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Written By : Amit
Title :
Comparator and Reflection using Java

Description :
Java and CompareTo
More...

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Description :
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Tags/Keywords : Java-and-CompareTo - Comparator-and-Reflection-using-Java - Coding-Tips-on-Java Example
Author : Amit
Date (Year/Month/Date): 2009-01-25
Comparator and Reflection 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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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?

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.





Using the same example as that of the 
earlier Question,
let us take the Student class
as the class to be compared for sorting
based on two field "name" and "age".

--------------------------------------
 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.




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