Java – Comparator interface

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Java Comparator interface is used to order the objects of user-defined class.

This interface is found in java.util package and contains 2 methods compare(Object obj1,Object obj2) and equals(Object element).

It provides multiple sorting sequence i.e. you can sort the elements on the basis of any data member, for example rollno, name, age or anything else.

compare() method

public int compare(Object obj1,Object obj2): compares the first object with second object.


Collections class

Collections class provides static methods for sorting the elements of collection. If collection elements are of Set or Map, we can use TreeSet or TreeMap. But we cannot sort the elements of List. Collections class provides methods for sorting the elements of List type elements also.

Method of Collections class for sorting List elements

public void sort(List list, Comparator c): is used to sort the elements of List by the given Comparator.

 

Java Comparator Example (Non-generic Old Style)

Let’s see the example of sorting the elements of List on the basis of age and name. In this example, we have created 4 java classes:

  1. Student.java
  2. AgeComparator.java
  3. NameComparator.java
  4. Simple.java

Student.java

This class contains three fields rollno, name and age and a parameterized constructor.

package collections.comparatordemo;

public class Student {
    int rollno;
    String name;
    int age;

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

AgeComparator.java

This class defines comparison logic based on the age. If age of first object is greater than the second, we are returning positive value, it can be any one such as 1, 2 , 10 etc. If age of first object is less than the second object, we are returning negative value, it can be any negative value and if age of both objects are equal, we are returning 0.

package collections.comparatordemo;

import java.util.*;

public class AgeComparator implements Comparator {
    public int compare(Object o1, Object o2) {
        Student s1 = (Student) o1;
        Student s2 = (Student) o2;

        if (s1.age == s2.age)
            return 0;
        else if (s1.age > s2.age)
            return 1;
        else
            return -1;
    }
}

NameComparator.java

This class provides comparison logic based on the name. In such case, we are using the compareTo() method of String class, which internally provides the comparison logic.

package collections.comparatordemo;

import java.util.*;

public class NameComparator implements Comparator {
    public int compare(Object o1, Object o2) {
        Student s1 = (Student) o1;
        Student s2 = (Student) o2;

        return s1.name.compareTo(s2.name);
    }
}

Simple.java

In this class, we are printing the objects values by sorting on the basis of name and age.

package collections.comparatordemo;

import java.util.*;
import java.io.*;

public class Simple {
    public static void main(String args[]) {

        ArrayList al = new ArrayList();
        al.add(new Student(101, "Vijay", 23));
        al.add(new Student(106, "Ajay", 27));
        al.add(new Student(105, "Jai", 21));

        System.out.println("Sorting by Name...");

        Collections.sort(al, new NameComparator());
        Iterator itr = al.iterator();
        while (itr.hasNext()) {
            Student st = (Student) itr.next();
            System.out.println(st.rollno + " " + st.name + " " + st.age);
        }

        System.out.println("sorting by age...");

        Collections.sort(al, new AgeComparator());
        Iterator itr2 = al.iterator();
        while (itr2.hasNext()) {
            Student st = (Student) itr2.next();
            System.out.println(st.rollno + " " + st.name + " " + st.age);
        }


    }
}
       Sorting by Name...
       106 Ajay 27
       105 Jai 21
       101 Vijay 23
       
       Sorting by age...       
       105 Jai 21
       101 Vijay 23
       106 Ajay 27

Java Comparator Example (Generic)

Student.java

package collections.comparatordemo.two;

public class Student {
    int rollno;
    String name;
    int age;

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

AgeComparator.java

package collections.comparatordemo.two;

import java.util.*;

public class AgeComparator implements Comparator<Student> {
    public int compare(Student s1, Student s2) {
        if (s1.age == s2.age)
            return 0;
        else if (s1.age > s2.age)
            return 1;
        else
            return -1;
    }
}

NameComparator.java

This class provides comparison logic based on the name. In such case, we are using the compareTo() method of String class, which internally provides the comparison logic.

package collections.comparatordemo.two;

import java.util.*;

public class NameComparator implements Comparator<Student> {
    public int compare(Student s1, Student s2) {
        return s1.name.compareTo(s2.name);
    }
}

Simple.java

In this class, we are printing the objects values by sorting on the basis of name and age.

package collections.comparatordemo.two;

import java.util.*;
import java.io.*;

public class Simple {
    public static void main(String args[]) {

        ArrayList<Student> al = new ArrayList<Student>();
        al.add(new Student(101, "Vijay", 23));
        al.add(new Student(106, "Ajay", 27));
        al.add(new Student(105, "Jai", 21));

        System.out.println("Sorting by Name...");

        Collections.sort(al, new NameComparator());
        for (Student st : al) {
            System.out.println(st.rollno + " " + st.name + " " + st.age);
        }

        System.out.println("sorting by age...");

        Collections.sort(al, new AgeComparator());
        for (Student st : al) {
            System.out.println(st.rollno + " " + st.name + " " + st.age);
        }

    }
}
Output:Sorting by Name...
       106 Ajay 27
       105 Jai 21
       101 Vijay 23

       Sorting by age...       
       105 Jai 21
       101 Vijay 23
       106 Ajay 27

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