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一、收集Stream流到集合和指定集和中

Stream 流提供了一个 collect() 方法，可以收集流中的数据到【集合】或者【数组】中去。

//1.收集数据到list集合中
stream.collect(Collectors.toList())
//2.收集数据到set集合中
stream.collect(Collectors.toSet())
//3.收集数据到指定的集合中
stream.collect(Collectors.toCollection(Supplier<C> collectionFactory))

1、示例

    public void test2() {//Stream 流Stream<String> stream = Stream.of("aaa", "bbb", "ccc", "bbb");//收集流中的数据到集合中//1.收集流中的数据到 listList<String> list = stream.collect(Collectors.toList());System.out.println(list);//Stream 流stream = Stream.of("aaa", "bbb", "ccc", "bbb");//2.收集流中的数据到 setSet<String> collect = stream.collect(Collectors.toSet());System.out.println(collect);//Stream 流stream = Stream.of("aaa", "bbb", "ccc", "bbb");//3.收集流中的数据(ArrayList)(不收集到list,set等集合中,而是)收集到指定的集合中ArrayList<String> arrayList = stream.collect(Collectors.toCollection(ArrayList::new));System.out.println(arrayList);//Stream 流stream = Stream.of("aaa", "bbb", "ccc", "bbb");//4.收集流中的数据到 HashSetHashSet<String> hashSet = stream.collect(Collectors.toCollection(HashSet::new));System.out.println(hashSet);//Stream 流stream = Stream.of("aaa", "bbb", "ccc", "bbb");//distinct()不重复List<String> distinctCollect = stream.distinct().collect(Collectors.toList());System.out.println(distinctCollect);}

2、结果

    public void test3() {List<People> peopleList = new ArrayList<People>();People people1 = new People("张三", 18);People people2 = new People("王五", 19);peopleList.add(people1);peopleList.add(people2);List<People> collect = peopleList.stream().collect(Collectors.toCollection(ArrayList<People>::new));System.out.println(collect);}

二、收集 Stream 流中的数据到数组中

1、示例

    void test2() {List<People> peopleList = new ArrayList<People>();People people1 = new People(1, "小王", 1);People people2 = new People(1, "小李", 2);peopleList.add(people1);peopleList.add(people2);//1.使用无参,收集到数组,返回值为 Object[](Object类型将不好操作)Object[] objects = peopleList.stream().toArray();for (Object o : objects) {//此处无法使用.length() 等方法System.out.println("data:" + o);}//2.使用有参,可以指定将数据收集到指定类型数组,方便后续对数组的操作People[] people = peopleList.stream().toArray(People[]::new);for (People str : people) {System.out.println(str.getName());}}

2、结果

三、Stream流中数据聚合/分组/分区/拼接操作

//最大值
Collectors.maxBy();
//最小值
Collectors.minBy();
//总和
Collectors.summingInt();/Collectors.summingDouble();/Collectors.summingLong();
//平均值
Collectors.averagingInt();/Collectors.averagingDouble();/Collectors.averagingLong();
//总个数
Collectors.counting();

1、聚合操作

    @Testvoid test3() {List<People> peopleList = new ArrayList<>();peopleList.add(new People(1, "小王", 1));peopleList.add(new People(3, "小李", 3));peopleList.add(new People(2, "小张", 2));peopleList.add(new People(4, "小皇", 4));//取最大值Optional<People> maxCollect1 = peopleList.stream().collect(Collectors.maxBy(new Comparator<People>() {@Overridepublic int compare(People o1, People o2) {//比大小前值比后值大则返回非负数if (o1.getJgid() > o2.getJgid()) {return 0;} else {return -1;}
//                        return o1.getJgid() - o2.getJgid();}}));//或者使用lambda表达式取最大值Optional<People> maxCollect2 = peopleList.stream().collect(Collectors.maxBy((s1, s2) -> s1.getJgid() - s2.getJgid()));//或者使用lambda表达式取最小值Optional<People> minCollect = peopleList.stream().collect(Collectors.maxBy((s1, s2) -> s1.getJgid() - s2.getJgid()));//取jgid总和Integer sumCollect1 = peopleList.stream().collect(Collectors.summingInt(new ToIntFunction<People>() {@Overridepublic int applyAsInt(People value) {return value.getJgid();}}));//或者使用lambda表达式取jgid总和Integer sumCollect2 = peopleList.stream().collect(Collectors.summingInt(People::getJgid));Double avgScore1 = peopleList.stream().collect(Collectors.averagingInt(new ToIntFunction<People>() {@Overridepublic int applyAsInt(People value) {return value.getJgid();}}));//或者使用lambda表达式取jgid平均值Double avgScore2 = peopleList.stream().collect(Collectors.averagingInt(People::getJgid));//(聚合)统计数量Long count = peopleList.stream().collect(Collectors.counting());System.out.println("最大值:" + maxCollect1.get());System.out.println("最大值:" + maxCollect2.get());System.out.println("最小值:" + minCollect.get());System.out.println("jgid总和:" + sumCollect1);System.out.println("jgid总和:" + sumCollect2);System.out.println("分数平均值:"+avgScore1);System.out.println("分数平均值:"+avgScore2);System.out.println("数量为:"+count);}

2、分组操作

当我们使用 Stream 流处理数据后，可以根据某个属性来将数据进行分组。

//接收一个 Function 参数
groupingBy(Function<? super T, ? extends K> classifier)

    @Testvoid test4() {List<People> peopleList = new ArrayList<>();peopleList.add(new People(1, "小王", 1));peopleList.add(new People(3, "小李", 2));peopleList.add(new People(2, "小张", 2));peopleList.add(new People(4, "小皇", 1));//根据年龄分组Map<Integer, List<People>> jgidMap1 = peopleList.stream().collect(Collectors.groupingBy(new Function<People, Integer>() {@Overridepublic Integer apply(People people) {return people.getJgid();}}));//或者使用lambda表达式Map<Integer, List<People>> jgidMap2 = peopleList.stream().collect(Collectors.groupingBy(People::getJgid));System.out.println("根据年龄分组");jgidMap1.forEach((key, value) -> {System.out.println(key + "---->" + value);});//jgid奇数一组偶数一组Map<String, List<People>> jgidMap3 = peopleList.stream().collect(Collectors.groupingBy(s -> {if (s.getJgid() % 2 == 0) {return "jgid为偶数";} else {return "jgid为奇数";}}));System.out.println("jgid奇数一组偶数一组");jgidMap3.forEach((key, value) -> {System.out.println(key + "---->" + value);});//根据jgid分组且规定为最大一组(规约：reducing)Map<Integer, Optional<People>> jgidMap4 = peopleList.stream().collect(Collectors.groupingBy(People::getJgid,Collectors.reducing(BinaryOperator.maxBy(Comparator.comparing(People::getJgid)))));System.out.println("jgid分组后最大值");jgidMap4.forEach((key, value) -> {System.out.println(key + "---->" + value);});}
}

3、多级分组操作

当我们使用 Stream 流处理数据后，可以根据某个属性来将数据进行分组。

//接收两个参数: 1.Function 参数  2.Collector多级分组
groupingBy(Function<? super T, ? extends K> classifier,Collector<? super T, A, D> downstream) 

    @Testvoid test5() {List<People> peopleList = new ArrayList<>();peopleList.add(new People(1, "小王", 1));peopleList.add(new People(2, "小李", 2));peopleList.add(new People(3, "小张", 2));peopleList.add(new People(4, "小皇", 1));Map<Integer, Map<String, List<People>>> collect = peopleList.stream().collect(Collectors.groupingBy(People::getJgid,Collectors.groupingBy(people -> {if (people.getId() % 2 == 0) {return "id是偶数";} else {return "id是奇数";}})));collect.forEach((key1, value1) -> {System.out.println("机构id" + "---->" + key1);value1.forEach((key2, value2) -> {System.out.println("\t" + key2 + "---->" + value2);});});}

4、分区操作

分区操作，通过使用 Collectors.partitioningBy() ，根据返回值是否为 true，把集合分为两个列表，一个 true 列表，一个 false 列表。
分组和分区的区别就在：分组可以有多个组。分区只会有两个区( true 和 false)

//1.一个参数
partitioningBy(Predicate<? super T> predicate)//2.两个参数(多级分区)
partitioningBy(Predicate<? super T> predicate, Collector<? super T, A, D> downstream)

    @Testvoid test6() {List<People> peopleList = new ArrayList<>();peopleList.add(new People(1, "小王", 1));peopleList.add(new People(2, "小李", 2));peopleList.add(new People(3, "小张", 2));peopleList.add(new People(4, "小皇", 1));Map<Boolean, List<People>> collect = peopleList.stream().collect(Collectors.partitioningBy(s -> {return s.getJgid() % 2 == 0;}));collect.forEach((key, value) -> {System.out.println(key + "---->" + value);});}

5、拼接操作

Collectors.joining() 会根据指定的连接符，将所有元素连接成一个字符串。

//无参数--等价于 joining("");
joining()
//一个参数
joining(CharSequence delimiter)
//三个参数(连接符+前缀+后缀)
joining(CharSequence delimiter, CharSequence prefix,CharSequence suffix)

    @Testvoid test7() {List<People> peopleList = new ArrayList<>();peopleList.add(new People(1, "小王", 1));peopleList.add(new People(2, "小李", 2));peopleList.add(new People(3, "小张", 2));peopleList.add(new People(4, "小皇", 1));String collect1 = peopleList.stream().map(People::getName).collect(Collectors.joining());System.out.println(collect1);String collect2 = peopleList.stream().map(People::getName).collect(Collectors.joining(","));System.out.println(collect2);String collect3 = peopleList.stream().map(People::getName).collect(Collectors.joining("_", "开始<", ">结束"));System.out.println(collect3);}