Testing Classes

Context

A class should be unit tested before integrating it with a larger program. Bugs are easier to find and fix without dealing with complex interactions between classes. By including a main method in each class to test just that class, the testing can be accomplished in an easy and uniform manner.

Example

public class Date
{  private int year;
   private int month;
   private int day;

   public Date(int aYear, int aMonth, int aDay)
   {  this.year = aYear;
      this.month = aMonth;
      this.day = aDay;
   }

   /** Is this date before date d? */
   public boolean before(Date d)
   {  return (this.year < d.year) || 
             (this.year == d.year && this.month < d.month) ||
             (this.year == d.year && this.month == d.month && this.day < d.day);
   }

   /** Convert a Date object to a String */
   public String toString()
   {  return this.year + "/" + this.month + "/" + this.day;
   }


   /** Test the class. */
   public static void main(String[] args)
   {  Date d1 = new Date(2005, 12, 25);
      Date d2 = new Date(2005, 12, 24);

      String t1 = d1 + " before " + d2;
      String t2 = d2 + " before " + d1;
      String t3 = d1 + " before " + d1;

      test("t1", d1.before(d2), false);
      test("t2", d2.before(d1), true);
      test("t3", d1.before(d1), false);
   }

   private static void test(String msg, boolean answer, boolean expected)
   {  if (answer == expected)
      {  System.out.print("passed ");
      } else
      {  System.out.println("FAILED ");
      }
      System.out.println("Test: '" + msg + "'");
      System.out.println("       Expected: '" + expected + "'");
      System.out.println("       Got:      '" + expected + "'");
      System.out.println();
   }
}

Pattern

public class <name-of-class>
{  <instance-variables>
   <methods>

   /** Test the class. */
   public static void main(String[] args)
   {
      <code-to-test-class>
      System.out.println(<message-to-report-test-results>);
   }
}

Notes

• An auxiliary function is often useful, such as test in the Example. Such methods must be declared static unless an object on this class is instantiated and test is invoked via that object.
• It is useful to have the test code determine if the test actually passed, rather than simply printing out values which then need to be verified by hand or may be meaningless to the programmer when testing again at a later date.
• '||' means 'or'
• When the the test strings t1 to t3 are being initialized, d1 and d2 are automatically converted to Strings using the toString method.

Related Patterns

• Console Application