• If there is a natural order for the values of the type, the interface and class should also provide a means for finding out whether one value of the type is less than another value.
• Some parts of the Collections API work better if your class implements the Comparable interface.

This interface is simply:

public interface Comparable
{
   public int compareTo(Object pObject);
}

In order for Date and DateImpl to do this, we need to change the header of the interface Date to be:

public interface Date extends Comparable

and to add the following declaration to the class declaration for DateImpl:

public int compareTo(final Object pObject)
{
   DateImpl tDateImpl = (DateImpl)pObject;
   int tResult = iYear - tDateImpl.iYear;
   if (tResult==0) {
      tResult = iMonth - tDateImpl.iMonth;
      if (tResult==0) {
         tResult = iDay - tDateImpl.iDay;
      }
   }
   return tResult;
}

The Comparable interface became part of Java when Java 2 was released in December 1998. Although most of the books that teach Java as a first language were prepared before then, a few have since been updated for Java 2. None of the ones that I've seen discuss the possibility of a class implementing the Comparable interface.