In this article, we will discuss how to sort list of Objects on the basis of specific integer parameter/field using Java 8 Comparator’s static method comparingInt() which accepts ToIntFunction functional interface

1. Comparator.comparingInt() method :

This static method accepts a function that extracts an int sort key from a type T
Returns a Comparator that compares by that sort key
The returned comparator is serializable if the specified function is also serializable
Method signature :-
- static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor)
Where
- T is the type of element to be compared
- keyExtractor is the function used to extract the integer sort key
Exception :- Throws NullPointerException, if the argument is null

2. Comparator.comparingInt() examples :

2.1 Sort Product list according to their Id

Product class is defined with 4 attributes namely id, name, quantity and their price
Along with 4 attributes, parameterized constructor, getters/setters and toString() method is defined – removed for brevity
Member variable id is of type primitive int

Product.java

package net.bench.resources.comparator.comparingint;

public class Product {

	// member variables
	private int id;
	private String name;
	private long quantity;
	private double price;

	// 4-arg parameterized constructor

	// getters and setters

	// toString() method
}

SortProductListById.java

A list contains Product information for 5 as per insertion-order
Sorting :- we are going to sort the Product list in the ascending order of Product Id using Stream‘s sorted() method passing Comparator.comparingInt() as argument
Comparator.comparingInt() accepts ToIntFunction which means it accepts key of integer type only

package net.bench.resources.comparator.comparingint;

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

public class SortProductListById {

	// List of Products
	private static List<Product> getProductList() {

		return Arrays.asList(

				new Product(102, "Rice", 502L, 58.19),
				new Product(104, "Oil", 208L, 164.75),
				new Product(103, "Lentils", 803L, 102.45),
				new Product(105, "Vegetables", 303L, 45.50),
				new Product(101, "Wheat", 1089L, 36.89)
				);
	}

	public static void main(String[] args) {

		// 1. get Product list
		List<Product> products = getProductList();


		// 1.1 print to console
		System.out.println("Product list as per Insertion-order :-\n");
		products.forEach(System.out::println); // iterating/printing



		// 2. sorting Product list in ascending-order by its Id
		System.out.println("\n\nSorted Product list in ascending-order of Id :-\n");


		// 2.1 sorting/iterating/printing
		products
		.stream() // 1. get sequential stream
		.sorted(Comparator.comparingInt(Product::getId)) // 2. integer sorting
		.forEach(System.out::println); // 3. iterate/printing
	}
}

Output:

Product list as per Insertion-order :-

Product [id=102, name=Rice, quantity=502, price=58.19]
Product [id=104, name=Oil, quantity=208, price=164.75]
Product [id=103, name=Lentils, quantity=803, price=102.45]
Product [id=105, name=Vegetables, quantity=303, price=45.5]
Product [id=101, name=Wheat, quantity=1089, price=36.89]


Sorted Product list in ascending-order of Id :-

Product [id=101, name=Wheat, quantity=1089, price=36.89]
Product [id=102, name=Rice, quantity=502, price=58.19]
Product [id=103, name=Lentils, quantity=803, price=102.45]
Product [id=104, name=Oil, quantity=208, price=164.75]
Product [id=105, name=Vegetables, quantity=303, price=45.5]

2.2 Sort Student list according to their Age

Student class is defined with 3 attributes namely rollNumber, name and their age
Along with 3 attributes, parameterized constructor, getters/setters and toString() method is defined – removed for brevity
Member variable rollNumber and age is of type primitive int

Student.java

package net.bench.resources.comparator.comparingint;

public class Student {

	// member variables
	private int rollNumber;
	private String name;
	private int age;

	// 3-arg parameterized constructors

	// getters & setters

	// toString()
}

SortStudentListByAge.java

A list contains 5 Student information as per insertion-order
Sorting :- we are going to sort the Student list in the ascending order of their age using Collections.sort() method passing Comparator.comparingInt() as 2^nd argument and original list as 1^st argument
Comparator.comparingInt() accepts ToIntFunction which means it accepts key of integer type only

package net.bench.resources.comparator.comparingint;

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

public class SortStudentListByAge {

	// List of Students
	private static List<Student> getStudentList() {

		return Arrays.asList(

				new Student(2033, "Viraj", 29),
				new Student(2020, "Rishi", 25),
				new Student(2004, "Aditya", 33),
				new Student(2015, "Krish", 19),
				new Student(2037, "Suresh", 34)
				);
	}

	public static void main(String[] args) {

		// 1. get Student list
		List<Student> studentList = getStudentList();


		// 1.1 print to console
		System.out.println("Student list as per Insertion-order :-\n");
		studentList.forEach(System.out::println); // iterating/printing



		// 2. sorting Student list in ascending-order of their Age
		System.out.println("\n\nSorted Student list by their Age :-\n");


		// 2.1 sorting using Collections.sort() & Comparator
		Collections.sort(
				studentList, // original list
				Comparator.comparingInt(Student::getAge) //integer sorting
				);


		// 2.2 print to console
		studentList.forEach(System.out::println); // iterate/printing
	}
}

Output:

Student list as per Insertion-order :-

Student [rollNumber=2033, name=Viraj, age=29]
Student [rollNumber=2020, name=Rishi, age=25]
Student [rollNumber=2004, name=Aditya, age=33]
Student [rollNumber=2015, name=Krish, age=19]
Student [rollNumber=2037, name=Suresh, age=34]


Sorted Student list by their Age :-

Student [rollNumber=2015, name=Krish, age=19]
Student [rollNumber=2020, name=Rishi, age=25]
Student [rollNumber=2033, name=Viraj, age=29]
Student [rollNumber=2004, name=Aditya, age=33]
Student [rollNumber=2037, name=Suresh, age=34]

2.3 Throws NullPointerException if argument is NULL

If the input argument to Comparator.comparingInt() method is null then it throws NullPointerException as shown in the below illustration
Check How to sort List and Arrays with null values to handle list with null values while sorting

SortProductListWithNullPresent.java

package net.bench.resources.comparator.comparingint;

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

public class SortProductListWithNullPresent {

	// List of Products
	private static List<Product> getProductList() {

		return Arrays.asList(

				new Product(102, "Rice", 502L, 58.19),
				new Product(104, "Oil", 208L, 164.75),
				new Product(103, "Lentils", 803L, 102.45),
				new Product(105, "Vegetables", 303L, 45.50),
				new Product(101, "Wheat", 1089L, 36.89)
				);
	}

	public static void main(String[] args) {

		// 1. get Product list
		List<Product> products = getProductList();


		// 2. sorting Product list in ascending-order by its Id
		System.out.println("Exception - Sorted Product list with Null value :-\n");


		// 3. sorting/iterating/printing
		products
		.stream() // 1. get sequential stream
		.sorted(Comparator.comparingInt(null)) // 2. null
		.forEach(System.out::println); // 3. iterate/printing
	}
}

Output:

Exception - Sorted Product list with Null value :-

Exception in thread "main" java.lang.NullPointerException
	at java.util.Objects.requireNonNull(Objects.java:203)
	at java.util.Comparator.comparingInt(Comparator.java:488)
	at net.bench.resources.comparator.comparingint.SortProductListWithNullPresent
.main(SortProductListWithNullPresent.java:35)

References:

Happy Coding !!
Happy Learning !!

BenchResources.Net

Java tutorial for beginners

1. Comparator.comparingInt() method :

2. Comparator.comparingInt() examples :

2.1 Sort Product list according to their Id

2.2 Sort Student list according to their Age

2.3 Throws NullPointerException if argument is NULL

References:

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