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Tags/Keywords : Java Comparator, Reflection, Common Comparator, Java Reflection, example, code, article, tutorial
Author : ISHTEK
Date (Year/Month/Date): 2009-07-15
Comparator using Java Technology and Reflection API

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.


How to write a comparator that can be used for any class that is
to be sorted, without changing the comparator or the class to be compared?

Suppose one has to sort all student objects in a list, then we know this can be
done by using either by defining java.lang.Comparable or 
java.util.Comparator interfaces.
By using java.util.Collections.sort method, one can do sorting on list of 
objects.

In this article I have tried to come up with another ways of providing
comparator that can be used with any number of JavaBeans, those are to be
comparated and sorted. By using this approach, I think, one should be able
to reduce somplexities of defining compareTo methods in many POJO, and
not to define many comparators for multiple POJOs (Those are to be sorted).

Explanation :
In this very scenario, the Comparator has to use reflection from java.lang.reflect
package to dynamically/runtime detection of the corresponding field to be used
for sorting and compare. Discussing with an example will help us understand this.

Example case study:
let us take the Student class as the class to be compared for sorting
based on two field "name" and "age".

Student.java
--------------------------------------
 private String name;
 private int age;
 public Student(String name, int age)
 {
 this.name = name;
 this.age = age;
 }
 public String getName()
 {
 return name;
 }
 public void setName(String name)
 {
 this.name = name;
 }
 public int getAge()
 {
 return age;
 }
 public void setAge(int age)
 {
 this.age = age;
 }
---------------------------------------

This Student class doesn't implement java.lang.Comparable interface, then
the comparator (suppose the name is TestComparator):
This TestComparator implements java.util.Comparator, and implement compareTo
method. Input to this TestComparator are the field (on which the sorting will
take place), and the type of sorting (Whether ascending or descending).

There are two important information required for feeding required values to the 
compareTo method, one is the value of the field and the other one is the Java 
Type of this field. So is sorting is to be done based on Student.name,
then required information are
value of name field - "test name"
type of name field - java.lang.String

And if sorting is to be done based on Student.age, then these are
value of age field - 23 (example age of student)
type of age field - int
To get these two inputs, we have two methods getValue and getType

---------------------------------------
 private Object getValue(Object a)
 {
 Object obj = null;
 Class[] obj1 = null;
 Object[] obj2 = null;
 String methodName = "get"+fieldName.replace(fieldName.substring(0,1)
 ,fieldName.substring(0,1).toUpperCase());

 try{
 obj = a.getClass().getMethod(methodName,obj1).invoke(a,obj2);

 }catch(Exception ex){
 ex.printStackTrace();
 }
 return obj;
 }
----------------------------------------

fieldName - the field to be used for sorting.
methodName - to get the accessible method
name for the attribute, like if attribute
name is "age", the methodName will
be "getAge". obj - gets the value
that the methodName returns from object.

----------------------------------------
 private Object getType(Object a)
 {
 Class[] obj1 = null;
 Object obj = null;
 String methodName = "get"+fieldName.replace(fieldName.substring(0,1),
 fieldName.substring(0,1).toUpperCase());
 try{
 obj = a.getClass().getMethod(methodName,obj1).getReturnType()
                   .getName().toString();
 }catch(Exception ex){
 ex.printStackTrace();
 }
 return obj;
 }
-----------------------------------------

This method returns the Java type of the methodName.

Client code using Java Technology, includes creation of Student Object 
multiple times

1.	Student student1=new Student("name1",34);
	Student student2=new Student("aname2",64);
	Student student3=new Student("zname3",27);

2. create a list
		List list = new ArrayList();
3. add all the three students
		list.add(student1);
		list.add(student2);
		list.add(student3);
4. sort this list using Collections

	Collections.sort(list,new TestComparator("name",1));

	1 = for descending order
	0 = for ascending
5. Not this list is sorted Students by name
   in descending order.

Now the complete source of this TestComparator is as follows:
TestComparator.java

/**
*  This source is provided as is, without any warranty
*  and /or guaranty of any kind.
*  Copyright (C) 2008, ISHTIAK, All Rights Reserved.
*  You can use it for Personal Learning purpose only.
*  source: http://www.interview-questions-tips-forum.net
*/

import java.util.Comparator;

public class TestComparator implements Comparator
{
    private String fieldName;
    public TestComparator(String argFieldName, int a) {
        this.fieldName = argFieldName;
	}
    private Object getValue(Object a)
    {
      Object obj = null;
      Class[] obj1 = null;
      Object[] obj2 = null;
      String methodName = "get"+fieldName.replace(fieldName.substring(0,1)
                                 ,fieldName.substring(0,1).toUpperCase());

     try{
        obj = a.getClass().getMethod(methodName,obj1).invoke(a,obj2);

     }catch(Exception ex){
        ex.printStackTrace();
     }
      return obj;
   }
   private Object getType(Object a)
   {
     Class[] obj1 = null;
     Object obj = null;
     String methodName = "get"+fieldName.replace(fieldName.substring(0,1),
     fieldName.substring(0,1).toUpperCase());
     try{
         obj = a.getClass().getMethod(methodName,obj1)
                           .getReturnType().getName().toString();
     }catch(Exception ex){
       ex.printStackTrace();
     }
     return obj;
   }

  public int compare(Object obj1, Object obj2) {
	  int status=0;
	  //System.out.println(getType(obj2));
    if(getType(obj2).equals("java.lang.String")) {
		status = ((String)getValue(obj1)).compareTo((String)getValue(obj2));
	} 
    if(getType(obj2).equals("int")) {
		status = ((Integer)getValue(obj1)).compareTo((Integer)getValue(obj2));
	} 
	return status;
  }
}


The client to test this application is as follows:
TestClient.java

/**
*  This source is provided as is, without any warranty
*  and /or guaranty of any kind.
*  Copyright (C) 2008, ISHTIAK, All Rights Reserved.
*  You can use it for Personal Learning purpose only.
*  source: http://www.interview-questions-tips-forum.net
*/

import java.util.*;
public class TestClient {
  public TestClient() {
	      //Some random test data.

	  Student student1=new Student("name1",34);
	  Student student2=new Student("aname2",64);
	  Student student3=new Student("zname3",27);

    List list = new ArrayList();
    list.add(student1);
		list.add(student2);
		list.add(student3);
    //test code for sorting Students based on name field.
    Collections.sort(list,new TestComparator("name",1));
    System.out.println("Sorted based on Student.name :");
    Iterator  itr = list.iterator();
	while(itr.hasNext()) {
		System.out.println(((Student)itr.next()).getName());
	}

    //test code for sorting Students based on age field.
    Collections.sort(list,new TestComparator("age",1));
    System.out.println("Sorted based on Student.age :");
    Iterator  itr1 = list.iterator();
	while(itr1.hasNext()) {
		System.out.println(((Student)itr1.next()).getAge());
	}
  }
  public static void main(String args[]) {
    new TestClient();
  }
}


As of now this code is at very preliminary stage, and I am working on it to make 
this a framework suitable enough to be used in any project/product.

if interested in participating with this initiative, please provide your
comments (those are really valuable to me).

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