| This interface imposes a total ordering on the objects of each class that
implements it. This ordering is referred to as the class's natural
ordering, and the class's compareTo method is referred to as
its natural comparison method.
Lists (and arrays) of objects that implement this interface can be sorted
automatically by Collections.sort (and Arrays.sort).
Objects that implement this interface can be used as keys in a sorted map
or elements in a sorted set, without the need to specify a comparator.
The natural ordering for a class C is said to be consistent
with equals if and only if (e1.compareTo((Object)e2) == 0) has
the same boolean value as e1.equals((Object)e2) for every
e1 and e2 of class C. Note that null
is not an instance of any class, and e.compareTo(null) should
throw a NullPointerException even though e.equals(null)
returns false.
It is strongly recommended (though not required) that natural orderings be
consistent with equals. This is so because sorted sets (and sorted maps)
without explicit comparators behave "strangely" when they are used with
elements (or keys) whose natural ordering is inconsistent with equals. In
particular, such a sorted set (or sorted map) violates the general contract
for set (or map), which is defined in terms of the equals
method.
For example, if one adds two keys a and b such that
(!a.equals((Object)b) && a.compareTo((Object)b) == 0) to a sorted
set that does not use an explicit comparator, the second add
operation returns false (and the size of the sorted set does not increase)
because a and b are equivalent from the sorted set's
perspective.
Virtually all Java core classes that implement comparable have natural
orderings that are consistent with equals. One exception is
java.math.BigDecimal, whose natural ordering equates
BigDecimal objects with equal values and different precisions
(such as 4.0 and 4.00).
For the mathematically inclined, the relation that defines
the natural ordering on a given class C is:
{(x, y) such that x.compareTo((Object)y) <= 0}.
The quotient for this total order is:
{(x, y) such that x.compareTo((Object)y) == 0}.
It follows immediately from the contract for compareTo that the
quotient is an equivalence relation on C, and that the
natural ordering is a total order on C. When we say that a
class's natural ordering is consistent with equals, we mean that the
quotient for the natural ordering is the equivalence relation defined by
the class's equals(Object) method:
{(x, y) such that x.equals((Object)y)}.
This interface is a member of the
Java Collections Framework. | 