* * Lists (and arrays) of objects that implement this interface can be sorted * automatically by {@link Collections#sort(List) Collections.sort} (and * {@link Arrays#sort(Object[]) Arrays.sort}). Objects that implement this * interface can be used as keys in a {@linkplain SortedMap sorted map} or as * elements in a {@linkplain SortedSet sorted set}, without the need to * specify a {@linkplain Comparator comparator}.
* * The natural ordering for a class C is said to be consistent * with equals if and only if e1.compareTo(e2) == 0 has * the same boolean value as e1.equals(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(b) && a.compareTo(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(y) <= 0}.
* The quotient for this total order is:
* {(x, y) such that x.compareTo(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 {@link Object#equals(Object) equals(Object)} method:
* {(x, y) such that x.equals(y)}.
*
* This interface is a member of the
*
* Java Collections Framework.
*
* @param The implementor must ensure sgn(x.compareTo(y)) ==
* -sgn(y.compareTo(x)) for all x and y. (This
* implies that x.compareTo(y) must throw an exception iff
* y.compareTo(x) throws an exception.)
*
* The implementor must also ensure that the relation is transitive:
* (x.compareTo(y)>0 && y.compareTo(z)>0) implies
* x.compareTo(z)>0.
*
* Finally, the implementor must ensure that x.compareTo(y)==0
* implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for
* all z.
*
* It is strongly recommended, but not strictly required that
* (x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any
* class that implements the Comparable interface and violates
* this condition should clearly indicate this fact. The recommended
* language is "Note: this class has a natural ordering that is
* inconsistent with equals."
*
* In the foregoing description, the notation
* sgn(expression) designates the mathematical
* signum function, which is defined to return one of -1,
* 0, or 1 according to whether the value of
* expression is negative, zero or positive.
*
* @param o the object to be compared.
* @return a negative integer, zero, or a positive integer as this object
* is less than, equal to, or greater than the specified object.
*
* @throws NullPointerException if the specified object is null
* @throws ClassCastException if the specified object's type prevents it
* from being compared to this object.
*/
public int compareTo(T o);
}