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Tags/Keywords : Using-Java-Technology - Java-Comparable-and-Comparator - Coding-Tips-on-Java Example
Author : Amit
Date (Year/Month/Date): 2009-01-27
Java Comparable and Comparator using Java Technology

Please be informed that NONE of the design/code from this
page is claiming to be some sort of best practices and we DO NOT expect
any of our visitor/reader of this page to assume this as some sort of
best practice for any context and should not be using this 
as it is without appropriate evaluation to their, so to say, 
specific programming context.

This page intends only to provide bit and piece of known ways  for
doing some sort of example and may not be fit for any other purpose.




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Java Questions and My Opinions
What is the sorting implementation 
Java Technology provides and how to
use it effectively?

Explaination:
There are two ways sorting of objects
can be done, one that is decided while
coding and the objects to be sorted are
designed and developed, taking sorting
implementation into consideration.
Like the objects to be sorted should 
have its class implement java.lang.Comparable
interface and implement compareTo(Object obj)
method. Another way is to write a comparator
class, that implements java.util.Comparator
interface and implements compare(Object obj1,
Object obj2) and this comparator is bound to
a specific type of comparing requirement, 
but in this way of comparison, the original
object is used as it is.

Let us take an example to understand it even
more clearly.

Suppose there is a class Student, and it has
two fields name and age.

Now let us consider the first case, where our
Student class implements java.lang.Comparable,
and has compareTo method implemented as follows:
------------------------------------------------
public class Student implements Comparable
{
   String name;
   int age;
   public Student(String name, int age)
   {
      this.name = name;
      this.age = age;
   }
    public int compareTo(Object obj)
   {
    //if sorting is required to be based on name.
    //descending alphabetical order, as it returns negative
    //value.
    return -(this.name.compareTo(((Student)obj).name));
   }
}
-------------------------------------------------
Here the comparison is based on name and alphabetically
descending order.

Now if multiple objects of Student class are created
and added to a Set implementation like TreeSet, 
then by default all the items will be
automatically sorted as descending alphabetically
by Student.name.

Suppose this Student class doesn't implement 
Comparable and is part of a JAR file, and developer
is instructed not to change this Student class,
but to list all the Student Objects descending 
alphabetically by Student.name.
Then in this case, the second approach 
(as already discussed) comes to rescue.
Here Another class is introduced, known as 
comparator, as this class implements 
java.util.Comparator and this comparator
class implements compareTo(Object obj1, Object obj2)
method, and this comparator is not generic
but bound to Student class only, as it is 
going to hard code comparison logic based on 
the Student.name, as follows:

 public int compare(Object obj1,Object obj2)
 {
    return -((((Student)obj1).name)
          .compareTo(((Student)obj2).name));
 }

And java.util.Collections.sort(List objList,
Comparator comparator) can be used to sort 
objList (Student objects stored in a java.util.ArrayList),
as per the comparator passed as argument.

In this above example, Student class is hard
coded in the compare method of the comparator,
but this means for every class to be sorted, one
has to create a comparator, and this means a 
lot of comparators in the application and growing,
there could be some alternative elegant approach
to this, I have written a generic comparator using
reflection to dynamically use the class object to
come up with the appropriate compare implementation.
It will be good if we shall walk through this example.

Please try this by some small sample program, and 
this concept is really interesting to explore.

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