Search⌘ K
AI Features
Home
Courses
Java Masterclass for Software Developers

Debugging a Program in Development

Explore techniques to debug Java programs in development by using assertions, trace methods, and correcting switch statement errors. Understand how to test individual methods and trace program execution to identify and fix bugs effectively.

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. ...