Laboratory 7: The Leap Year Applet

March 14, 15, 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 if-else and assignment statements.
3. To give practice using basic arithmetic and relational operators.

Problem statement

Problem Decomposition

Implementation

Step 1. Set Up.

• scramble.html is renamed leapyear.html
• ScrambleApplet.java is renamed LeapYearApplet.java
• ScrambleGUIPanel.java is renamed LeapYearGUIPanel.java

Step 2. Adapting the Applet and Panel Classes.

Modify both LeapYearApplet and LeapYearGUIPanel so they interface with the LeapYear class. Here are a list of required changes:

• 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 so that it inputs a year in the text field and reports in the display whether or not the year is a leap year whenever the "Check Year" button is pressed.

  One new thing this week is that you have to convert the String that you input from the text field into an int. Use the following code. For example, if str is storing "2004", the following expression will convert it to the int value 2004:

       Integer.parseInt(str)
  
  

Step 2. Adapting the Applet and Panel Classes.

• Something New Define the LeapYear class. Let's model this class after the Math class. If you look at page 215 of the text, you will find an outline of the Math class definition. The Math class is a final class, which means it can't be subclassed, and it has a private constructor method, which means it can't be instantiated. The reason for this is that all of its methods are static. This allows us to use mathematical methods without having to create Math instances, a convenience.

  Why do we follow this design for the LeapYear class? The reason is that we only need to define one method, isLeapYear(), which takes an int parameter, representing the year, and returns a boolean that represents whether the year is a leap year. The only information needed by this method is the year itself. So this class doesn't need instance variables. However, to be able to use the isLeapYear() method from the GUI, we will have to declare it public static. Then we will be able to reference it as follows:

      ... LeapYear.isLeapYear(2000) ...
      ... LeapYear.isLeapYear(aYear) ...
  

• Writing the isLeapYear() Method. One of the big challenges of this week's lab is writing a correct implementation of the isLeapYear method. This method will require a nested if/else statement, to handle the different cases in the definition. It will also require you to use the modulus operator (%) to define "divisible by". For example, the following expression tests whether the integer N is divisible by 2:

         N % 2 == 0
  

Hand In.

Before handing in your programs, be sure to document your source code.

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