/*

Java Comparator example.

This Java Comparator example describes how java.util.Comparator interface is implemented to compare Java user defined classe's objects.

These Java Comparator is passed to Collection's sorting method ( for example Collections.sort method)to perform sorting of Java user defined classe's objects.

*/

import java.util.*;

/*

java.util.Comparator interface declares two methods,

1) public int compare(Object object1, Object object2) and

2) boolean equals(Object object)

*/

/*

We will compare objects of the Employee class using custom comparators

on the basis of employee age and name.

*/

class Employee{

private int age;

private String name;

public void setAge(int age){       

this.age=age;

}

public int getAge(){         

return this.age;

}

public void setName(String name){        

this.name=name;

}

public String getName(){

return this.name;

}    

}

/*

User defined java comaprator.

To create custom java comparator Implement Comparator interface and

define compare method.

The below given comparator compares employees on the basis of their age.

*/

class AgeComparator implements Comparator{

public int compare(Object emp1, Object emp2){

//parameter are of type Object, so we have to downcast it to Employee objects

int emp1Age = ( (Employee) emp1).getAge();

int emp2Age = ( (Employee) emp2).getAge();

if( emp1Age > emp2Age )

return 1;

else if( emp1Age < emp2Age )

return -1;

else

return 0;

}

}

/*

The below given comparator compares employees on the basis of their name.

*/

class NameComparator implements Comparator{

public int compare(Object emp1, Object emp2){

//parameter are of type Object, so we have to downcast it to Employee objects

String emp1Name = ( (Employee) emp1 ).getName();

String emp2Name = ( (Employee) emp2 ).getName();

//uses compareTo method of String class to compare names of the employee

return emp1Name.compareTo(emp2Name);

}

}

/*

This Java comparator example compares employees on the basis of

their age and name and sort it in that order.

*/

public class JavaComparatorExample{

public static void main(String args[]){

/*

Employee array which will hold employees

*/

Employee employee[] = new Employee[2];

//set different attributes of the individual employee.

employee[0] = new Employee();

employee[0].setAge(40);

employee[0].setName("Joe");

employee[1] = new Employee();

employee[1].setAge(20);

employee[1].setName("Mark");

System.out.println("Order of employee before sorting is");

//print array as is.

for(int i=0; i < employee.length; i++){

System.out.println( "Employee " + (i+1) + " name :: " + employee[i].getName() + ", Age :: " + employee[i].getAge());

}

/*

Sort method of the Arrays class sorts the given array.

Signature of the sort method is,

static void sort(Object[] object, Comparator comparator)

IMPORTANT: All methods defined by Arrays class are static. Arrays class

serve as a utility class.

*/

/*

Sorting array on the basis of employee age by passing AgeComparator

*/

Arrays.sort(employee, new AgeComparator());

System.out.println("\n\nOrder of employee after sorting by employee age is");

for(int i=0; i < employee.length; i++){

System.out.println( "Employee " + (i+1) + " name :: " + employee[i].getName() + ", Age :: " + employee[i].getAge());

}

/*

Sorting array on the basis of employee Name by passing NameComparator

*/

Arrays.sort(employee, new NameComparator());

System.out.println("\n\nOrder of employee after sorting by employee name is");     

for(int i=0; i < employee.length; i++){

                  System.out.println( "Employee " + (i+1) + " name :: " + employee[i].getName() + ", Age :: " + employee[i].getAge());

}

}

}

/*

OUTPUT of the above given Java Comparable Example would be :

Order of employee before sorting is

Employee 1 name :: Joe, Age :: 40

Employee 2 name :: Mark, Age :: 20

Order of employee after sorting by employee age is

Employee 1 name :: Mark, Age :: 20

Employee 2 name :: Joe, Age :: 40

Order of employee after sorting by employee name is

Employee 1 name :: Joe, Age :: 40

Employee 2 name :: Mark, Age :: 20

*/