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 byName = new Comparator() {
  @Override
  public int compare(Developer o1, Developer o2) {
   return o1.getName().compareTo(o2.getName());
  }
 };
2. Lambda expression equivalent.
 Comparator 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.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 listDevs = getDevelopers();

  System.out.println("Before Sort");
  for (Developer developer : listDevs) {
   System.out.println(developer);
  }
  
  //sort by age
  Collections.sort(listDevs, new Comparator() {
   @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 getDevelopers() {

  List result = new ArrayList();

  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() {
  @Override
  public int compare(Developer o1, Developer o2) {
   return o1.getAge() - o2.getAge();
  }
 });
 
 //sort by name 
 Collections.sort(listDevs, new Comparator() {
  @Override
  public int compare(Developer o1, Developer o2) {
   return o1.getName().compareTo(o2.getName());
  }
 });
    
 //sort by salary
 Collections.sort(listDevs, new Comparator() {
  @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() {
  @Override
  public int compare(Developer o1, Developer o2) {
   return o2.getAge() - o1.getAge();
  }
 });
Lambda expression example :
TestSorting.java
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 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 getDevelopers() {

  List result = new ArrayList();

  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() {
  @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() {
  @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() {
  @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 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 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