYearGUIPanel.java is renamed PrimesGUIPanel.java

Step 2. Adapting the Applet and Panel Classes.

As you did last week, modify both PrimesApplet and PrimesGUIPanel so they interface with the PrimeFinder class and have the appropriate components and documentation.

• Since you've renamed the applet and GUI files, you need to rename the class names and the constructor names.

• Modify the names of the text fields, buttons, and other components so that they are appropriate for this problem. For example, the user will be prompted to input a year not a guess.

• Modify the actionPerformed() method in the GUI class so that it inputs an integer in the text field and then displays a list of all primes less than that integer in the text area.

  As in last week's lab, you have to convert the String that is input into the text field into an int by using the Integer.parseInt() method. Consult your solution to last week's lab if you've forgotten how to do this.

Step 3. Designing the Loop Algorithms.

• findPrimesLessThanN() Method This method requires a loop that will go through all the values less than or equal to its parameter and test whether it is a prime or not. If it is a prime, it should append that value to a String that will accumulate the prime numbers.

  Design Questions: Is this a counting or non-counting loop? What Java loop statement should you use here?

  Here's some code to illustrate how to append the values 0..4 to a String variable using a for-loop.

      String s = "";    // Initialize a string to the empty string
      for (int k = 0; k < 5; k++)
          s = s + k + "\t";     // Append k and a tab to the string
      System.out.println(s);    // String s now equals 0 1 2 3 4
  

  Note: This is not the most efficient way to append values to a String. A better approach would use a StringBuffer, which we'll learn about in Chapter 7.

• isPrime() Method This method requires a loop to test whether its parameter is a prime or not. To determine whether N is prime we can attempt to divide N by 2, then 3, then 4, and so on up to N-1. If N is divisible by any of those values, it is not prime. Otherwise, it is prime. You should see from this description that we can stop testing for primality as soon as we find a number less than N that evenly divides N.

  Design Questions: Is this a counting or non-counting loop? What type of loop should you use? Don't forget every loop must have an initializer, a loop-entry condition and an updater that makes progress toward its stopping condition. How will you represent these?

Hand In.

Before handing in your programs, be sure to document your source code. Every method must have a comment block that describes the method, its parameters, and its return value.

Have your work checked by the laboratory instructor or TA. Print out and hand in a copy of your PrimesGUIPanel.java and PrimeFinder.java source code.