java Comparator Example

http://www.javadeveloper.co.in/java-example/java-comparator-example.html

Submitted by Andr on Tue, 10/26/2010 - 20:12
Java Examples
package examples;

/*
Java Comparator example.
This Java Comparator example describes how java.util.Comparator
interface is implemented to compare Java custom class objects.

This Java Comparator is passed to Collection's sorting method
(for example Collections.sort method) to perform sorting of Java custom
class 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 comparator.
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 them 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       
        serves 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
*/