1. Comparable Vs Comparator
Both are used to sort the objects of the class.
Packages:
Comparable is in: java.lang
Comparator is in: java.util
Syntax:
Interface Comparable<T>
T- the type of object that this object may be compared to
Interface Comparator<T>
T- the type of object that this object may be compared to
Class implementing Comparable interface need to override
int compareTO(T o)
which compares this obj with specified object for order.
Class implementing Comparator interface need to override
int compare(T o1, T o2)
compares its two arguments for order.
boolean equals(Object o)
indicates whether some other object is "equal to" this comparator.
Comparable
When we create list(or array) of object of user defined class, we cannot call
Collection.sort() method to sort object in the list, because they do not have
natural ordering.
To define natural ordering we implement Comparable interface on the Object we
want to compare our class to.
Implementing class must override compareTo(Object o) method
Example:
class Employee implements Comparable<Employee>
{
int id;
String name;
int compareTo(Employee emp)
{
return (this.id<emp.id) ? -1 : (this.id>emp.id) ? 1 : 0;
}
}

2. When -1: this returned that means: this object’s id is less than emp.id
When 1: this returned that means: this object’s id is greater than emp.id
When 0: both objects are equal.
OR
return this.id.compareTo(emp.id);
This will return result in ascending order.
To print in descending order:
return –this.id.compareTo(emp.id); // - negative sign returns decending
Comparator
The class whose objects to be sorted do not need to implement this interface.
Some third class can implement this interface to sort. E.g.,
EmployeeSortByIdComparator class can implement Comparator interface to sort
collection of employee object by id.
Using Comparator interface, we can write different sorting based on different
attributes of objects to be sorted.
Parameter
Comparable
Comparator
Sorting Logic
Sorting logic must be in same
class whose objects are being
sorted. Hence this is called
natural ordering of objects
Sorting logic is in separate
class. Hence we can write
different sorting based on
different attributes of objects
to be sorted. E.g. Sorting
using id,name etc.
Implementation
Class whose objects to be sorted
must implement this interface.
e.g. Employee class needs to
implement comparable to
collection of country object by id
Sorting method
int compareTo(Object o1)
This method compares this object
with o1 object and returns a
integer. Its value has following
meaning
positive – this object is greater
than o1
zero – this object equals to o1
Class whose objects to be
sorted do not need to
implement this interface.
Some other class can
implement this interface.
E.g.EmployeeSortByIdComparator
class can implement
Comparator interface to sort
collection of country object by
id
int compare(Object o1,Object
o2)
This method compares o1 and
o2 objects. and returns a
integer.Its value has following
meaning.
positive – o1 is greater than
o2

3. Calling method
negative – this object is less than
o1
Collections.sort(List)
Here objects will be sorted on the
basis of CompareTo method
zero – o1 equals to o2
negative – o1 is less than o1
Collections.sort(List,
Comparator)
Here objects will be sorted on
the basis of Compare method
in Comparator