I just wanted to copy it in case this link disappears. Thanks Kamal!
Java Comparators and Comparables? What are they? How do we use them? This is a question
we received from one of our readers. This article will discuss the java.util.Comparator
and java.lang.Comparable in details with a set of sample codes for further
clarifications.
Prerequisites
- Basic Java knowledge
System Requirements
- JDK installed
What are Java Comparators and Comparables?
As both namessuggest (and you may have guessed), these are used for comparing objects in Java. Using
these concepts; Java objects can be
sorted according to a predefined order.
Two of these concepts can be explained as follows.
Comparable
A comparable object is capable of comparingitself with another object. The class itself must implements the java.lang.Comparable
interface in order to be able to compare its instances.
Comparator
A comparator object is capable of comparingtwo different objects. The class is not comparing its instances, but some other
class's instances. This comparator class must implement the
java.util.Comparator interface.
Do we need to compare objects?
The simplest answer is yes. When there is alist of objects, ordering these objects into different orders becomes a must in some
situations. For example; think of displaying a list of employee objects in a web page.
Generally employees may be displayed by sorting them using the employee id. Also there
will be requirements to sort them according to the name or age as well. In these
situations both these (above defined) concepts will become handy.
How to use these?
There are two interfaces in Java tosupport these concepts, and each of these has one method to be implemented by
user.
Those are;
java.lang.Comparable: int compareTo(Object o1)
This method compares this object with o1 object. Returned int value has the following
meanings.
- positive : this object is greater than o1
- zero : this object equals to o1
- negative : this object is less than o1
java.util.Comparator: int compare(Object o1, Objecto2)
This method compares o1 and o2 objects. Returned int value has the following meanings.
- positive : o1 is greater than o2
- zero : o1 equals to o2
- negative : o1 is less than o1
java.util.Collections.sort(List) and java.util.Arrays.sort(Object[])
methods can be used to sort using natural ordering of objects.
java.util.Collections.sort(List, Comparator) and
java.util.Arrays.sort(Object[], Comparator) methods can be used if a Comparator
is available for comparison.
The above explained Employee example is a good candidate for explaining these two
concepts. First we'll write a simple Java bean to represent the
Employee.
public class Employee { private int empId; private String name; private int age; public Employee(int empId, String name, int age) { // set values on attributes } // getters & setters }
Next we'll create a list of Employees for using in different sorting
requirements. Employees are added to a List without any specific order in the following
class.
import java.util.*; public class Util { public static List<Employee> getEmployees() { List<Employee> col = new ArrayList<Employee>(); col.add(new Employee(5, "Frank", 28)); col.add(new Employee(1, "Jorge", 19)); col.add(new Employee(6, "Bill", 34)); col.add(new Employee(3, "Michel", 10)); col.add(new Employee(7, "Simpson", 8)); col.add(new Employee(4, "Clerk",16 )); col.add(new Employee(8, "Lee", 40)); col.add(new Employee(2, "Mark", 30)); return col; } }
Sorting in natural
ordering
Employee's natural ordering would be done according tothe employee id. For that, above Employee class must be altered to add the comparing
ability as follows.
public class Employee implements Comparable<Employee> { private int empId; private String name; private int age; /** * Compare a given Employee with this object. * If employee id of this object is * greater than the received object, * then this object is greater than the other. */ public int compareTo(Employee o) { return this.empId - o.empId ; } }
The new compareTo() method does the trick of implementing the natural ordering of the
instances. So if a collection of Employee objects is sorted using
Collections.sort(List) method; sorting happens according to the ordering done inside
this method.
We'll write a class to test this natural ordering mechanism.
Following class use the Collections.sort(List) method to sort the given list in natural
order.
import java.util.*; public class TestEmployeeSort { public static void main(String[] args) { List coll = Util.getEmployees(); Collections.sort(coll); // sort method printList(coll); } private static void printList(List<Employee> list) { System.out.println("EmpId\tName\tAge"); for (Employee e: list) { System.out.println(e.getEmpId() + "\t" + e.getName() + "\t" + e.getAge()); } } }
Run the above class and examine the output. It will be as follows. As you can see, the
list is sorted correctly using the employee id. As empId is an int value, the employee
instances are ordered so that the int values ordered from 1 to 8.
EmpId Name Age 1 Jorge 19 2 Mark 30 3 Michel 10 4 Clerk 16 5 Frank 28 6 Bill 34 7 Simp 8 8 Lee 40
Sorting by other fields
If we need to sort using otherfields of the employee, we'll have to change the Employee
class's compareTo() method to use those fields. But then
we'll loose this empId based sorting mechanism. This is not a good
alternative if we need to sort using different fields at different occasions. But no
need to worry; Comparator is there to save us.
By writing a class that implements the java.util.Comparator interface, you can sort
Employees using any field as you wish even without touching the Employee class itself;
Employee class does not need to implement java.lang.Comparable or java.util.Comparator
interface.
Sorting by name field
Following EmpSortByName class is used to sortEmployee instances according to the name field. In this class, inside the compare()
method sorting mechanism is implemented. In compare() method we get two Employee
instances and we have to return which object is greater.
public class EmpSortByName implements Comparator<Employee>{ public int compare(Employee o1, Employee o2) { return o1.getName().compareTo(o2.getName()); } }
Watch out: Here, String class's compareTo() method is used in
comparing the name fields (which are Strings).
Now to test this sorting mechanism, you must use the Collections.sort(List, Comparator)
method instead of Collections.sort(List) method. Now change the TestEmployeeSort class
as follows. See how the EmpSortByName comparator is used inside sort method.
import java.util.*; public class TestEmployeeSort { public static void main(String[] args) { List coll = Util.getEmployees(); //Collections.sort(coll); //use Comparator implementation Collections.sort(coll, new EmpSortByName()); printList(coll); } private static void printList(List<Employee> list) { System.out.println("EmpId\tName\tAge"); for (Employee e: list) { System.out.println(e.getEmpId() + "\t" + e.getName() + "\t" + e.getAge()); } } }
Now the result would be as follows. Check whether the employees are sorted correctly by
the name String field. You'll see that these are sorted
alphabetically.
EmpId Name Age 6 Bill 34 4 Clerk 16 5 Frank 28 1 Jorge 19 8 Lee 40 2 Mark 30 3 Michel 10 7 Simp 8
Sorting by empId field
Even the ordering by empId (previously done usingComparable) can be implemented using Comparator; following class
does that.
public class EmpSortByEmpId implements Comparator<Employee>{ public int compare(Employee o1, Employee o2) { return o1.getEmpId() - o2.getEmpId(); } }
Explore further
Do not stop here. Work on the followingsby yourselves and sharpen knowledge on these concepts.
- Sort employees using name, age, empId in this order (ie: when names are equal,
try age and then next empId) - Explore how & why equals() method and compare()/compareTo() methods must be
consistence.
