Jul 25, 2021by @kodeazy

what is comparator in java and how it works?

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java

Comparator is an interface used to sort objects of user defined class.
Comparator provides multiple sorting sequences where as comparable provides singles sorting sequence.
It is found in java.util package.
It has two methods compare() and equals().

Below is an example of how to use compare() method for sorting objects in Comparator interface.

class SortByName implements Comparator<ComparatorEmployee>{
public int compare(ComparatorEmployee e1,ComparatorEmployee e2){
    return e1.name.compareTo(e2.name);
}
}

Above method helps to sort the objects based on name of the employee.here compareTo() method returns negitive,positive, zero values, based on these values objects are swapped.
Here e1.name.compareTo(e2.name) allows to print the Objects by sorting in Asc order.To print in Desc order modify it as e2.name.compareTo(e1.name).
Below is an Example of using Comparator.

import java.util.*;
class ComparatorEmployee{
    int age;
    int salary;
    String name;
    ComparatorEmployee(int age,int salary,String name){
        this.age=age;
        this.salary=salary;
        this.name=name;

    }
   
}
class SortByName implements Comparator<ComparatorEmployee>{
    public int compare(ComparatorEmployee e1,ComparatorEmployee e2){
        return e1.name.compareTo(e2.name);
    }
}
class SortByage implements Comparator<ComparatorEmployee>{
    public int compare(ComparatorEmployee e1,ComparatorEmployee e2){
        if(e1.age>e2.age)
            return 1;
        else if(e1.age<e2.age)
            return -1;
        else
            return 0;
    }
}
class ComparatorExample{
    public static void main(String args[]){
        List<ComparatorEmployee> al=new ArrayList<ComparatorEmployee>();
        al.add(new ComparatorEmployee(27, 103, "Suresh"));
        al.add(new ComparatorEmployee(23, 102, "Ramesh"));
        al.add(new ComparatorEmployee(21, 105, "Mahesh"));
        al.add(new ComparatorEmployee(29, 104, "Gopi"));
        System.out.println("Sorting by Name");
        Collections.sort(al,new SortByName());
        al.forEach(emplyeDetails->{
            System.out.println("Name is:"+emplyeDetails.name+" Age is :"+emplyeDetails.age+"Salary is"+emplyeDetails.salary);
        });
        System.out.println("---------------------------------------------------------");
        System.out.println("Sorting by Age");
        Collections.sort(al,new SortByage());
        al.forEach(emplyeDetails->{
            System.out.println("Name is:"+emplyeDetails.name+" Age is :"+emplyeDetails.age+"Salary is"+emplyeDetails.salary);
        });
    }
}

Output:

Sorting by Name
Name is:Gopi Age is :29Salary is104
Name is:Mahesh Age is :21Salary is105
Name is:Ramesh Age is :23Salary is102
Name is:Suresh Age is :27Salary is103
---------------------------------------------------------
Sorting by Age
Name is:Mahesh Age is :21Salary is105
Name is:Ramesh Age is :23Salary is102
Name is:Suresh Age is :27Salary is103
Name is:Gopi Age is :29Salary is104

To understand working of custom comparato click here