This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class'snatural ordering, and the class'scompareTo method is referred to as itsnatural comparison method.

Lists (and arrays) of objects that implement this interface can be sorted automatically byCollections.sort (andArrays.sort). Objects that implement this interface can be used as keys in asorted map or as elements in asorted set, without the need to specify acomparator.

The natural ordering for a classC is said to beconsistent with equals if and only ife1.compareTo(e2) == 0 has the same boolean value ase1.equals(e2) for everye1 ande2 of classC. Note thatnull is not an instance of any class, ande.compareTo(null) should throw aNullPointerException even thoughe.equals(null) returnsfalse.

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 theequals method.

For example, if one adds two keysa andb such that(!a.equals(b) && a.compareTo(b) == 0) to a sorted set that does not use an explicit comparator, the secondadd operation returns false (and the size of the sorted set does not increase) becausea andb are equivalent from the sorted set's perspective.

Virtually all Java core classes that implementComparable have natural orderings that are consistent with equals. One exception isjava.math.BigDecimal, whose natural ordering equatesBigDecimal objects with equal values and different precisions (such as 4.0 and 4.00).

For the mathematically inclined, therelation that defines the natural ordering on a given class C is:

       {(x, y) such that x.compareTo(y) <= 0}.

Thequotient for this total order is:

       {(x, y) such that x.compareTo(y) == 0}.

It follows immediately from the contract forcompareTo that the quotient is anequivalence relation onC, and that the natural ordering is atotal order onC. When we say that a class's natural ordering isconsistent with equals, we mean that the quotient for the natural ordering is the equivalence relation defined by the class'sequals(Object) method:

     {(x, y) such that x.equals(y)}.

This interface is a member of the Java Collections Framework.