Java 8 Lambda : Comparator example

In this example, we will show you how to use Java 8 Lambda expression to write a Comparator to sort a List.

1. Classic Comparator example.

	Comparator<Developer> byName = new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getName().compareTo(o2.getName());
	};

2. Lambda expression equivalent.

	Comparator<Developer> byName = 
		(Developer o1, Developer o2)->o1.getName().compareTo(o2.getName());

1. Sort without Lambda

Example to compare the Developer objects using their age. Normally, you use Collections.sort and pass an anonymous Comparator class like this :

package com.mkyong.java8;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class TestSorting {
	public static void main(String[] args) {
		List<Developer> listDevs = getDevelopers();
		System.out.println("Before Sort");
		for (Developer developer : listDevs) {
			System.out.println(developer);
		//sort by age
		Collections.sort(listDevs, new Comparator<Developer>() {
			@Override
			public int compare(Developer o1, Developer o2) {
				return o1.getAge() - o2.getAge();
		});
		System.out.println("After Sort");
		for (Developer developer : listDevs) {
			System.out.println(developer);
	private static List<Developer> getDevelopers() {
		List<Developer> result = new ArrayList<Developer>();
		result.add(new Developer("mkyong", new BigDecimal("70000"), 33));
		result.add(new Developer("alvin", new BigDecimal("80000"), 20));
		result.add(new Developer("jason", new BigDecimal("100000"), 10));
		result.add(new Developer("iris", new BigDecimal("170000"), 55));
		return result;

Output

Before Sort
Developer [name=mkyong, salary=70000, age=33]
Developer [name=alvin, salary=80000, age=20]
Developer [name=jason, salary=100000, age=10]
Developer [name=iris, salary=170000, age=55]
After Sort
Developer [name=jason, salary=100000, age=10]
Developer [name=alvin, salary=80000, age=20]
Developer [name=mkyong, salary=70000, age=33]
Developer [name=iris, salary=170000, age=55]

When the sorting requirement is changed, you just pass in another new anonymous Comparator class :

	//sort by age
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getAge() - o2.getAge();
	});
	//sort by name	
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getName().compareTo(o2.getName());
	});
	//sort by salary
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getSalary().compareTo(o2.getSalary());
	});				

It works, but, do you think it is a bit weird to create a class just because you want to change a single line of code?

2. Sort with Lambda

In Java 8, the List interface is supports the sort method directly, no need to use Collections.sort anymore.

        //List.sort() since Java 8
	listDevs.sort(new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o2.getAge() - o1.getAge();
	});	

Lambda expression example :

package com.mkyong.java8;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;
public class TestSorting {
	public static void main(String[] args) {
		List<Developer> listDevs = getDevelopers();
		System.out.println("Before Sort");
		for (Developer developer : listDevs) {
			System.out.println(developer);
		System.out.println("After Sort");
		//lambda here!
		listDevs.sort((Developer o1, Developer o2)->o1.getAge()-o2.getAge());
		//java 8 only, lambda also, to print the List
		listDevs.forEach((developer)->System.out.println(developer));
	private static List<Developer> getDevelopers() {
		List<Developer> result = new ArrayList<Developer>();
		result.add(new Developer("mkyong", new BigDecimal("70000"), 33));
		result.add(new Developer("alvin", new BigDecimal("80000"), 20));
		result.add(new Developer("jason", new BigDecimal("100000"), 10));
		result.add(new Developer("iris", new BigDecimal("170000"), 55));
		return result;

Output

Before Sort
Developer [name=mkyong, salary=70000, age=33]
Developer [name=alvin, salary=80000, age=20]
Developer [name=jason, salary=100000, age=10]
Developer [name=iris, salary=170000, age=55]
After Sort
Developer [name=jason, salary=100000, age=10]
Developer [name=alvin, salary=80000, age=20]
Developer [name=mkyong, salary=70000, age=33]
Developer [name=iris, salary=170000, age=55]

3. More Lambda Examples

3.1 Sort By age

	//sort by age
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getAge() - o2.getAge();
	});
	//lambda
	listDevs.sort((Developer o1, Developer o2)->o1.getAge()-o2.getAge());
	//lambda, valid, parameter type is optional
	listDevs.sort((o1, o2)->o1.getAge()-o2.getAge());

3.2 Sort by name

	//sort by name
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getName().compareTo(o2.getName());
	});
	//lambda
	listDevs.sort((Developer o1, Developer o2)->o1.getName().compareTo(o2.getName()));		
	//lambda
	listDevs.sort((o1, o2)->o1.getName().compareTo(o2.getName()));		

3.3 Sort by salary

	//sort by salary
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getSalary().compareTo(o2.getSalary());
	});				
	//lambda
	listDevs.sort((Developer o1, Developer o2)->o1.getSalary().compareTo(o2.getSalary()));
	//lambda
	listDevs.sort((o1, o2)->o1.getSalary().compareTo(o2.getSalary()));

3.4 Reversed sorting.

3.4.1 Lambda expression to sort a List using their salary.

	Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
	listDevs.sort(salaryComparator);

Output

Developer [name=mkyong, salary=70000, age=33]
Developer [name=alvin, salary=80000, age=20]
Developer [name=jason, salary=100000, age=10]
Developer [name=iris, salary=170000, age=55]

3.4.2 Lambda expression to sort a List using their salary, reversed order.

	Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
	listDevs.sort(salaryComparator.reversed());

Output

Developer [name=iris, salary=170000, age=55]
Developer [name=jason, salary=100000, age=10]
Developer [name=alvin, salary=80000, age=20]
Developer [name=mkyong, salary=70000, age=33]

References

1. Start Using Java Lambda Expressions
2. Oracle : Lambda Expressions
3. Oracle : Comparator