How to sort collections in Java 8?

Introduction

In this post, we will explore different ways to sort Java collections in a certain order. Java platform provides a Collections framework that represents a group of objects. All the Java classes those implement the Collection interface have by default implemented the sort method i.e.

Collections.sort(list/set/Array)

Java classes like String, Date, Integer, Double, Float, Long, Integer etc implements the Comparable interface by default. Java provides us with the following two interfaces to compare and sort the custom objects

Comparable Interface 

public interface Comparable<T> {
    public int compareTo(T o);
}

Java 8 also provides the built in Comparator.comparing method that returns an instance of Comparable.

Comparator Interface

public interface Comparator<T> {
    int compare(T o1, T o2);
}
In this post we will use the following student class for practice.

public class Student{

 public String name;
 public Integer age;
 
 public Student() {
  
 }
 
 public Student(String name,Integer age) {
  this();
  this.name = name;
  this.age = age;
 }
 public String getName() {
  return name;
 }
 public Integer getAge() {
  return age;
 }

 @Override
 public String toString() {
  // TODO Auto-generated method stub
  return "Name: " + name + " ,Age: " + age;
 }
}

Java 8 Example of Comparable Interface using lambda expressions

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class ComparableExample{

 public static void main(String[] args) {
  List students = new ArrayList();
  
  students.add(new Student("One",4));
  students.add(new Student("Two",12));
  students.add(new Student("Three",9));
  
  students.sort( (s1,s2) -> s1.age.compareTo(s2.age));
  
  //OR Java 8 Comparator.comparing method 
  Collections.sort(students, Comparator.comparing(Student::getAge));
  
  students.forEach(System.out::println);
 }
}

Java 8 Example of Comparator Interface using lambda expressions

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

public class ComparatorExample{

 public static void main(String[] args) {
  List students = new ArrayList();
  
  students.add(new Student("One",4));
  students.add(new Student("Two",12));
  students.add(new Student("Three",9));
  
  Comparator ageComparator = (s1,s2) -> s1.age - s2.age;
  
  students.sort(ageComparator);
  students.forEach(System.out::println);
  
  //Reverse Order Sorting
  
  students.sort(ageComparator.reversed());
  System.out.println("Reverse Order Sorted List");
  students.forEach(System.out::println);
 }
}

Note that we have reverse sorted the collection using .reversed() method.

Conclusion

In this post, we learned how to sort collection using Comparable interface, Comparator interface and Java 8 provided Comparator.comparing method. Hope this post will help you in dealing with sorting logic :)