In this article, we will discuss how to sort list of Objects on multiple field/parameters using static method Comparator.comparing() and default method thenComparing() for custom/reverse sorting
2-level attribute sorting of an Object :
If our requirement is to sort list of objects on 2 different attributes of an Object like Product or Customer in custom sorting order then,
- We can use Comparator.comparing() method for 1st level custom/reverse sorting which will return Comparator
- Then we can invoke thenComparing() method for 2nd level custom/reverse sorting on the returned Comparator
Overloaded thenComparing() method :
There are 3 overloaded thenComparing() method
- We already discussed 1st variant (Function) & 2nd variant (Comparator) in the previous article which accepts only one argument
- Here, in this article we will discuss 3rd variant (Function, Comparator) which accepts 2-arguments
1. Comparator.thenComparing() method :
- This is the 3rd variant which returns a lexicographic-order comparator with a function that extracts a key to be compared with the given
Comparator
- Method signature :-
- default <U> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator)
- Where
- T is the type of element to be compared
- keyExtractor is the function used to extract the sort key
- U is the type of sort key
- keyComparator is the
Comparator
used to compare the sort key
- Exception :- Throws NullPointerException, if the argument is null
- Check Java doc
2. Comparator.thenComparing() examples :
- 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
Product.java
package net.bench.resources.comparator.thencomparing.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 first by Name in ascending-order & then by Id by descending-order
- A list contains Product information for 5 as per insertion-order with some product names are same
- 2-level attribute Sorting :-
- First, we are going to sort the Product list according to alphabetial order of its name
- Second, we are going to compare their Ids and sort in descending-order, if name of the Products are same for 2nd level sorting
SortProductListByNameAscAndThenByIdDesc.java
package net.bench.resources.comparator.thencomparing.custom;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
public class SortProductListByNameAscAndThenByIdDesc {
// List of Products
private static List<Product> getProductList() {
return Arrays.asList(
new Product(101, "Wheat", 1089L, 36.89),
new Product(104, "Rice", 208L, 164.75),
new Product(103, "Lentils", 803L, 102.45),
new Product(102, "Wheat", 502L, 58.19),
new Product(105, "Rice", 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("Original Product list as per Insertion-order :-\n");
products.forEach(System.out::println); // iterate/printing
// 2. Sort Product list first by Name in ASC & then by Id in DESC
System.out.println("\n\nSort Product list first by Name in ASC"
+ " & then by Id in DESC :-\n");
// 2.1 sorting/iterating/printing
products
.stream() // 1. get sequential stream
.sorted(
Comparator.comparing(Product::getName) // 1. name sorting ASC
.thenComparing(
Product::getId, // id sorting
(int1, int2) -> int2-int1 // reverse
)
)
.forEach(System.out::println); // 3. iterate/printing
}
}
Output:
Original Product list as per Insertion-order :-
Product [name=Wheat, id=101, quantity=1089, price=36.89]
Product [name=Rice, id=104, quantity=208, price=164.75]
Product [name=Lentils, id=103, quantity=803, price=102.45]
Product [name=Wheat, id=102, quantity=502, price=58.19]
Product [name=Rice, id=105, quantity=303, price=45.5]
Sort Product list first by Name in ASC & then by Id in DESC :-
Product [name=Lentils, id=103, quantity=803, price=102.45]
Product [name=Rice, id=105, quantity=303, price=45.5]
Product [name=Rice, id=104, quantity=208, price=164.75]
Product [name=Wheat, id=102, quantity=502, price=58.19]
Product [name=Wheat, id=101, quantity=1089, price=36.89]
2.2 Sort Product list first by Name in ascending-order & then by Quantity in descending-order
- A list contains Product information for 5 as per insertion-order with some product names are same
- 2-level attribute Sorting :-
- First, we are going to sort the Product list according to alphabetial order of its name
- Second, we are going to compare their quantities and sort in descending-order, if name of the Products are same for 2nd level sorting
SortProductListByNameAscAndThenByQuantityDesc.java
package net.bench.resources.comparator.thencomparing.custom;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
public class SortProductListByNameAscAndThenByQuantityDesc {
// List of Products
private static List<Product> getProductList() {
return Arrays.asList(
new Product(101, "Wheat", 1089L, 36.89),
new Product(104, "Rice", 208L, 164.75),
new Product(103, "Lentils", 803L, 102.45),
new Product(102, "Wheat", 502L, 58.19),
new Product(105, "Rice", 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("Original Product list as per Insertion-order :-\n");
products.forEach(System.out::println); // iterating/printing
// 2. Sort Product list first by Name in ASC & then by Quantity in DESC
System.out.println("\n\nSort Product list first by Name in ASC"
+ " & then by Quantity in DESC :-\n");
// 2.1 sorting/iterating/printing
products
.stream()
.sorted(
Comparator.comparing(Product::getName) // 1. name sorting ASC
.thenComparing(
Product::getQuantity, // quantity sorting
(lng1, lng2) -> Long.compare(lng2, lng1) // reverse
)
)
.forEach(product -> System.out.println(
"Product [name=" + product.getName()
+ ", quantity=" + product.getQuantity()
+ ", id=" + product.getId()
+ ", price=" + product.getPrice()
+ "]"
)); // customized printing
}
}
Output:
Original Product list as per Insertion-order :-
Product [name=Wheat, id=101, quantity=1089, price=36.89]
Product [name=Rice, id=104, quantity=208, price=164.75]
Product [name=Lentils, id=103, quantity=803, price=102.45]
Product [name=Wheat, id=102, quantity=502, price=58.19]
Product [name=Rice, id=105, quantity=303, price=45.5]
Sort Product list first by Name & then by Quantity :-
Product [name=Lentils, quantity=803, id=103, price=102.45]
Product [name=Rice, quantity=303, id=105, price=45.5]
Product [name=Rice, quantity=208, id=104, price=164.75]
Product [name=Wheat, quantity=1089, id=101, price=36.89]
Product [name=Wheat, quantity=502, id=102, price=58.19]
2.3 Sort Product list first by Name in descending-order & then by Price in descending-order
- A list contains Product information for 5 as per insertion-order with some product names are same
- 2-level attribute Sorting :-
- First, we are going to sort the Product list according to reverse alphabetial order of its name
- Second, we are going to compare their prices and sort in descending-order, if name of the Products are same for 2nd level sorting
SortProductListByNameDescAndThenByPriceDesc.java
package net.bench.resources.comparator.thencomparing.custom;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
public class SortProductListByNameDescAndThenByPriceDesc {
// List of Products
private static List<Product> getProductList() {
return Arrays.asList(
new Product(101, "Wheat", 1089L, 36.89),
new Product(104, "Rice", 208L, 164.75),
new Product(103, "Lentils", 803L, 102.45),
new Product(102, "Wheat", 502L, 58.19),
new Product(105, "Rice", 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("Original Product list as per Insertion-order :-\n");
products.forEach(System.out::println); // iterating/printing
// 2. Sort Product list first by Name in DESC & then by Price in DESC
System.out.println("\n\nSort Product list first by Name in DESC"
+ " & then by Price in DESC :-\n");
// 2.1 sorting/iterating/printing
products
.stream()
.sorted(
Comparator.comparing(
Product::getName, // name sorting
(str1, str2) -> str2.compareTo(str1) // reverse
) // 1.
.thenComparing(
Product::getPrice, // price sorting
(dbl1, dbl2) -> Double.compare(dbl2, dbl1) // reverse
)
)
.forEach(product -> System.out.println(
"Product [name=" + product.getName()
+ ", price=" + product.getPrice()
+ ", id=" + product.getId()
+ ", quantity=" + product.getQuantity()
+ "]"
)); // customized printing
}
}
Output:
Original Product list as per Insertion-order :-
Product [name=Wheat, id=101, quantity=1089, price=36.89]
Product [name=Rice, id=104, quantity=208, price=164.75]
Product [name=Lentils, id=103, quantity=803, price=102.45]
Product [name=Wheat, id=102, quantity=502, price=58.19]
Product [name=Rice, id=105, quantity=303, price=45.5]
Sort Product list first by Name in DESC & then by Price in DESC :-
Product [name=Wheat, price=58.19, id=102, quantity=502]
Product [name=Wheat, price=36.89, id=101, quantity=1089]
Product [name=Rice, price=164.75, id=104, quantity=208]
Product [name=Rice, price=45.5, id=105, quantity=303]
Product [name=Lentils, price=102.45, id=103, quantity=803]
2.4 Throws NullPointerException if argument is NULL
- If the input argument to thenComparing() 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.thencomparing.custom;
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(
null,
new Product(101, "Wheat", 1089L, 36.89),
new Product(104, "Rice", 208L, 164.75),
new Product(103, "Lentils", 803L, 102.45),
new Product(102, "Wheat", 502L, 58.19),
new Product(105, "Rice", 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("Original Product list as per Insertion-order :-\n");
products.forEach(System.out::println); // iterate/printing
// 2. Sort Product list first by Name in ASC & then by Id in DESC
System.out.println("\n\nSort Product list first by Name in ASC"
+ " & then by Id in DESC :-\n");
// 2.1 sorting/iterating/printing
products
.stream() // 1. get sequential stream
.sorted(
Comparator.comparing(Product::getName) // 1. name sorting ASC
.thenComparing(
Product::getId, // id sorting
(int1, int2) -> int2-int1 // reverse
)
)
.forEach(System.out::println); // 3. iterate/printing
}
}
Output:
Original Product list as per Insertion-order :-
null
Product [name=Wheat, id=101, quantity=1089, price=36.89]
Product [name=Rice, id=104, quantity=208, price=164.75]
Product [name=Lentils, id=103, quantity=803, price=102.45]
Product [name=Wheat, id=102, quantity=502, price=58.19]
Product [name=Rice, id=105, quantity=303, price=45.5]
Sort Product list first by Name in ASC & then by Id in DESC :-
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$1(Comparator.java:216)
at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
at java.util.TimSort.sort(TimSort.java:220)
at java.util.Arrays.sort(Arrays.java:1512)
at java.util.stream.SortedOps$SizedRefSortingSink.end(SortedOps.java:348)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
at java.util.stream.ForEachOps$ForEachOp$OfRef
.evaluateSequential(ForEachOps.java:174)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
at net.bench.resources.comparator.thencomparing.custom.SortProductListWithNullPresent
.main(SortProductListWithNullPresent.java:48)
References:
- https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html
- https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#thenComparing-java.util.function.Function-java.util.Comparator-
- https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html
- https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html
- https://docs.oracle.com/javase/8/docs/api/java/util/Collection.html
Happy Coding !!
Happy Learning !!