Structuring Data with Java - Java - page 266

gaining
mainly

And this is all as it should be.

On the other hand, you may be writing a class and want to build in the comparison function-
ality so that you don't always have to remember to pass the Comparator with it. In this case,
you can directly implement the java.lang.Comparable interface. The String class; the
wrapper classes Byte , Character , Double , Float , Long , Short , and Integer , BigInteger ,
and BigDecimal from java.math ; File from java.io ; java.util.Date ; and
java.text.CollationKey all implement this interface, so arrays or Collections of these
types can be sorted without providing a Comparator . Classes that implement Comparable
are said to have a “natural” ordering. The documentation strongly recommends that a class's
natural ordering be consistent with its equals() method, and it is consistent with equals()
if and only if e1.compareTo((Object)e2)==0 has the same Boolean value as
e1.equals((Object)e2) for every instance, e1 and e2 of the given class. This means that if
you implement Comparable , you should also implement equals() , and the logic of
equals() should be consistent with the logic of the compareTo() method. Here, for ex-
ample, is part of the appointment class Appt from a hypothetical scheduling program:

// public class Appt implements Comparable {
// Much code and variables omitted - see online version
//-----------------------------------------------------------------
// METHODS - COMPARISON
//-----------------------------------------------------------------
/** compareTo method, from Comparable interface.
* Compare this Appointment against another, for purposes of sorting.
* <P>Only text, and date and time participate, not repetition!
* (Repetition has to do with recurring events, e.g.,
* "Meeting every Tuesday at 9").
* This methods is consistent with equals().
* @return -1 if this<a2, +1 if this>a2, else 0.
*/
@Override
public public int int compareTo ( Appt a2 ) {

iif ( year < a2 . year )
return return - 1 ;

iif ( year > a2 . year )
return

return + 1 ;
iif ( month < a2 . month )
return return - 1 ;