To sort the objects of user-defined classes, a comparator interface is utilized. A comparator object can compare two items of the same type.
This interface may be found in java.util package and includes two methods:

• compare(Object obj1,Object obj2)
• equals (Object element).

It supports different sorting sequences, which means you can sort the items based on any data member, such as rollno, name, age, or anything else.

Syntax: 

class MyComparator implements Comparator<Class-name>{}
public int compare(Object obj1, Object obj2)

The following are the rules for utilizing the Comparator interface:

• You must implement the Comparator interface if you want to sort the elements of a collection.
  
• If you do not mention the type of the object in your Comparator interface, it will presume that you are sorting objects of type Object. As a result, while overriding the compare() method, you must specify the parameter type as Object only.
• If you want to order the user-defined type elements, you must provide the user-defined type generically while implementing the Comparator interface.
• If you do not specify the user-defined type when implementing the interface, it will default to Object and you will be unable to compare the user-defined type components in the collection.

Example:

Student class:

class Student {
 
    // Attributes of a student
    int rollno;
    String name, address;
 
    // Constructor
    public Student(int rollno, String name, String address)
    {
 
        // This keyword refers to current instance itself
        this.rollno = rollno;
        this.name = name;
        this.address = address;
    }
 public String toString()
    {
 
        // Returning attributes of Student
        return this.rollno + " " + this.name + " "
            + this.address;
    }
}

Sortbyroll Class:

Class to sort the entries according to the specified roll numbers.

class Sortbyroll implements Comparator<Student> {
 
    // Method
    // Sorting in ascending order of roll number
    public int compare(Student a, Student b)
    {
 
        return a.rollno - b.rollno;
    }
}
 

Main class:

class Coderz {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating an empty ArrayList of Student type
        ArrayList<Student> ar = new ArrayList<Student>();
 
        // Adding entries in above List
        // using add() method
        ar.add(new Student(111, "Rabecca", "London"));
        ar.add(new Student(102, "Tom", "Seoul"));
        ar.add(new Student(123, "Tiffany", "Jaipur"));
        ar.add(new Student(99, "John", "Hongkong"));
 
        // Display message on console for better readability
        System.out.println("Unsorted");
 
        // Iterating over entries to print them
        for (int i = 0; i < ar.size(); i++)
            System.out.println(ar.get(i));
 
        // Sorting student entries by roll number
        Collections.sort(ar, new Sortbyroll());
 
        // Display message on console for better readability
        System.out.println("\nSorted by rollno");
 
        // Again iterating over entries to print them
        for (int i = 0; i < ar.size(); i++)
            System.out.println(ar.get(i));
    }
}

Output:

Unsorted
111 Rabecca London
102 Tom Seoul
123 Tiffany Jaipur
99 John Hongkong

Sorted by rollno
99 John Hongkong
102 Tom Seoul
111 Rabecca London
123 Tiffany Jaipur

Note: also read about the Comparable Interface in Java

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