Comparator custom sorting java

Java 8 – Comparator.comparing() method for custom/reverse sorting

In this article, we will discuss how to sort list of Objects on the basis of any one parameter/field using Java 8 Comparator’s static method comparing() which accepts 2-arguments for custom/reverse sorting

Comparator.comparing() is the overloaded method where 1 st variant accepts only-one argument and 2 nd variant accepts 2-arguments

Already in the previous article we have discussed along with various examples for the 1 st overloaded method, check Comparator.comparing() method for natural-order sorting

Here, we are going to discuss 2 nd overloaded method which helps in customized sorting or reverse-order sorting of numbers or reverse-order alphabetical sorting of String elements

1. Comparator.comparing() method :

2. Comparator.comparing() examples :

• Product class is defined with 4 attributes namely id, name, quantity and their price
• Along with 4 attributes, 4-arguments parameterized constructor, getters/setters and toString() method is defined – removed for brevity

Product.java

package net.bench.resources.comparator.comparing.custom; 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 >

2.1 Sort Product list in reverse alphabetical order of its name

SortProductListByNameInReverseOrder.java

package net.bench.resources.comparator.comparing.custom; import java.util.Arrays; import java.util.Comparator; import java.util.List; public class SortProductListByNameInReverseOrder < // List of Products private static ListgetProductList() < return Arrays.asList( new Product(101, "Wheat", 1089L, 36.89), new Product(102, "Rice", 502L, 58.19), new Product(103, "Lentils", 803L, 102.45), new Product(104, "Oil", 208L, 164.75), new Product(105, "Vegetables", 303L, 45.50) ); >// main() method public static void main(String[] args) < // 1. get Product list Listproducts = 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 alphabetical order by its Name System.out.println("\n\nSorted Product list by" + " REVERSE alphabetical order of Name :-\n"); // 2.1 sorting/iterating/printing products .stream() // 1. get sequential stream .sorted( Comparator.comparing( Product::getName, // name sorting (str1, str2) -> str2.compareTo(str1) // reverse ) ) .forEach(System.out::println); // 3. iterate/printing > >

Product list as per Insertion-order :- 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] Sorted Product list by REVERSE alphabetical order of Name :- Product [id=101, name=Wheat, quantity=1089, price=36.89] Product [id=105, name=Vegetables, quantity=303, price=45.5] 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]

2.2 Sort Product list in descending-order of Product Id

SortProductListByIdInReverseOrder .java

package net.bench.resources.comparator.comparing.custom; import java.util.Arrays; import java.util.Comparator; import java.util.List; public class SortProductListByIdInReverseOrder < // List of Products private static ListgetProductList() < 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) ); >// main() method public static void main(String[] args) < // 1. get Product list Listproducts = 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 descending-order of Id :-\n"); // 2.1 sorting/iterating/printing products .stream() // 1. get sequential stream .sorted( Comparator.comparing( Product::getId, // Id sorting (int1, int2) -> int2-int1 // reverse ) ) .forEach(System.out::println); // 3. iterate/printing > >

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

2.3 Sort Product list in descending-order of its quantity

SortProductListByQuantityInReverseOrder.java

package net.bench.resources.comparator.comparing.custom; import java.util.Arrays; import java.util.Collections; import java.util.Comparator; import java.util.List; public class SortProductListByQuantityInReverseOrder < // List of Products private static ListgetProductList() < return Arrays.asList( new Product(101, "Wheat", 1089L, 36.89), new Product(102, "Rice", 502L, 58.19), new Product(103, "Lentils", 803L, 102.45), new Product(104, "Oil", 208L, 164.75), new Product(105, "Vegetables", 303L, 45.50) ); >// main() method public static void main(String[] args) < // 1. get Product list ListproductList = getProductList(); // 1.1 print to console System.out.println("Product list as per Insertion-order :-\n"); productList.forEach(System.out::println); // iterating/printing // 2. sorting Product list in ascending-order by its Quantity System.out.println("\n\nSorted Product list" + " in descending-order of Quantity :-\n"); // 2.1 sorting using Collections.sort() & Comparator Collections.sort( productList, // original list Comparator.comparing( Product::getQuantity, // quantity sorting (lng1, lng2) -> Long.compare(lng2, lng1) // reverse ) ); // 2.2 iterate & printing productList.forEach(System.out::println); // iterating/printing > >

Product list as per Insertion-order :- 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] Sorted Product list in descending-order of Quantity :- Product [id=101, name=Wheat, quantity=1089, price=36.89] Product [id=103, name=Lentils, quantity=803, price=102.45] Product [id=102, name=Rice, quantity=502, price=58.19] Product [id=105, name=Vegetables, quantity=303, price=45.5] Product [id=104, name=Oil, quantity=208, price=164.75]

2.4 Sort Product list in decreasing order of its price

• A list contains Product information for 5 as per insertion-order
• Sorting :- we are going to sort the Product list on the basis of decreasing price using Collections.sort() method passing reverse Comparator as 2 nd argument and original list as 1 st argument

SortProductListByPriceInReverseOrder.java

package net.bench.resources.comparator.comparing.custom; import java.util.Arrays; import java.util.Collections; import java.util.Comparator; import java.util.List; public class SortProductListByPriceInReverseOrder < // List of Products private static ListgetProductList() < return Arrays.asList( new Product(101, "Wheat", 1089L, 36.89), new Product(102, "Rice", 502L, 58.19), new Product(103, "Lentils", 803L, 102.45), new Product(104, "Oil", 208L, 164.75), new Product(105, "Vegetables", 303L, 45.50) ); >// main() method public static void main(String[] args) < // 1. get Product list ListproductList = getProductList(); // 1.1 print to console System.out.println("Product list as per Insertion-order :-\n"); productList.forEach(System.out::println); // iterating/printing // 2. sorting Product list in increasing-order of price System.out.println("\n\nSorted Product list in decreasing-order of price :- \n"); // 2.1 sorting using Collections.sort() & Comparator Collections.sort( productList, // original list Comparator.comparing( Product::getPrice, // price sorting (dbl1, dbl2) -> Double.compare(dbl2, dbl1) // reverse ) ); // 2.2 iterate & printing productList.forEach(System.out::println); // iterating/printing > >

Product list as per Insertion-order :- 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] Sorted Product list in decreasing-order of price :- Product [id=104, name=Oil, quantity=208, price=164.75] Product [id=103, name=Lentils, quantity=803, price=102.45] Product [id=102, name=Rice, quantity=502, price=58.19] Product [id=105, name=Vegetables, quantity=303, price=45.5] Product [id=101, name=Wheat, quantity=1089, price=36.89]

2.5 Throws NullPointerException if argument is NULL

• If the input argument to Comparator.comparing() 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.comparing.custom; import java.util.Arrays; import java.util.Comparator; import java.util.List; public class SortProductListWithNullPresent < // List of Products private static ListgetProductList() < 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 Listproducts = 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.comparing(null)) // 2. null .forEach(System.out::println); // 3. iterate/printing > >

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.comparing(Comparator.java:467) at net.bench.resources.comparator.comparing.custom.SortProductListWithNullPresent .main(SortProductListWithNullPresent.java:35)

3. Sorting String elements ignoring cases :

• In below illustration, we are sorting String elements in alphabetical order ignoring cases

SortStringElementsIgnoringCases.java

package net.bench.resources.comparator.comparing.custom; import java.util.Arrays; import java.util.Comparator; import java.util.List; public class SortStringElementsIgnoringCases < public static void main(String[] args) < // 1. list of Strings Listnames = Arrays.asList( "rafael", "messi", "argus", "zen", "Martin", "Roger", "Ben" ); // 1.1 original String elements - insertion-order System.out.println("Original String elemetns" + " as per Insertion-order :- \n"); names.forEach(System.out::println); // 2. String comparator ignoring cases Comparator stringComparator = Comparator .comparing( String::valueOf, // name comparison String.CASE_INSENSITIVE_ORDER // ignore cases ); // 2.1 print to console System.out.println("\n\nSorted String list" + " in alphabetical-order ignoring cases :- \n"); // 2.2 sorting/iterate/printing names .stream() .sorted(stringComparator) .forEach(System.out::println); > >

Original String elemetns as per Insertion-order :- rafael messi argus zen Martin Roger Ben Sorted String list in alphabetical-order ignoring cases :- argus Ben Martin messi rafael Roger zen

References:

Happy Coding !!
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