public interface Comparable
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.
A class's natural ordering 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.