jaroslav@52: * jaroslav@52: * Lists (and arrays) of objects that implement this interface can be sorted jaroslav@52: * automatically by {@link Collections#sort(List) Collections.sort} (and jaroslav@52: * {@link Arrays#sort(Object[]) Arrays.sort}). Objects that implement this jaroslav@52: * interface can be used as keys in a {@linkplain SortedMap sorted map} or as jaroslav@52: * elements in a {@linkplain SortedSet sorted set}, without the need to jaroslav@52: * specify a {@linkplain Comparator comparator}.
jaroslav@52: * jaroslav@52: * The natural ordering for a class C is said to be consistent jaroslav@52: * with equals if and only if e1.compareTo(e2) == 0 has jaroslav@52: * the same boolean value as e1.equals(e2) for every jaroslav@52: * e1 and e2 of class C. Note that null jaroslav@52: * is not an instance of any class, and e.compareTo(null) should jaroslav@52: * throw a NullPointerException even though e.equals(null) jaroslav@52: * returns false.
jaroslav@52: * jaroslav@52: * It is strongly recommended (though not required) that natural orderings be jaroslav@52: * consistent with equals. This is so because sorted sets (and sorted maps) jaroslav@52: * without explicit comparators behave "strangely" when they are used with jaroslav@52: * elements (or keys) whose natural ordering is inconsistent with equals. In jaroslav@52: * particular, such a sorted set (or sorted map) violates the general contract jaroslav@52: * for set (or map), which is defined in terms of the equals jaroslav@52: * method.
jaroslav@52: * jaroslav@52: * For example, if one adds two keys a and b such that jaroslav@52: * (!a.equals(b) && a.compareTo(b) == 0) to a sorted jaroslav@52: * set that does not use an explicit comparator, the second add jaroslav@52: * operation returns false (and the size of the sorted set does not increase) jaroslav@52: * because a and b are equivalent from the sorted set's jaroslav@52: * perspective.
jaroslav@52: * jaroslav@52: * Virtually all Java core classes that implement Comparable have natural jaroslav@52: * orderings that are consistent with equals. One exception is jaroslav@52: * java.math.BigDecimal, whose natural ordering equates jaroslav@52: * BigDecimal objects with equal values and different precisions jaroslav@52: * (such as 4.0 and 4.00).
jaroslav@52: * jaroslav@52: * For the mathematically inclined, the relation that defines jaroslav@52: * the natural ordering on a given class C is:
jaroslav@52: * {(x, y) such that x.compareTo(y) <= 0}.
jaroslav@52: * The quotient for this total order is:
jaroslav@52: * {(x, y) such that x.compareTo(y) == 0}.
jaroslav@52: *
jaroslav@52: *
jaroslav@52: * It follows immediately from the contract for compareTo that the
jaroslav@52: * quotient is an equivalence relation on C, and that the
jaroslav@52: * natural ordering is a total order on C. When we say that a
jaroslav@52: * class's natural ordering is consistent with equals, we mean that the
jaroslav@52: * quotient for the natural ordering is the equivalence relation defined by
jaroslav@52: * the class's {@link Object#equals(Object) equals(Object)} method:
jaroslav@52: * {(x, y) such that x.equals(y)}.
jaroslav@52: *
jaroslav@52: * This interface is a member of the
jaroslav@52: *
jaroslav@52: * Java Collections Framework.
jaroslav@52: *
jaroslav@52: * @param The implementor must ensure sgn(x.compareTo(y)) ==
jaroslav@52: * -sgn(y.compareTo(x)) for all x and y. (This
jaroslav@52: * implies that x.compareTo(y) must throw an exception iff
jaroslav@52: * y.compareTo(x) throws an exception.)
jaroslav@52: *
jaroslav@52: * The implementor must also ensure that the relation is transitive:
jaroslav@52: * (x.compareTo(y)>0 && y.compareTo(z)>0) implies
jaroslav@52: * x.compareTo(z)>0.
jaroslav@52: *
jaroslav@52: * Finally, the implementor must ensure that x.compareTo(y)==0
jaroslav@52: * implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for
jaroslav@52: * all z.
jaroslav@52: *
jaroslav@52: * It is strongly recommended, but not strictly required that
jaroslav@52: * (x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any
jaroslav@52: * class that implements the Comparable interface and violates
jaroslav@52: * this condition should clearly indicate this fact. The recommended
jaroslav@52: * language is "Note: this class has a natural ordering that is
jaroslav@52: * inconsistent with equals."
jaroslav@52: *
jaroslav@52: * In the foregoing description, the notation
jaroslav@52: * sgn(expression) designates the mathematical
jaroslav@52: * signum function, which is defined to return one of -1,
jaroslav@52: * 0, or 1 according to whether the value of
jaroslav@52: * expression is negative, zero or positive.
jaroslav@52: *
jaroslav@52: * @param o the object to be compared.
jaroslav@52: * @return a negative integer, zero, or a positive integer as this object
jaroslav@52: * is less than, equal to, or greater than the specified object.
jaroslav@52: *
jaroslav@52: * @throws NullPointerException if the specified object is null
jaroslav@52: * @throws ClassCastException if the specified object's type prevents it
jaroslav@52: * from being compared to this object.
jaroslav@52: */
jaroslav@52: public int compareTo(T o);
jaroslav@52: }