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.

Java8Example1.java

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.

Java8Example2.java

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.

Item.java

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.

Java8Examples3.java

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.

Java8Examples4.java

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)