The Comparator is an interface available in the java.util package. The java Comparator is used to order the objects of user-defined classes. The java Comparator can compare two objects from two different classes.

Using the java Comparator, we can sort the elements based on data members of a class. For example, we can sort based on roolNo, age, salary, marks, etc.

The Comparator interface has the following methods.

The Comparator can be used in the following three ways.

• Using a seperate class that implements Comparator interface.
• Using anonymous class.
• Using lamda expression.

Using a seperate class

We use the following steps to use Comparator with a seperate class.

• Step - 1: Create the user-defined class.
• Step - 2: Create a class that implements Comparator interface.
• Step - 3: Implement the comapare( ) method of Comparator interface inside the above defined class(step - 2).
• Step - 4: Create the actual class where we use the Compatator object with sort method of Collections class.
• Step - 5: Create the object of Compatator interface using the class crearted in step - 2.
• Step - 6: Call the sort method of Collections class by passing the object created in step - 6.
• Step - 7: Use a for-each (any loop) to print the sorted information.

Let's consider an example program to illustrate Comparator using a separate class.

import java.util.*;

class Student{

	String name;
	float percentage;
	
	Student(String name, float percentage){
		this.name = name;
		this.percentage = percentage;
	}
	
}

class PercentageComparator implements Comparator<Student>{
	public int compare(Student stud1, Student stud2) {
		if(stud1.percentage < stud2.percentage)
			return 1;
		return -1;
	}
}

public class StudentCompare{
	
	public static void main(String args[]) {
	
		ArrayList<Student> studList = new ArrayList<Student>();
		
		studList.add(new Student("Gouthami", 90.61f));
		studList.add(new Student("Raja", 83.55f));
		studList.add(new Student("Honey", 85.55f));
		studList.add(new Student("Teja", 77.56f));
		studList.add(new Student("Varshith", 80.89f));
		
		Comparator<Student> com = new PercentageComparator();
		
		Collections.sort(studList, com);
		
		System.out.println("Avg % --> Name");
		System.out.println("---------------------");
		for(Student stud:studList) {
			System.out.println(stud.percentage + " --> " + stud.name);
		}		
	}	
}

Using anonymous class

We use the following steps to use Comparator with anonymous class.

• Step - 1: Create the user-defined class.
• Step - 2: Create the actual class where we use the Compatator object with sort method of Collections class.
• Step - 3: Create the object of Compatator interface using anonymous class and implement compare method of Comparator interface.
• Step - 4: Call the sort method of Collections class by passing the object created in step - 6.
• Step - 5: Use a for-each (any loop) to print the sorted information.

Let's consider an example program to illustrate Comparator using a separate class.

import java.util.*;

class Student{

	String name;
	float percentage;
	
	Student(String name, float percentage){
		this.name = name;
		this.percentage = percentage;
	}
	
}

public class StudentCompare{
	
	public static void main(String args[]) {
	
		ArrayList<Student> studList = new ArrayList<Student>();
		
		studList.add(new Student("Gouthami", 90.61f));
		studList.add(new Student("Raja", 83.55f));
		studList.add(new Student("Honey", 85.55f));
		studList.add(new Student("Teja", 77.56f));
		studList.add(new Student("Varshith", 80.89f));
		
        Comparator<Student> com = new Comparator<Student>() {
			public int compare(Student stud1, Student stud2) {
				if(stud1.percentage < stud2.percentage)
					return 1;
				return -1;
			}
		};
		
		Collections.sort(studList, com);
		
		System.out.println("Avg % --> Name");
		System.out.println("---------------------");
		for(Student stud:studList) {
			System.out.println(stud.percentage + " --> " + stud.name);
		}		
	}	
}

Using lamda expression

We use the following steps to use Comparator with lamda expression.

• Step - 1: Create the user-defined class.
• Step - 2: Create the actual class where we use the Compatator object with sort method of Collections class.
• Step - 3: Create the object of Compatator interface using lamda expression and implement the code for compare method of Comparator interface.
• Step - 4: Call the sort method of Collections class by passing the object created in step - 6.
• Step - 5: Use a for-each (any loop) to print the sorted information.

Let's consider an example program to illustrate Comparator using a separate class.

import java.util.*;

class Student{

	String name;
	float percentage;
	
	Student(String name, float percentage){
		this.name = name;
		this.percentage = percentage;
	}	
}

public class StudentCompare{
	
	public static void main(String args[]) {
	
		ArrayList<Student> studList = new ArrayList<Student>();
		
		studList.add(new Student("Gouthami", 90.61f));
		studList.add(new Student("Raja", 83.55f));
		studList.add(new Student("Honey", 85.55f));
		studList.add(new Student("Teja", 77.56f));
		studList.add(new Student("Varshith", 80.89f));
		
        Comparator<Student> com = (stud1, stud2) -> {
				if(stud1.percentage < stud2.percentage)
					return 1;
				return -1;
		};
		
		Collections.sort(studList, com);
		
		System.out.println("Avg % --> Name");
		System.out.println("---------------------");
		for(Student stud:studList) {
			System.out.println(stud.percentage + " --> " + stud.name);
		}		
	}	
}