Java 8 Lambda Comparator example for Sorting List of Custom Objects

We have already seen how to sort an Arraylist of custom Objects without using lambda expression. In this tutorial we will see how to sort a list of custom objects using lambda expression in Java.

Before we see the complete example, lets see what is the difference between using lambda vs without using lambda:

Without using Lambda expression: Before Java 8 we sort like this:

If you are not familiar with the Comparator, refer this guide: Java Comparator with example.

Here we are sorting a list of objects of Student class by name.

Comparator sortingByName = new Comparator() {
   @Override
   public int compare(Student s1, Student s2) {
	return s1.getName().compareTo(s2.getName());
   }
};

Using Lambda expression: The Java 8 equivalent code using Lambda expression would look like this:

Comparator sortingByName =
(Student s1, Student s2)->s1.getName().compareTo(s2.getName());

Note: In Java 8, the List interface supports the sort() method so you need not to use the Comparator.sort(), instead you can use the List.sort() method. The above code can be further re-written as:
Here studentlist is the instance of the list containing Student objects (see the complete example below).

studentlist.sort((Student s1, Student s2)->s1.getName().compareTo(s2.getName()));

Java 8 – Sorting with Lambda Expression

import java.util.ArrayList;
import java.util.List;
class Student {
   String name; 
   int age; 
   int id; 
   public String getName() {
      return name; 
   } 
   public int getAge() { 
      return age; 
   } 
   public int getId() { 
      return id; 
   } 
   Student(String n, int a, int i){ 
      name = n; 
      age = a; 
      id = i; 
   } 
   @Override public String toString() {     
      return ("Student[ "+"Name:"+this.getName()+             
              " Age: "+ this.getAge() +                     
              " Id: "+ this.getId()+"]"); 
   }
}
public class Example {
   public static void main(String[] args) {
      List<Student> studentlist = new ArrayList<Student>();
      studentlist.add(new Student("Jon", 22, 1001)); 
      studentlist.add(new Student("Steve", 19, 1003)); 
      studentlist.add(new Student("Kevin", 23, 1005)); 
      studentlist.add(new Student("Ron", 20, 1010)); 
      studentlist.add(new Student("Lucy", 18, 1111));
      System.out.println("Before Sorting the student data:"); 
 
      //java 8 forEach for printing the list 
      studentlist.forEach((s)->System.out.println(s));

      System.out.println("After Sorting the student data by Age:"); 

      //Lambda expression for sorting by age 
      studentlist.sort((Student s1, Student s2)->s1.getAge()-s2.getAge()); 

      //java 8 forEach for printing the list
      studentlist.forEach((s)->System.out.println(s));         

      System.out.println("After Sorting the student data by Name:"); 
      //Lambda expression for sorting the list by student name       
      studentlist.sort((Student s1, Student s2)->s1.getName().compareTo(s2.getName())); 
      studentlist.forEach((s)->System.out.println(s));        
      System.out.println("After Sorting the student data by Id:");        
      //Lambda expression for sorting the list by student id 
      studentlist.sort((Student s1, Student s2)->s1.getId()-s2.getId()); 
      studentlist.forEach((s)->System.out.println(s)); 
   }
}

Output:

Before Sorting the student data:
Student[ Name:Jon Age: 22 Id: 1001]
Student[ Name:Steve Age: 19 Id: 1003]
Student[ Name:Kevin Age: 23 Id: 1005]
Student[ Name:Ron Age: 20 Id: 1010]
Student[ Name:Lucy Age: 18 Id: 1111]

After Sorting the student data by Age:
Student[ Name:Lucy Age: 18 Id: 1111]
Student[ Name:Steve Age: 19 Id: 1003]
Student[ Name:Ron Age: 20 Id: 1010]
Student[ Name:Jon Age: 22 Id: 1001]
Student[ Name:Kevin Age: 23 Id: 1005]

After Sorting the student data by Name:
Student[ Name:Jon Age: 22 Id: 1001]
Student[ Name:Kevin Age: 23 Id: 1005]
Student[ Name:Lucy Age: 18 Id: 1111]
Student[ Name:Ron Age: 20 Id: 1010]
Student[ Name:Steve Age: 19 Id: 1003]

After Sorting the student data by Id:
Student[ Name:Jon Age: 22 Id: 1001]
Student[ Name:Steve Age: 19 Id: 1003]
Student[ Name:Kevin Age: 23 Id: 1005]
Student[ Name:Ron Age: 20 Id: 1010]
Student[ Name:Lucy Age: 18 Id: 1111]

Java 8 – Sorting in reverse order

To sort the list in the reverse(descending) order, just change the order of the arguments like this:

Sorting the list of objects of Student class by Name in reverse order:

studentlist.sort((Student s1, Student s2)->s2.getName().compareTo(s1.getName()));

Similarly, the list can be sorted in reverse order by student age and id like this:

//Lambda expression for sorting the list by student age in reverse order 
studentlist.sort((Student s1, Student s2)->s2.getAge()-s1.getAge());
//Lambda expression for sorting the list by student id in reverse order 
studentlist.sort((Student s1, Student s2)->s2.getId()-s1.getId());

Related Tutorials:

Java 8 – Sorting with Lambda Expression

Java 8 – Sorting in reverse order

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