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Comparator interface

Comparator interface is used to order(sort) the objects in the collection in your own way. It gives you the ability to decide how elements will be sorted and stored within collection and map. Comparators can be passed to a sort method (such as Collections.sort or Arrays.sort) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such as sorted sets or sorted maps), or to provide an ordering for collections of objects that don’t have a natural ordering.

Rules for using Comparator interface:

• If you want to sort the elements of a collection, you need to implement Comparator interface.
• If you do not specify the type of the object in your Comparator interface, then, by default, it assumes that you are going to sort the objects of type Object. Thus, when you override the compare() method ,you will need to specify the type of the parameter as Object only.
• If you want to sort the user-defined type elements, then while implementing the Comparator interface, you need to specify the user-defined type generically. If you do not specify the user-defined type while implementing the interface, then by default, it assumes Object type and you will not be able to compare the user-defined type elements in the collection

Example

Let’s create Student class:

public final class Student {

    private final int id;
    private final String name;

    Student(int id, String name) {
        this.id = id;
        this.name = name;
    }

    int getId() {
        return id;
    }

    String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

Then create StudentIdComparator class:

import java.util.Comparator;

public final class StudentIdComparator implements Comparator<Student> {

    @Override
    public int compare(Student student1, Student student2) {
        if (student1.getId() == student2.getId()) return 0;
        else if (student1.getId() > student2.getId()) return 1;
        else return -1;
    }
}

Example of usage StudenIdComparator

public class Test {

   public static void main(String[] args) {
        List<Student> studentList = new ArrayList<>();
        studentList.add(new Student(3, "Donald"));
        studentList.add(new Student(2, "Joseph"));
        studentList.add(new Student(1, "Mike"));
        Collections.sort(studentList, new StudentIdComparator());
        System.out.println("Sorted ArrayList:");
        System.out.println(studentList);

        Set<Student> studentSet = new TreeSet<>(new StudentIdComparator());
        studentSet.add(new Student(6, "Daniel"));
        studentSet.add(new Student(5, "Sam"));
        studentSet.add(new Student(4, "Kyrie"));
        System.out.println("Sorted TreeSet:");
        System.out.println(studentSet);
   }
}

Output:

Sorted ArrayList:
[Student{id=1, name='Mike'}, Student{id=2, name='Joseph'}, Student{id=3, name='Donald'}]
Sorted TreeSet:
[Student{id=4, name='Kyrie'}, Student{id=5, name='Sam'}, Student{id=6, name='Daniel'}]

Note:

• When we are sorting elements in a collection using Comparator interface, we need to pass the class object that implements Comparator interface.
• To sort a TreeSet collection, this object needs to be passed in the constructor of TreeSet.
• If any other collection, like ArrayList, was used, then we need to call sort method of Collections class and pass the name of the collection and this object as a parameter.

Links

https://www.studytonight.com/java/comparators-interface-in-java.php
https://www.edureka.co/blog/comparable-in-java/
https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html
https://www.javatpoint.com/Comparator-interface-in-collection-framework

credit goes to @swayangjit