Lambda收集器示例

Java

万世威

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 2018/11/06   Share

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

1 2	Double aDouble = appleList.stream().collect(averagingInt(Apple::getNum)); // 25.0

summarizingInt:求最大、最小、平均值等

1 2	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

1 2	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'}]}

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CATALOG

1. Lambda收集器示例
1. 1.1. Collectors常用方法
2. 1.2. 示例

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