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 :

TestSorting.java

package com.mycareerrepublic.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("mycareerrepublic", 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=mycareerrepublic, 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=mycareerrepublic, 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 :

TestSorting.java

package com.mycareerrepublic.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("mycareerrepublic", 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=mycareerrepublic, 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=mycareerrepublic, 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=mycareerrepublic, 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=mycareerrepublic, salary=70000, age=33]

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About Mariano

I'm Ethan Mariano a software engineer by profession and reader/writter by passion.I have good understanding and knowledge of AngularJS, Database, javascript, web development, digital marketing and exploring other technologies related to Software development.