Lambda收集器示例

Collectors常用方法

工厂方法	返回类型	作用
toSet	Set	把流中所有项目收集到一个 Set，删除重复项
toList	List	收集到一个 List 集合中
toCollection	Collection	把流中所有项目收集到给定的供应源创建的集合menuStream.collect(toCollection(), ArrayList::new)
counting	Long	计算流中元素的个数
summingInt	Integer	对流中项目的一个整数属性求和
averagingInt	Double	计算流中项目 Integer 属性的平均值
summarizingInt	IntSummaryStatistics	收集关于流中项目 Integer 属性的统计值，例如最大、最小、 总和与平均值
joining	String	连接对流中每个项目调用 toString 方法所生成的字符串collect(joining(", "))
maxBy	Optional	一个包裹了流中按照给定比较器选出的最大元素的 Optional， 或如果流为空则为 Optional.empty()
minBy	Optional	一个包裹了流中按照给定比较器选出的最小元素的 Optional， 或如果流为空则为 Optional.empty()
reducing	归约操作产生的类型	从一个作为累加器的初始值开始，利用 BinaryOperator 与流 中的元素逐个结合，从而将流归约为单个值累加int totalCalories = menuStream.collect(reducing(0, Dish::getCalories, Integer::sum));
collectingAndThen	转换函数返回的类型	包裹另一个收集器，对其结果应用转换函数int howManyDishes = menuStream.collect(collectingAndThen(toList(), List::size))
groupingBy	Map<K, List>	根据项目的一个属性的值对流中的项目作问组，并将属性值作 为结果 Map 的键
partitioningBy	Map<Boolean,List>	根据对流中每个项目应用谓词的结果来对项目进行分区

示例

Apple 类

public class Apple {
    private Integer id;
    private String name;
    private Integer num;
    private String color;

    public Apple() {}
    
    public Apple(Integer id, String name, Integer num, String color) {
        this.id = id;
        this.name = name;
        this.num = num;
        this.color = color;
    }

    // 省略 setter、getter方法

    @Override
    public String toString() {
        return "Apple{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", num=" + num +
                ", color='" + color + '\'' +
                '}';
    }
}

数据

List<Apple> appleList = new ArrayList<>();

Apple apple1 =  new Apple(1,"苹果1",10, "red");
Apple apple2 = new Apple(1,"苹果2",20, "green");
Apple apple3 =  new Apple(2,"香蕉",30, "yellow");
Apple apple4 =  new Apple(3,"荔枝",40, "red");

appleList.add(apple1);
appleList.add(apple2);
appleList.add(apple3);
appleList.add(apple4);

常用收集器示例

• toList

// 获取数量大于25的Apple
List<Apple> collect = appleList.stream()
                .filter(apple -> apple.getNum() > 25)
                .collect(toList());
//
[Apple{id=2, name='香蕉', num=30, color='yellow'}, 
 Apple{id=3, name='荔枝', num=40, color='red'}]

• toSet

// 获取颜色信息
Set<String> collect = appleList.stream()
                .map(Apple::getColor)
                .collect(toSet());
// [red, green, yellow]

• toCollection

List<Apple> collect = appleList.stream()
                .filter(x->x.getNum()>18)
                .collect(toCollection(ArrayList::new));
/-----------------------------/
[Apple{id=1, name='苹果2', num=20, color='green'}, 
 Apple{id=2, name='香蕉', num=30, color='yellow'}, 
 Apple{id=3, name='荔枝', num=40, color='red'}]

• counting：统计数量

Long aLong = appleList.stream()
                .filter(x -> x.getNum() > 18)
                .collect(counting());
// 另一种写法
Long aLong = appleList.stream()
                .filter(x -> x.getNum() > 18)
                .count();
// 3

• summingInt：对一个属性求和

Integer integer = appleList.stream()
                .collect(summingInt(p -> p.getNum()));
// 另一种写法
Integer integer = appleList.stream()
                .mapToInt(Apple::getNum).sum();
// 100

• averagingInt：求平均值

Double aDouble = appleList.stream().collect(averagingInt(Apple::getNum));
// 25.0

• summarizingInt:求最大、最小、平均值等

IntSummaryStatistics collect1 = appleList.stream().collect(summarizingInt(Apple::getNum));
// IntSummaryStatistics{count=4, sum=100, min=10, average=25.000000, max=40}

• joining：对流中元素调用toString，拼接成字符串

String s = appleList.stream().map(Apple::getName).collect(joining("--"));
// 苹果1--苹果2--香蕉--荔枝

• maxBy | minBy : 求最大最小元素

Optional<Apple> collect = appleList.stream()
                .collect(maxBy(comparing(Apple::getNum)));
// 另一种写法
Optional<Apple> collect = appleList.stream()
                .max(comparing(Apple::getNum))
// Apple{id=3, name='荔枝', num=40, color='red'}

• reducing：规约操作

Integer integer = appleList.stream()
                .collect(reducing(0, Apple::getNum, Integer::sum));
// 另一种写法
Optional<Integer> reduce = appleList.stream()
                .map(Apple::getNum)
                .reduce(Integer::sum);
// 100

• groupingBy | partitioningBy ：分组操作

Map<Integer, List<Apple>> groupBy = appleList.stream()
                .collect(Collectors.groupingBy(Apple::getId));
// {
	1=[Apple{id=1, name='苹果1', num=10, color='red'}, 
     Apple{id=1, name='苹果2', num=20, color='green'}], 
  2=[Apple{id=2, name='香蕉', num=30, color='yellow'}], 
	3=[Apple{id=3, name='荔枝', num=40, color='red'}]}

// partitioningBy：条件分组
Map<Boolean, List<Apple>> collect1 = appleList.stream()
  					.collect(partitioningBy(x -> x.getNum() > 24));
// {
	false=[Apple{id=1, name='苹果1', num=10, color='red'}, 
         Apple{id=1, name='苹果2', num=20, color='green'}],
  true=[Apple{id=2, name='香蕉', num=30, color='yellow'}, 
        Apple{id=3, name='荔枝', num=40, color='red'}]}