Java Reference
In-Depth Information
21.3. Ordering with
Comparable
and
Comparator
The interface
java.lang.Comparable<T>
can be implemented by any class
whose objects can be sorted. The interface has a single method:
public int
compareTo(T other)
Returns a value that is less than, equal to, or greater than zero
as this object is less than, equal to, or greater than the
other
object. This method should return zero only if
equals
with the
same object would return
true
. If the objects are not mutually
comparable (such as an
Integer
with a
String
), a
ClassCastEx-
ception
is thrown.
The ordering defined by
compareTo
is a class's
natural ordering,
that is,
the ordering that is most natural to objects of the class. It is also a
total
ordering,
meaning that any two objects of the same type must be mutu-
ally comparablethat is, you must be able to compare them in either order
and always get the same result as to which is the larger. Similarly, the
equals
method defines a
natural equivalence.
Many existing classes are
Comparable
, including
String
,
java.io.File
,
java.util.Date
, and all the primitive wrapper class types.
If a given class does not implement
Comparable
or if its natural ordering
is wrong for some purpose, you can often provide a
java.util.Comparator
object instead. The
Comparator<T>
interface has the method
public int
compare(T o1, T o2)
Provides
an
ordering
in
the
same
manner
as
Compar-
able.compareTo
for the two provided objects.
You can use
Comparable
and
Comparator
objects to sort and search
List
ob-
jects with the
Collections
class's static methods
sort
and
binarySearch
.
