In this article, we will discuss how to sort list of Objects on multiple field/parameters using static method Comparator.comparing() and default method thenComparingDouble()

Comparing.comparing() method is used to sort any type of key for 1^st level sorting
thenComparingDouble() accepts ToDoubleFunction functional interface which means it is used to sort double-type key for 2^nd level sorting

1. thenComparingDouble() method :

Returns a lexicographic-order comparator with a function that extracts a double sort key
This default method accepts a function that extracts a double sort key from a type T
Method signature :-
- default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)
Where
- T is the type of element to be compared
- keyExtractor is the function used to extract the double sort key
Exception :- Throws NullPointerException, if the argument is null

2. thenComparingDouble() examples :

2.1 Sort Product list first by Name & then by Price

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 price is of type primitive double

Product.java

package net.bench.resources.comparator.thencomparingdouble;

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
}

SortProductListByNameAndThenByPrice.java

A list contains Product information for 5 as per insertion-order with some product names are same
2-level attribute Sorting :-
- Comparator.comparing() :- we are going to sort the Product list according to alphabetial order of its name for the 1^st level sorting
- thenComparingDouble() :- then we are going to compare their prices, if name of the Products are same for 2^nd level sorting
thenComparingDouble() method accepts ToDoubleFunction which means it accepts key of double type only

package net.bench.resources.comparator.thencomparingdouble;

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

public class SortProductListByNameAndThenByPrice {

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

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

	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); // iterate/printing



		// 2. Sort Product list first by Name & then by Price
		System.out.println("\n\nSort Product list"
				+ " first by Name & then by Price :-\n");


		// 2.1 sorting/iterating/printing
		products
		.stream() // 1. get sequential stream
		.sorted(
				Comparator.comparing(Product::getName) // 1. name sorting
				.thenComparingDouble(Product::getPrice) // 2. double sorting
				)
		.forEach(product -> System.out.println(
				"Product [name=" + product.getName()
				+ ", price=" + product.getPrice() 
				+ ", id=" + product.getId()
				+ ", quantity=" + product.getQuantity()
				+ "]"
				)); // customized printing
	}
}

Output:

Product list as per Insertion-order :-

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


Sort Product list first by Name & then by Price :-

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

2.2 Sort Student list first by Name & then by Percentage

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

Student.java

package net.bench.resources.comparator.thencomparingdouble;

public class Student {

	// member variables
	private int rollNumber;
	private String name;
	private double percentage;

	// 3-arg parameterized constructors

	// getters & setters

	// toString()
}

SortStudentListByNameAndThenByPercentage.java

A list contains Student information for 5 as per insertion-order with few students having same names
2-level attribute Sorting :-
- Comparator.comparing() :- we are going to sort the Student list according to alphabetial order of its name for the 1^st level sorting
- thenComparingDouble() :- then we are going to compare their percentages, if name of the Students are same for 2^nd level sorting
thenComparingDouble() method accepts ToDoubleFunction which means it accepts key of double type only

package net.bench.resources.comparator.thencomparingdouble;

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

public class SortStudentListByNameAndThenByPercentage {

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

		return Arrays.asList(

				new Student(2033, "Viraj", 65.67),
				new Student(2020, "Krish", 91.97),
				new Student(2004, "Rishi", 78.13),
				new Student(2015, "Krish", 35.32),
				new Student(2037, "Viraj", 51.02)
				);
	}

	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. Sort Student list first by Name & then by Percentage
		System.out.println("\n\nSort Student list"
				+ " first by Name & then by Percentage :-\n");


		// 2.1 sorting using Collections.sort() & Comparator
		Collections.sort(studentList, // original list
				Comparator.comparing(Student::getName) // 1. name sorting
				.thenComparingDouble(Student::getPercentage) // 2. double sorting
				);


		// 2.2 print to console
		studentList.forEach(student -> System.out.println(
				"Student [name=" + student.getName()
				+ ", percentage=" + student.getPercentage() 
				+ ", rollNumber=" + student.getRollNumber() 
				+ "]"
				)); // customized printing
	}
}

Output:

Student list as per Insertion-order :-

Student [rollNumber=2033, name=Viraj, percentage=65.67]
Student [rollNumber=2020, name=Krish, percentage=91.97]
Student [rollNumber=2004, name=Rishi, percentage=78.13]
Student [rollNumber=2015, name=Krish, percentage=35.32]
Student [rollNumber=2037, name=Viraj, percentage=51.02]


Sort Student list first by Name & then by Percentage :-

Student [name=Krish, percentage=35.32, rollNumber=2015]
Student [name=Krish, percentage=91.97, rollNumber=2020]
Student [name=Rishi, percentage=78.13, rollNumber=2004]
Student [name=Viraj, percentage=51.02, rollNumber=2037]
Student [name=Viraj, percentage=65.67, rollNumber=2033]

2.3 Throws NullPointerException if argument is NULL

If the input argument to thenComparingDouble() 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

SortStudentListWithNullPresent.java

package net.bench.resources.comparator.thencomparingdouble;

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

public class SortStudentListWithNullPresent {

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

		return Arrays.asList(
				null,
				new Student(2033, "Viraj", 65.67),
				new Student(2020, "Krish", 91.97),
				new Student(2004, "Rishi", 78.13),
				new Student(2015, "Krish", 35.32),
				new Student(2037, "Viraj", 51.02)
				);
	}

	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. Sort Student list first by Name & then by Percentage
		System.out.println("\n\nSort Student list"
				+ " first by Name & then by Percentage :-\n");


		// 2.1 sorting using Collections.sort() & Comparator
		Collections.sort(studentList, // original list
				Comparator.comparing(Student::getName) // 1. name sorting
				.thenComparingDouble(Student::getPercentage) // 2. double sorting
				);


		// 2.2 print to console
		studentList.forEach(student -> System.out.println(
				"Student [name=" + student.getName()
				+ ", percentage=" + student.getPercentage() 
				+ ", rollNumber=" + student.getRollNumber() 
				+ "]"
				)); // customized printing
	}
}

Output:

Student list as per Insertion-order :-

null
Student [rollNumber=2033, name=Viraj, percentage=65.67]
Student [rollNumber=2020, name=Krish, percentage=91.97]
Student [rollNumber=2004, name=Rishi, percentage=78.13]
Student [rollNumber=2015, name=Krish, percentage=35.32]
Student [rollNumber=2037, name=Viraj, percentage=51.02]


Sort Student list first by Name & then by Percentage :-

Exception in thread "main" java.lang.NullPointerException
	at java.util.Comparator.lambda$comparing$77a9974f$1(Comparator.java:469)
	at java.util.Comparator.lambda$thenComparing$36697e65$1(Comparator.java:216)
	at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
	at java.util.TimSort.sort(TimSort.java:220)
	at java.util.Arrays.sort(Arrays.java:1438)
	at java.util.Arrays$ArrayList.sort(Arrays.java:3895)
	at java.util.Collections.sort(Collections.java:175)
	at net.bench.resources.comparator.thencomparingdouble.SortStudentListWithNullPresent
.main(SortStudentListWithNullPresent.java:41)

References:

Happy Coding !!
Happy Learning !!

BenchResources.Net

Java tutorial for beginners

1. thenComparingDouble() method :

2. thenComparingDouble() examples :

2.1 Sort Product list first by Name & then by Price

2.2 Sort Student list first by Name & then by Percentage

2.3 Throws NullPointerException if argument is NULL

References:

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