Collectors 类

利用java8新特性，可以用简洁高效的代码来实现一些数据处理。

定义1个Apple对象：

public class Apple {

    private Integer id;
    private String name;
    private BigDecimal money;
    private Integer num;
    public Apple(Integer id, String name, BigDecimal money, Integer num) {
        this.id = id;
        this.name = name;
        this.money = money;
        this.num = num;
    }
    ... //省略getse和toString 方法

}

public class Main {

    public static void main(String[] args) {
        List<Apple> appleList = new ArrayList<>();//存放apple对象集合
        get(appleList);

        //List 以ID分组 Map<Integer,List<Apple>>
        Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));

        System.err.println("groupBy:"+groupBy);
        //groupBy:{1=[Apple [id=1, name=苹果1, money=3.25, num=10], Apple [id=1, name=苹果2, money=1.35, num=20]], 2=[Apple [id=2, name=香蕉, money=2.89, num=30]], 3=[Apple [id=3, name=荔枝, money=9.99, num=40]]}


        /**
         * List -> Map
         * 需要注意的是：
         * toMap 如果集合对象有重复的key，会报错Duplicate key ....
         *  apple1,apple12的id都为1。
         *  可以用 (k1,k2)->k1 来设置，如果有重复的key,则保留key1,舍弃key2
         */
        Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));
        System.out.println(appleMap.toString());
        // {1=Apple [id=1, name=苹果1, money=3.25, num=10], 2=Apple [id=2, name=香蕉, money=2.89, num=30], 3=Apple [id=3, name=荔枝, money=9.99, num=40]}


        //过滤出符合条件的数据
        List<Apple> filterList = appleList.stream().filter(a -> a.getName().equals("香蕉")).collect(Collectors.toList());
        System.err.println("filterList:"+filterList);
        // filterList:[Apple [id=2, name=香蕉, money=2.89, num=30]]


        //计算 总金额
        BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
        System.err.println("totalMoney:"+totalMoney);
        // totalMoney:17.48

    }

    public static void get(List<Apple> appleList){
        Apple apple1 =  new Apple(1,"苹果1",new BigDecimal("3.25"),10);
        Apple apple12 = new Apple(1,"苹果2",new BigDecimal("1.35"),20);
        Apple apple2 =  new Apple(2,"香蕉",new BigDecimal("2.89"),30);
        Apple apple3 =  new Apple(3,"荔枝",new BigDecimal("9.99"),40);

        appleList.add(apple1);
        appleList.add(apple12);
        appleList.add(apple2);
        appleList.add(apple3);
    }
}

5、查找流中最大 最小值

Collectors.maxBy 和 Collectors.minBy 来计算流中的最大或最小值。

Optional<Dish> maxDish = Dish.menu.stream().collect(Collectors.maxBy(Comparator.comparing(Dish::getCalories)));
maxDish.ifPresent(System.out::println);
 
Optional<Dish> minDish = Dish.menu.stream().collect(Collectors.minBy(Comparator.comparing(Dish::getCalories)));
minDish.ifPresent(System.out::println);

6、去重

import static java.util.Comparator.comparingLong;
import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.toCollection;
 
// 根据id去重
// 根据id去重
List<Apple> unique = appleList.stream().collect(
collectingAndThen(toCollection(() -> new TreeSet<>(comparingLong(Apple::getId))), ArrayList::new)
);
System.out.println(unique);
// [Apple [id=1, name=苹果1, money=3.25, num=10], Apple [id=2, name=香蕉, money=2.89, num=30], Apple [id=3, name=荔枝, money=9.99, num=40]]

下表展示 Collectors 类的静态工厂方法。

原文链接： https://mp.weixin.qq.com/s?__biz=MzAxNjk4ODE4OQ==&mid=2247492204&idx=2&sn=3cffb3b417554e4e6592aeacba945402&chksm=9beec31eac994a08b4dd5991f5f750a086f73cc2f75e359e9124555e26eb0c9696ce94036495&mpshare=1&scene=24&srcid=082582rwLd6h5jEyWuRt7gUW&sharer_sharetime=1598325433286&sharer_shareid=c4a6148c7c8ffeaa9d3745b5ff23e33c#rd