The document discusses the differences between the Comparable and Comparator interfaces in Java. Comparable allows objects to be sorted based on natural ordering by implementing the compareTo() method. Comparator allows objects to be compared and sorted based on arbitrary criteria by implementing the compare() method. Guidelines are provided for when to use each interface, such as using Comparable for a class's natural ordering and Comparator when multiple orderings are possible or the class cannot be modified. Examples of implementing each are also included.

1. Java Comparable vs Comparator
Sujit Kumar
Zenolocity LLC © 2013-2023

2. java.lang.Comparable Interface
• Allows an object of a class to be compared to
objects of the same class using a 1 or more
attributes of the same class.
• compareTo method is the ONLY member of
the Comparable interface. Allows objects of a
class to be sorted by natural ordering.
• Implementing Comparable allows:
• calling Collections.sort & Collections.binarySearch
• calling Arrays.sort and Arrays.binarySearch
• using objects as keys in a TreeMap
• using objects as elements in a TreeSet

3. compareTo method
• Return value is int, 0 => equal, -ve => less than and +ve
=> greater than
• anticommutation :
x.compareTo(y) = - (y.compareTo(x))
• exception symmetry :
x.compareTo(y) throws exactly the same exceptions
as y.compareTo(x)
• transitivity :
if x.compareTo(y) > 0 and y.compareTo(z) > 0,
then x.compareTo(z) > 0 (and same for less than)
if x.compareTo(y) == 0, then x.compareTo(z) has the same sign
as y.compareTo(z)

4. Example Implementation of
compareTo
public int compareTo(Employee emp) {
int result = this.id.compareTo(emp.getId());
if (result == 0) {
result = this.age > emp.age ? 1 : this.age < emp.age
? -1 : 0;
}
return result;
}

5. Performance Considerations
• One can greatly increase the performance
of compareTo by comparing first on attributes
which are most likely to differ.
• Preferable to always sort items in an RDBMS
first using ORDER BY instead of in memory in
java using Comparable.

6. java.util.Comparator Interface
• public int compare (Object o1, Object o2);
• Logical difference between Comparator and
Comparable is :
Comparator in Java compares any two objects,
while Comparable interface compares the
"this" reference with the object specified.
• Return value semantics is same as the
compareTo method of Comparable interface.

7. Guidelines on when to use
Comparable and Comparator
• For natural (intuitive) ordering use Comparable. Use
Comparator if you want to have an ordering different
from the natural order. If there is more than one
intuitive comparison possible, use a Comparator.
• Use comparable if class is in your control. Use
Comparator if class is NOT in your control and you cannot
make the class author implement Comparable.
• Comparable should be used when you compare instances
of same class (homogeneous). Comparator can be used
to compare instances of same or different classes
(homogeneous and heterogeneous).

8. Guidelines (continued…)
• Comparator has a distinct advantage of being
self descriptive. Examples:
• If you are writing Comparator to compare two
Employees based upon the salary then name
that comparator as SalaryComparator.
• If you are writing Comparator to compare two
Employees based upon the age then name
that comparator as AgeComparator.

9. Examples of Comparator
public static Comparator<Employee> EmployeeAgeComparator
= new Comparator<Employee>() {
public int compare(Employee emp1, Employee emp2) {
return emp1.age.compareTo(emp2);
}
};
public static Comparator<Employee> EmployeeNameComparator
= new Comparator<Employee>() {
public int compare(Employee emp1, Employee emp2) {
return emp1.name.compareTo(emp2)
}
};

10. How to Remember?
• Am I (this) Comparable to (compareTo) my
friend?
• Can I compare (Comparator) apples to
oranges?