Stream.Collect 收集器的使用案例

public static void main(String[] args) {
    Person p1 = new Person("zhangsan",20);Person p2 = new Person("lisi",30);Person p3 = new Person("wangwu",40);Person p4 = new Person("赵六",40);List<Person> list = Arrays.asList(p1, p2, p3, p4);System.out.println("获取流中所有元素");List<Person> collect = list.stream().collect(Collectors.toList());collect.forEach(System.out::println);//收集统计数据的对象 例如 最大最小平均总和  这个对象只返回数据 不返回对象 例如最小：返回20  而不是这个personIntSummaryStatistics intSummaryStatistics = list.stream().collect(Collectors.summarizingInt(Person::getAge));System.out.println("-----------------------------");System.out.println("统计流中元素个数");System.out.println("方式一stream.count() : " + list.stream().count());System.out.println("方式二stream.collect(Collectors.counting()): " + list.stream().collect(Collectors.counting()));System.out.println("方式三：" + intSummaryStatistics.getCount());System.out.println("-------------------");System.out.println("获取流中 年龄最小的元素");System.out.print("获取最小方式一：");list.stream().collect(Collectors.minBy(Comparator.comparingInt(Person::getAge))).ifPresent(System.out::println);System.out.println("获取最小方式二：" + intSummaryStatistics.getMin());System.out.println("-------------------");System.out.println("获取流中 年龄最大的元素");System.out.print("获取最大方式一：");list.stream().collect(Collectors.maxBy(Comparator.comparingInt(Person::getAge))).ifPresent(System.out::println);System.out.println("获取最大方式二：" + intSummaryStatistics.getMax());System.out.println("-------------------");//年龄平均数Double average = list.stream().collect(Collectors.averagingInt(Person::getAge));System.out.println("平均年龄方式一：" + average);System.out.println("平均年龄方式二：" + intSummaryStatistics.getAverage());System.out.println("-------------------");System.out.println("年龄总和方式一：" + intSummaryStatistics.getSum());Integer sum = list.stream().collect(Collectors.summingInt(Person::getAge));System.out.println("年龄总和方式二：" + sum);System.out.println("-------------------");System.out.println("拼接所有人的性名");String personNames = list.stream().map(Person::getName).collect(Collectors.joining(" "));System.out.println("所有人的性名：" + personNames);System.out.println("-------------------");System.out.println("一级分组");System.out.println("根据年龄分组");Map<Integer, List<Person>> map = list.stream().collect(Collectors.groupingBy(Person::getAge));System.out.println(map);System.out.println("二级分组 先根据年龄分组 在根据xing'm'z");Map<Integer, Map<String, List<Person>>> mapMap = list.stream().collect(Collectors.groupingBy(Person::getAge, Collectors.groupingBy(Person::getName)));System.out.println(mapMap);System.out.println("-------------------");System.out.println("分区 <30 , >=30");Map<Boolean, List<Person>> listMap = list.stream().collect(Collectors.partitioningBy(person -> person.getAge() < 30));System.out.println(listMap);System.out.println("-------------------");System.out.println("分区 <30 , >=30 后 >=30 这个分区再次分区 =30 和 >30");Map<Boolean, Map<Boolean, List<Person>>> map1 = list.stream().collect(Collectors.partitioningBy(person -> person.getAge() >= 30,Collectors.partitioningBy(person -> person.getAge() == 30)));System.out.println(map1);System.out.println("--------------");System.out.println("分区后统计数量");Map<Boolean, Long> map2 = list.stream().collect(Collectors.partitioningBy(person -> person.getAge() < 30, Collectors.counting()));System.out.println(map2);}