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?
