In this article, we will show you how to use Java 8 Stream Collectors to group by, count, sum and sort a List.

1. Group By, Count and Sort

1.1 Group by a List and display the total count of it.

package com.mkyong.java8;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
public class Java8Example1 {
    public static void main(String[] args) {
        //3 apple, 2 banana, others 1
        List<String> items =
                Arrays.asList("apple", "apple", "banana",
                        "apple", "orange", "banana", "papaya");
        Map<String, Long> result =
                items.stream().collect(
                        Collectors.groupingBy(
                                Function.identity(), Collectors.counting()
                );
        System.out.println(result);

output

	papaya=1, orange=1, banana=2, apple=3

1.2 Add sorting.

package com.mkyong.java8;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
public class Java8Example2 {
    public static void main(String[] args) {
        //3 apple, 2 banana, others 1
        List<String> items =
                Arrays.asList("apple", "apple", "banana",
                        "apple", "orange", "banana", "papaya");
        Map<String, Long> result =
                items.stream().collect(
                        Collectors.groupingBy(
                                Function.identity(), Collectors.counting()
                );
        Map<String, Long> finalMap = new LinkedHashMap<>();
        //Sort a map and add to finalMap
        result.entrySet().stream()
                .sorted(Map.Entry.<String, Long>comparingByValue()
                        .reversed()).forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));
        System.out.println(finalMap);

output

	apple=3, banana=2, papaya=1, orange=1

2. List Objects

Examples to ‘group by’ a list of user defined Objects.

2.1 A Pojo.

package com.mkyong.java8;
import java.math.BigDecimal;
public class Item {
    private String name;
    private int qty;
    private BigDecimal price;
    //constructors, getter/setters 

2.2 Group by the name + Count or Sum the Qty.

package com.mkyong.java8;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Java8Examples3 {
    public static void main(String[] args) {
        //3 apple, 2 banana, others 1
        List<Item> items = Arrays.asList(
                new Item("apple", 10, new BigDecimal("9.99")),
                new Item("banana", 20, new BigDecimal("19.99")),
                new Item("orang", 10, new BigDecimal("29.99")),
                new Item("watermelon", 10, new BigDecimal("29.99")),
                new Item("papaya", 20, new BigDecimal("9.99")),
                new Item("apple", 10, new BigDecimal("9.99")),
                new Item("banana", 10, new BigDecimal("19.99")),
                new Item("apple", 20, new BigDecimal("9.99"))
        );
        Map<String, Long> counting = items.stream().collect(
                Collectors.groupingBy(Item::getName, Collectors.counting()));
        System.out.println(counting);
        Map<String, Integer> sum = items.stream().collect(
                Collectors.groupingBy(Item::getName, Collectors.summingInt(Item::getQty)));
        System.out.println(sum);

output

//Group by + Count
	papaya=1, banana=2, apple=3, orang=1, watermelon=1
//Group by + Sum qty
	papaya=20, banana=30, apple=40, orang=10, watermelon=10

2.2 Group by Price – Collectors.groupingBy and Collectors.mapping example.

package com.mkyong.java8;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
public class Java8Examples4 {
    public static void main(String[] args) {
        //3 apple, 2 banana, others 1
        List<Item> items = Arrays.asList(
                new Item("apple", 10, new BigDecimal("9.99")),
                new Item("banana", 20, new BigDecimal("19.99")),
                new Item("orang", 10, new BigDecimal("29.99")),
                new Item("watermelon", 10, new BigDecimal("29.99")),
                new Item("papaya", 20, new BigDecimal("9.99")),
                new Item("apple", 10, new BigDecimal("9.99")),
                new Item("banana", 10, new BigDecimal("19.99")),
                new Item("apple", 20, new BigDecimal("9.99"))
                );
		//group by price
        Map<BigDecimal, List<Item>> groupByPriceMap = 
			items.stream().collect(Collectors.groupingBy(Item::getPrice));
        System.out.println(groupByPriceMap);
		// group by price, uses 'mapping' to convert List<Item> to Set<String>
        Map<BigDecimal, Set<String>> result =
                items.stream().collect(
                        Collectors.groupingBy(Item::getPrice,
                                Collectors.mapping(Item::getName, Collectors.toSet())
                );
        System.out.println(result);

output

	19.99=[
			Item{name='banana', qty=20, price=19.99}, 
			Item{name='banana', qty=10, price=19.99}
		], 
	29.99=[
			Item{name='orang', qty=10, price=29.99}, 
			Item{name='watermelon', qty=10, price=29.99}
		], 
	9.99=[
			Item{name='apple', qty=10, price=9.99}, 
			Item{name='papaya', qty=20, price=9.99}, 
			Item{name='apple', qty=10, price=9.99}, 
			Item{name='apple', qty=20, price=9.99}
//group by + mapping to Set
	19.99=[banana], 
	29.99=[orang, watermelon], 
	9.99=[papaya, apple]

References

1. Java 8 Stream Collectors JavaDoc
2. Java – How to sort a Map
3. Stackoverflow – Sort a Map by values (Java)