介绍常用的StreamAPI。
本文中的例子GitHub地址：https://github.com/Lee875083146/java8/blob/master/src/main/java/com/nopainanymore/java8/Stream/StreamOperation.java

例子中使用的类：


public class  {

    private String name;

    private Integer age;

    private String sex;

    private Integer stuId;

    private Integer classId;

    public () {
    }

    public (String name, Integer age, String sex, Integer stuId, Integer classId) {
        this.name = name;
        this.age = age;
        this.sex = sex;
        this.stuId = stuId;
        this.classId = classId;
    }
}

函数原型为<R, A> R collect(Collector<? super T, A, R> collector);，使用Collector作为参数，一般使用Collectors类实现的各种收集操作，例如toList，toSet等。Collectors类中还实现了一些有意思的静态方法例如groupingBy,partitioningBy等。

filter()

函数原型为Stream<T> filter(Predicate<? super T> predicate);，其作用是返回满足predicate元素的Stream。
例子：

private static void filter(List<Student> studentList) {
    log.info("StreamOperation- filter- before :{}", gson.toJson(studentList));
    List<Student> afterFilter = studentList
            .stream()
            .filter(student -> MALE.equals(student.getSex()))
            .collect(Collectors.toList());
    log.info("StreamOperation- filter- after:{}", gson.toJson(afterFilter));
}

执行结果：

23:55:33.560 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- filter- before :[{"name":"a1","age":21,"sex":"女","stuId":1401},{"name":"a2","age":20,"sex":"男","stuId":1402},{"name":"a3","age":21,"sex":"女","stuId":1403},{"name":"a4","age":20,"sex":"男","stuId":1404}]
23:55:33.563 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- filter- after:[{"name":"a2","age":20,"sex":"男","stuId":1402},{"name":"a4","age":20,"sex":"男","stuId":1404}]

使用filter达到了将性别为女的学生过滤。

map()

函数原型为<R> Stream<R> map(Function<? super T, ? extends R> mapper);，其作用返回当前所有元素执行mapper之后的结果组成的Stream。
例子：

private static void map(List<Student> studentList) {
    log.info("StreamOperation- map- before:{}", gson.toJson(studentList));
    List<String> nameString = studentList
            .stream()
            .map(Student::getName)
            .collect(Collectors.toList());
    log.info("StreamOperation- map- after:{}", gson.toJson(nameString));
}

执行结果：

12:15:24.636 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- map- before:[{"name":"a1","age":21,"sex":"女","stuId":1401},{"name":"a2","age":20,"sex":"男","stuId":1402},{"name":"a3","age":21,"sex":"女","stuId":1403},{"name":"a4","age":20,"sex":"男","stuId":1404}]
12:15:24.641 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- map- after:["a1","a2","a3","a4"]

通过map获取到了所有学生的姓名

mapToInt(), mapToDouble(), mapToLong()

三者的函数原型分别为IntStream mapToInt(ToIntFunction<? super T> mapper);，DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);，LongStream mapToLong(ToLongFunction<? super T> mapper);，分别返回IntStream、DoubleStream、LongStream三种基本类型流，基本类型流可以使用特殊的lambda表达式，同时支持一些聚合方法，如：sum()，average()等。
例子：

private static void mapToInt(List<Student> studentList) {
    log.info("StreamOperation- mapToInt- before:{}", gson.toJson(studentList));
    Double avgAge = studentList
            .stream()
            .mapToInt(Student::getAge)
            .average()
            .orElse(0);
    log.info("StreamOperation- mapToInt- result:{}", avgAge);
}

执行结果：

12:25:25.829 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- mapToInt- before:[{"name":"a1","age":21,"sex":"女","stuId":1401},{"name":"a2","age":20,"sex":"男","stuId":1402},{"name":"a3","age":21,"sex":"女","stuId":1403},{"name":"a4","age":20,"sex":"男","stuId":1404}]
12:25:25.833 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- mapToInt- result:20.5

使用mapToInte()将Student流转化为年龄的IntStream，使用average() 获取OptionalDouble类型的结果，求出了学生的平均年龄.

flatMap()

函数原型为<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);，flatMap可以将一个stream中的每一个元素对象转换为另一个stream中的另一种元素对象，因此可以将stream中的每个对象改造成零，一个或多个。flatMap操作的返回流包含这些改造后的对象。

例子：

private static void flatMap(List<List<Student>> classList) {
    log.info("StreamOperation- flatMap- before:{}", gson.toJson(classList));
    List<Student> students = classList
            .stream()
            .flatMap(Collection::stream)
            .collect(Collectors.toList());
    log.info("StreamOperation- flatMap- after:{}", gson.toJson(students));
}

执行结果：

12:49:49.925 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- flatMap- before:[[{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":1},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":1},{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":1},{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":1}],[{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":2},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":2},{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":2},{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":2}]]
12:49:49.934 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- flatMap- after:[{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":1},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":1},{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":1},{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":1},{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":2},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":2},{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":2},{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":2}]

通过flatMap将两个List<Student>组成的classList转化为一个List<Student>。

peek()

函数原型Stream<T> peek(Consumer<? super T> action);，返回包含流中所有元素的流并为流中所以元素执行所提供的操作，该方法存在的目的是debug，可以用此方法

例子：

private static void peek(List<Student> studentList) {
    log.info("StreamOperation- peek- before:{}", gson.toJson(studentList));
    Long numOfMale = studentList
            .stream()
            .peek(student -> log.info("first peek Student:{}", gson.toJson(student)))
            .filter(student -> MALE.equals(student.getSex()))
            .peek(student -> log.info("second peek Student:{}", gson.toJson(student)))
            .count();
    log.info("StreamOperation- peek- numOfMale:{}", numOfMale);
}

结果：

19:45:58.814 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- peek- before:[{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":1},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":1},{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":1},{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":1}]
19:45:58.820 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - first peek Student:{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":1}
19:45:58.820 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - first peek Student:{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":1}
19:45:58.820 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - second peek Student:{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":1}
19:45:58.820 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - first peek Student:{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":1}
19:45:58.820 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - first peek Student:{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":1}
19:45:58.820 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - second peek Student:{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":1}
19:45:58.820 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- peek- numOfMale:2

由日志打印出的信息我们也可以看到流构成的函数管道，将所有中间操作全部融合后执行。
例子中也使用了count()来统计性别是男的学生人数。

distinct()

函数原型Stream<T> distinct();，返回由该流中equals决定的唯一的元素组成的流，即用来去重。

例子：

private static void distinct(List<Student> studentList) {
    log.info("StreamOperation- distinct- before :{}", gson.toJson(studentList));
    List<Integer> classIdList = studentList
            .stream()
            .map(Student::getClassId)
            .distinct()
            .collect(Collectors.toList());
    log.info("StreamOperation- distinct- classIdList :{}", gson.toJson(classIdList));
}

执行结果：

20:14:59.964 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- distinct- before :[{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":1},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":1},{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":1},{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":1},{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":2},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":2},{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":2},{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":2}]
20:14:59.974 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- distinct- classIdList :[1,2]

通过map方法获取classId的集合，通过distinct去重获得不重复的班级。

sorted(comparator)

函数原型Stream<T> sorted(Comparator<? super T> comparator);，返回一个由comparator决定的排序之后的流。

例子：

private static void sorted(List<Student> studentList) {
    log.info("StreamOperation- sorted- before:{}", gson.toJson(studentList));
    List<Student> sorted = studentList
            .stream()
            .sorted((o1, o2) -> o2.getStuId() - o1.getStuId())
            .collect(Collectors.toList());
    log.info("StreamOperation- sorted- :{}", gson.toJson(sorted));
}

执行结果：

20:38:52.395 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- sorted- before:[{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":1},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":1},{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":1},{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":1}]
20:38:52.405 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- sorted- :[{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":1},{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":1},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":1},{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":1}]

传入比较学号的lambda表达式，为了逆序用后一个学号减去前一个学号。

limit()

函数原型Stream<T> limit(long maxSize);，截取流中前maxSize组成的元素组成新的流。

private static void limit(List<Student> studentList) {
    log.info("StreamOperation- limit- before:{}", gson.toJson(studentList));
    List<Student> limit = studentList
            .stream()
            .limit(2)
            .collect(Collectors.toList());
    log.info("StreamOperation- limit- after:{}", gson.toJson(limit));
}

执行结果：

20:40:22.430 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- limit- before:[{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":1},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":1},{"name":"a3","age":21,"sex":"女","stuId":1403,"classId":1},{"name":"a4","age":20,"sex":"男","stuId":1404,"classId":1}]
20:40:22.433 [main] INFO com.nopainanymore.java8.Stream.StreamOperation - StreamOperation- limit- after:[{"name":"a1","age":21,"sex":"女","stuId":1401,"classId":1},{"name":"a2","age":20,"sex":"男","stuId":1402,"classId":1}]

截取了前两个学生。

其他API

Stream中还有很多API在用到时，直接看源码及源码注释即可。

参考资料

Java 8 中的 Streams API 详解
 Java 8 Stream 教程
 函数纯度
 Java 8 Stream探秘
 JavaLambdaInternals

java8如何使用stream filter() map() mapToInt(), mapToDouble(), mapToLong() flatMap() peek() distinct() sorted(comparator) limit() 其他API 参考资料

filter()

map()

mapToInt(), mapToDouble(), mapToLong()

flatMap()

peek()

distinct()

sorted(comparator)

limit()

其他API

参考资料

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