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Java Masterclass for Software Developers

Debugging a Program in Development

In this lesson, we will develop a program to display a calendar and add statements to trace the program's execution to help us find a coding error.

We'll cover the following...

The calendar problem

Imagine a Java program that displays a complete calendar for any given year. We already saw how to find the day of the week for any given date and how to detect when a year is a leap year. In the previous lesson, we learned how to use a switch statement to get the number of days in a given month. Using these basic ideas, we could begin the definition of a class of calendars.

A first-draft solution

The program given below shows such a class still in development. The plan is to focus on computing the number of days in a month by writing and testing the private method getDaysInMonth. The class has temporary implementations, or stubs, for other methods in the class. A short main method in the class Driver tests the class for two different years, one of which is a leap year. Click the RUN button to see what happens.

Java
/** Calendar.java by F. M. Carrano
    A class to display a calendar for a given year.
*/
public class Calendar
{
   private final int year;
	
   public Calendar(int theYear)
   {
      year = theYear;
   } // End constructor
	
   /** Displays the 12 months in a year.	*/
   public void display()
   {
      System.out.println("For the year " + year + ", ");
      displayMonth(1);
      displayMonth(2);
      displayMonth(3);
      displayMonth(4);
      displayMonth(5);
      displayMonth(6);
      displayMonth(7);
      displayMonth(8);
      displayMonth(9);
      displayMonth(10);
      displayMonth(11);
      displayMonth(12);
   } // End display
	
   // Displays the calendar for a given month.
   private void displayMonth(int month) // STUB
   {
      int numberOfDays = getDaysInMonth(month);
      System.out.println("    Month " + month + " has " + numberOfDays + " days.");
   } // End displayMonth
	
   // Returns the number of days in a given month for a specific year.
   private int getDaysInMonth(int month)
   {
      int daysInMonth;
      switch (month)
      {
         // Sept, Apr, June, Nov
         case 9: case 4: case 6: case 11: 
            daysInMonth = 30;
            break;
         // Feb.
         case 2:                          
            if (isLeapYear())
               daysInMonth = 29;
            else
               daysInMonth = 28;
         // All other months
         default:
            daysInMonth = 31;
         } // end switch
		 
         return daysInMonth;
   } // End getDaysInMonth
   // Returns true if the year is a leap year; otherwise, returns false. 
   private boolean isLeapYear()
   {		
      // A year is a leap year if it is 
      //   a) Divisible by 4 but not divisible by 100, or
      //   b) Divisible by 400. 
      return ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
   } // End isLeapYear
	
   // Other methods are here . . .
 
} // End Calendar

Observe that the output is correct, except for month 2. February does not have 31 days, even in a leap year. Let’s look at what we have. The class begins with one data field, year, and a constructor that initializes it. The method display, which should display a complete calendar, produces only basic output for now. It can have a less tedious definition after we study the repetition statements in the next chapter. This method calls the private method displayMonth, which is a stub that in turn calls the private method getDaysInMonth. The latter method is the one we want to develop. Since getDaysInMonth calls the private method isLeapYear, writing a stub for isLeapYear would normally make sense. ...