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

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.