Suppose that, besides Date we have also created other classes that implement the DateIF interface, e.g.:

0779: public class DateA implements DateIF { ... }
0780: public class DateB implements DateIF { ... }

If the code of some client needs to refer to a class (e.g., a parameter of some method is a date), then it should use the name of the interface (e.g., DateIF) rather than the name of a class (e.g., Date). For example:

0781: private static boolean iIsLeap(final DateIF pDateIF) {
0782:    final int tYear = pDateIF.getYear();
0783:    return (tYear%400==0) || (tYear%4==0 && tYear%100!=0);
0784: }

The only time when the code of a client needs to refer to one of the classes is when it wants to create a date. Then it has to choose which implementation to use, e.g., to choose between one of the following statements:

0785: DateIF tDateIF = new Date();
0786: DateIF tDateIF = new DateA();
0787: DateIF tDateIF = new DateB();

The variable tDateIF is a reference variable that is of an interface type. It can refer to any object which is of a class that implements the interface.