Stream Collectors groupingBy 例子

1. Group By, Count and Sort

1.1 Group by a List and display the total count of it.（按列表分组，并显示其总数）

public class Java8Example1 {

    public static void main(String[] args) {

        //3 apple, 2 banana, others 1
        List items =
                Arrays.asList("apple", "apple", "banana",
                        "apple", "orange", "banana", "papaya");

        Map 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.（增加排序实现）

public class Java8Example2 {
    public static void main(String[] args) {

        //3 apple, 2 banana, others 1
        List items =
                Arrays.asList("apple", "apple", "banana",
                        "apple", "orange", "banana", "papaya");

        Map result =
                items.stream().collect(
                        Collectors.groupingBy(
                                Function.identity(), Collectors.counting()
                        )
                );

        Map finalMap = new LinkedHashMap<>();

        //Sort a map and add to finalMap
        result.entrySet().stream()
                .sorted(Map.Entry.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.（通过“用户定义的对象”列表进行分组的示例。）

2.1 A Pojo.

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. （name + Count分组或者 对 Qty求和分组）

public class Java8Examples3 {

    public static void main(String[] args) {

        //3 apple, 2 banana, others 1
        List 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 counting = items.stream().collect(
                Collectors.groupingBy(Item::getName, Collectors.counting()));

        System.out.println(counting);

        //求和
        Map sum = items.stream().collect(
                Collectors.groupingBy(Item::getName, Collectors.summingInt(Item::getQty)));

        System.out.println(sum);

        Map> env = items.stream().collect(
                Collectors.groupingBy(Item::getName));

        System.out.println(env);
    }
}

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
}

{
	papaya=[Item(name=papaya, qty=20, price=9.99)], banana=[Item(name=banana, qty=20, price=19.99), Item(name=banana, qty=10, price=19.99)], apple=[Item(name=apple, qty=10, price=9.99), Item(name=apple, qty=10, price=9.99), Item(name=apple, qty=20, price=9.99)], orang=[Item(name=orang, qty=10, price=29.99)], watermelon=[Item(name=watermelon, qty=10, price=29.99)]}
}