Java8分組(groupingBy)

1、分組，計數，排序

public class Java8Example1 {

    public static void main(String[] args) {

        List<String> items = Arrays.asList(
                "apple", "apple",
                "orange", "orange", "orange",
                "blueberry",
                "peach", "peach", "peach", "peach"
        );

        // 分組，計數
        Map<String, Long> result = items.stream()
                .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

        System.out.println(result);

        Map<String, Long> finalMap = new LinkedHashMap<>();
        // 排序
        result.entrySet().stream()
                .sorted(Map.Entry.<String, Long>comparingByValue().reversed())
                .forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));

        System.out.println(finalMap);
    }
}

 

2、以下例子使用的類

public class Item {

    private String name;

    private int qty;

    private BigDecimal price;

    public Item() {
    }

    public Item(String name, int qty, BigDecimal price) {
        this.name = name;
        this.qty = qty;
        this.price = price;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getQty() {
        return qty;
    }

    public void setQty(int qty) {
        this.qty = qty;
    }

    public BigDecimal getPrice() {
        return price;
    }

    public void setPrice(BigDecimal price) {
        this.price = price;
    }

    @Override
    public String toString() {
        return "Item{" +
                "name='" + name + '\'' +
                ", qty=" + qty +
                ", price=" + price +
                '}';
    }
}

 

3、分組，計數，計算數量

public class Java8Example2 {

    public static void main(String[] args) {

        List<Item> items = Arrays.asList(
                new Item("apple", 10, new BigDecimal(23.5)),
                new Item("apple", 20, new BigDecimal(32.5)),
                new Item("orange", 30, new BigDecimal(13.9)),
                new Item("orange", 20, new BigDecimal(33.5)),
                new Item("orange", 10, new BigDecimal(63.5)),
                new Item("orange", 50, new BigDecimal(41.5)),
                new Item("peach", 20, new BigDecimal(26.5)),
                new Item("peach", 30, new BigDecimal(42.5)),
                new Item("peach", 40, new BigDecimal(24.5)),
                new Item("peach", 10, new BigDecimal(12.5))
        );

        // 分組，計數
        Map<String, Long> counting = items.stream()
                .collect(Collectors.groupingBy(Item::getName, Collectors.counting()));
        System.out.println(counting);

        // 分組，計數，數量
        Map<String, Integer> sum = items.stream()
                .collect(Collectors.groupingBy(Item::getName, Collectors.summingInt(Item::getQty)));
        System.out.println(sum);

    }
}

 

4、通過價格分組

public class Java8Example3 {

    public static void main(String[] args) {

        List<Item> items = Arrays.asList(
                new Item("apple", 10, new BigDecimal(23.5)),
                new Item("apple", 20, new BigDecimal(32.5)),
                new Item("orange", 30, new BigDecimal(13.9)),
                new Item("orange", 20, new BigDecimal(32.5)),
                new Item("orange", 10, new BigDecimal(63.5)),
                new Item("orange", 50, new BigDecimal(41.5)),
                new Item("peach", 20, new BigDecimal(26.5)),
                new Item("peach", 30, new BigDecimal(32.5)),
                new Item("peach", 40, new BigDecimal(24.5)),
                new Item("peach", 10, new BigDecimal(12.5))
        );

        // 分組
        Map<BigDecimal, List<Item>> groupByPriceMap = items.stream()
                .collect(Collectors.groupingBy(Item::getPrice));
        System.out.println(groupByPriceMap);

        // 分組 轉化List->Set
        Map<BigDecimal, Set<String>> result = items.stream()
                .collect(Collectors.groupingBy(Item::getPrice, Collectors.mapping(Item::getName, Collectors.toSet())));
        System.out.println(result);
    }
}