Comparator Interface in Java with Examples

Comparator interface is used to order the objects of user-defined classes. A comparator object is capable of comparing two objects of two different classes. Following function compare obj1 with obj2

Syntax:

public int compare(Object obj1, Object obj2):

Suppose we have an array/arraylist of our own class type, containing fields like rollno, name, address, DOB etc and we need to sort the array based on Roll no or name?



Method 1: One obvious approach is to write our own sort() function using one of the standard algorithms. This solution requires rewriting the whole sorting code for different criterion like Roll No. and Name.

Method 2: Using comparator interface- Comparator interface is used to order the objects of user-defined class. This interface is present java.util package and contains 2 methods compare(Object obj1, Object obj2) and equals(Object element). Using comparator, we can sort the elements based on data members. For instance it may be on rollno, name, age or anything else.

Method of Collections class for sorting List elements is used to sort the elements of List by the given comparator.

// To sort a given list. ComparatorClass must implement 
// Comparator interface.
public void sort(List list, ComparatorClass c)

How does Collections.Sort() work?
Internally the Sort method does call Compare method of the classes it is sorting. To compare two elements, it asks “Which is greater?” Compare method returns -1, 0 or 1 to say if it is less than, equal, or greater to the other. It uses this result to then determine if they should be swapped for its sort.

Working Program:

// Java program to demonstrate working of Comparator
// interface
import java.util.*;
import java.lang.*;
import java.io.*;
  
// A class to represent a student.
class Student
{
    int rollno;
    String name, address;
  
    // Constructor
    public Student(int rollno, String name,
                               String address)
    {
        this.rollno = rollno;
        this.name = name;
        this.address = address;
    }
  
    // Used to print student details in main()
    public String toString()
    {
        return this.rollno + " " + this.name +
                           " " + this.address;
    }
}
  
class Sortbyroll implements Comparator<Student>
{
    // Used for sorting in ascending order of
    // roll number
    public int compare(Student a, Student b)
    {
        return a.rollno - b.rollno;
    }
}
  
class Sortbyname implements Comparator<Student>
{
    // Used for sorting in ascending order of
    // roll name
    public int compare(Student a, Student b)
    {
        return a.name.compareTo(b.name);
    }
}
  
// Driver class
class Main
{
    public static void main (String[] args)
    {
        ArrayList<Student> ar = new ArrayList<Student>();
        ar.add(new Student(111, "bbbb", "london"));
        ar.add(new Student(131, "aaaa", "nyc"));
        ar.add(new Student(121, "cccc", "jaipur"));
  
        System.out.println("Unsorted");
        for (int i=0; i<ar.size(); i++)
            System.out.println(ar.get(i));
  
        Collections.sort(ar, new Sortbyroll());
  
        System.out.println("\nSorted by rollno");
        for (int i=0; i<ar.size(); i++)
            System.out.println(ar.get(i));
  
        Collections.sort(ar, new Sortbyname());
  
        System.out.println("\nSorted by name");
        for (int i=0; i<ar.size(); i++)
            System.out.println(ar.get(i));
    }
}

Output:

Unsorted
111 bbbb london
131 aaaa nyc
121 cccc jaipur

Sorted by rollno
111 bbbb london
121 cccc jaipur
131 aaaa nyc

Sorted by name
131 aaaa nyc
111 bbbb london
121 cccc jaipu

By changing the return value in inside compare method you can sort in any order you want. eg.for descending order just change the positions of a and b in above compare method.

Sort collection by more than one field:

In previous articles, we have discussed how to sort list of objects on the basis of single field using Comparable and Comparator interface But, what if we have a requirement to sort ArrayList objects in accordance with more than one fields like firstly sort, according to student name and secondly sort according to student age.

Below is the implementation of above approach:

// Java program to demonstrate working of Comparator
// interface more than one field
  
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Comparator;
  
class Student {
  
    // instance member variables
    String Name;
    int Age;
  
    // parameterized constructor
    public Student(String Name, Integer Age) {
        this.Name = Name;
        this.Age = Age;
    }
  
    public String getName() {
        return Name;
    }
  
    public void setName(String Name) {
        this.Name = Name;
    }
  
    public Integer getAge() {
        return Age;
    }
  
    public void setAge(Integer Age) {
        this.Age = Age;
    }
  
    // overriding toString() method
    @Override
    public String toString() {
        return "Customer{" + "Name=" + Name + ", Age=" + Age + '}';
    }
  
    static class CustomerSortingComparator implements Comparator<Student> {
  
        @Override
        public int compare(Student customer1, Student customer2) {
  
            // for comparison
            int NameCompare = customer1.getName().compareTo(customer2.getName());
            int AgeCompare = customer1.getAge().compareTo(customer2.getAge());
  
            // 2-level comparison using if-else block
            if (NameCompare == 0) {
                return ((AgeCompare == 0) ? NameCompare : AgeCompare);
            } else {
                return NameCompare;
            }
        }
    }
  
    public static void main(String[] args) {
  
        // create ArrayList to store Student
        List<Student> al = new ArrayList<>();
  
        // create customer objects using constructor initialization
        Student obj1 = new Student("Ajay", 27);
        Student obj2 = new Student("Sneha", 23);
        Student obj3 = new Student("Simran", 37);
        Student obj4 = new Student("Ajay", 22);
        Student obj5 = new Student("Ajay", 29);
        Student obj6 = new Student("Sneha", 22);
  
        // add customer objects to ArrayList
        al.add(obj1);
        al.add(obj2);
        al.add(obj3);
        al.add(obj4);
        al.add(obj5);
        al.add(obj6);
  
        // before Sorting arraylist: iterate using Iterator
        Iterator<Student> custIterator = al.iterator();
  
        System.out.println("Before Sorting:\n");
        while (custIterator.hasNext()) {
            System.out.println(custIterator.next());
        }
  
        // sorting using Collections.sort(al, comparator);
        Collections.sort(al, new CustomerSortingComparator());
  
        // after Sorting arraylist: iterate using enhanced for-loop
        System.out.println("\n\nAfter Sorting:\n");
        for (Student customer : al) {
            System.out.println(customer);
        }
    }
}

Output:

Before Sorting:

Customer{Name=Ajay, Age=27}
Customer{Name=Sneha, Age=23}
Customer{Name=Simran, Age=37}
Customer{Name=Ajay, Age=22}
Customer{Name=Ajay, Age=29}
Customer{Name=Sneha, Age=22}


After Sorting:

Customer{Name=Ajay, Age=22}
Customer{Name=Ajay, Age=27}
Customer{Name=Ajay, Age=29}
Customer{Name=Simran, Age=37}
Customer{Name=Sneha, Age=22}
Customer{Name=Sneha, Age=23}

References:
http://www.dreamincode.net/forums/topic/169079-how-collectionssort-is-doing-its-stuff-here/
http://www.javatpoint.com/Comparator-interface-in-collection-framework

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