T
 the type of objects that this object may be compared topublic interface Comparable<T>
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
equals(Object)
method:
{(x, y) such that x.equals(y)}.
This interface is a member of the Java Collections Framework.
Modifier and Type  Method and Description 

int 
compareTo(T o)
Compares this object with the specified object for order.

int compareTo(T o)
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.
o
 the object to be compared.NullPointerException
 if the specified object is nullClassCastException
 if the specified object's type prevents it from being compared to this object.