How to Implement a Custom Comparator for a LinkedHashSet in Java?

Last Updated : 07 Feb, 2024

In Java, we cannot directly implement a custom comparator for a LinkedHashSet. But sometimes we need a requirement of custom comparator for example custom sorting. So, we can use some alternative approaches to fulfill the requirements.

In this article, we will be using a class for creating the custom comparator and this class will implement the Comparable interface for a LinkedHashSet in Java.

Approach:

In the first step, we have created a class which is the type of Employee, and implemented a Comparable interface.
It accepts a value of Employee type in a constructor and accepts UserIDs.
Then Override compareTo method.
We also Override a toString method for printing the output.
Now we create the LinkedHashSet of type Employee and add Employee ID.
Then we add these LinkedHashSet values to an ArrayList.
So, we can use Collection.Sort which sorts the elements by User IDs.
Then we print the output.

Program to Implement a Custom Comparator for a LinkedHashSet in Java

Suppose we have a LinkedHashSet of Employee IDs and want to perform a custom sorting, for this follow the below implementation.

Java

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedHashSet;
import java.util.List;

// We need to implement the Comparable interface for creating
// a custom comparator.
class Employee implements Comparable<Employee> {

    private int employeeId;

    public Employee(int id) {
        this.employeeId = id;
    }

    public int getId() {
        return this.employeeId;
    }

    // We override the toString method.
    public String toString() {
        return "Employee -> " + getId();
    }

    // Custom comparator
    public int compareTo(Employee otherEmployee) {
        return this.getId() - otherEmployee.getId();
    }
}

public class GFG {
    public static void main(String[] args) {
        LinkedHashSet<Employee> setOfEmployees = new LinkedHashSet<>();
        setOfEmployees.add(new Employee(3));
        setOfEmployees.add(new Employee(1));
        setOfEmployees.add(new Employee(2));

        // Convert the set to a list for sorting
        List<Employee> listUsers = new ArrayList<>(setOfEmployees);

        // Sort the list using the custom comparator
        Collections.sort(listUsers);

        // Print the sorted list
        System.out.println(listUsers);
    }
}

Output

[Employee -> 1, Employee -> 2, Employee -> 3]

Explanation of the Program:

In the above program, we have defined a class Employee that implements the Comparable interface. The compareTo method is implemented to compare Employee objects based on their employee IDs.
We create a LinkedHashSet named setOfEmployees to store Employee objects. Note that LinkedHashSet maintains insertion order.
We convert the LinkedHashSet to an ArrayList named listUsers to perform sorting. This is done to demonstrate sorting using the custom comparator.
We sort the listUsers using Collections.sort method, which uses the custom comparator defined in the Employee class.
Finally, we print the sorted list.