group by string java 8

1package com.mkyong.java8;
2
3import java.math.BigDecimal;
4import java.util.Arrays;
5import java.util.List;
6import java.util.Map;
7import java.util.Set;
8import java.util.stream.Collectors;
9
10public class Java8Examples4 {
11
12    public static void main(String[] args) {
13
14        //3 apple, 2 banana, others 1
15        List<Item> items = Arrays.asList(
16                new Item("apple", 10, new BigDecimal("9.99")),
17                new Item("banana", 20, new BigDecimal("19.99")),
18                new Item("orang", 10, new BigDecimal("29.99")),
19                new Item("watermelon", 10, new BigDecimal("29.99")),
20                new Item("papaya", 20, new BigDecimal("9.99")),
21                new Item("apple", 10, new BigDecimal("9.99")),
22                new Item("banana", 10, new BigDecimal("19.99")),
23                new Item("apple", 20, new BigDecimal("9.99"))
24                );
25
26		//group by price
27        Map<BigDecimal, List<Item>> groupByPriceMap =
28			items.stream().collect(Collectors.groupingBy(Item::getPrice));
29
30        System.out.println(groupByPriceMap);
31
32		// group by price, uses 'mapping' to convert List<Item> to Set<String>
33        Map<BigDecimal, Set<String>> result =
34                items.stream().collect(
35                        Collectors.groupingBy(Item::getPrice,
36                                Collectors.mapping(Item::getName, Collectors.toSet())
37                        )
38                );
39
40        System.out.println(result);
41
42    }
43}

1Map<BlogPostType, List<BlogPost>> postsPerType = posts.stream()
2  .collect(groupingBy(BlogPost::getType));