Java8 Stream流的常用方法和讲解

1 Stream概述

Java 8 是一个非常成功的版本,这个版本新增的 Stream,配合同版本出现的 Lambda ,给我们操作集合(Collection)提供了极大的便利。

那么什么是 Stream

Stream将要处理的元素集合看作一种流,在流的过程中,借助 Stream API对流中的元素进行操作,比如:筛选、排序、聚合等。

Stream可以由数组或集合创建,对流的操作分为两种:

  1. 中间操作,每次返回一个新的流,可以有多个。

  2. 终端操作,每个流只能进行一次终端操作,终端操作结束后流无法再次使用。终端操作会产生一个新的集合或值。

另外, Stream有几个特性:

案例一:英文字符串数组的元素全部改为大写。整数数组每个元素+3。

public class StreamTest {
    public static void main(String[] args) {
        String[] strArr = { "abcd", "bcdd", "defde", "fTr" };
        List strList = Arrays.stream(strArr).map(String::toUpperCase).collect(Collectors.toList());
​
        List intList = Arrays.asList(1, 3, 5, 7, 9, 11);
        List intListNew = intList.stream().map(x -> x + 3).collect(Collectors.toList());
​
        System.out.println("每个元素大写:" + strList);
        System.out.println("每个元素+3:" + intListNew);
    }
}

案例二:将员工的薪资全部增加1000。

public class StreamTest {
    public static void main(String[] args) {
        List personList = new ArrayList();
        personList.add(new Person("Tom", 8900, 23, "male", "New York"));
        personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
        personList.add(new Person("Anni", 8200, 24, "female", "New York"));
        personList.add(new Person("Owen", 9500, 25, "male", "New York"));
        personList.add(new Person("Alisa", 7900, 26, "female", "New York"));
​
        List personListNew = personList.stream().map(person -> {
            Person personNew = new Person(person.getName(), 0, 0, null, null);
            personNew.setSalary(person.getSalary() + 10000);
            return personNew;
        }).collect(Collectors.toList());
        System.out.println("一次改动前:" + personList.get(0).getName() + "-->" + personList.get(0).getSalary());
        System.out.println("一次改动后:" + personListNew.get(0).getName() + "-->" + personListNew.get(0).getSalary());
​
        List personListNew2 = personList.stream().map(person -> {
            person.setSalary(person.getSalary() + 10000);
            return person;
        }).collect(Collectors.toList());
        System.out.println("二次改动前:" + personList.get(0).getName() + "-->" + personListNew.get(0).getSalary());
        System.out.println("二次改动后:" + personListNew2.get(0).getName() + "-->" + personListNew.get(0).getSalary());
    }
}

案例三:将两个字符数组合并成一个新的字符数组。

public class StreamTest {
    public static void main(String[] args) {
        List list = Arrays.asList("m,k,l,a", "1,3,5,7");
        List listNew = list.stream().flatMap(s -> {
​
            String[] split = s.split(",");
            Stream s2 = Arrays.stream(split);
            return s2;
        }).collect(Collectors.toList());
​
        System.out.println("处理前的集合:" + list);
        System.out.println("处理后的集合:" + listNew);
    }
}

3.5 归约(reduce)

归约,也称缩减,顾名思义,是把一个流缩减成一个值,能实现对集合求和、求乘积和求最值操作。

案例一:求 Integer 集合的元素之和、乘积和最大值。

public class StreamTest {
    public static void main(String[] args) {
        List list = Arrays.asList(1, 3, 2, 8, 11, 4);
​
        Optional sum = list.stream().reduce((x, y) -> x + y);
​
        Optional sum2 = list.stream().reduce(Integer::sum);
​
        Integer sum3 = list.stream().reduce(0, Integer::sum);
​
        Optional product = list.stream().reduce((x, y) -> x * y);
​
        Optional max = list.stream().reduce((x, y) -> x > y ? x : y);
​
        Integer max2 = list.stream().reduce(1, Integer::max);
​
        System.out.println("list求和:" + sum.get() + "," + sum2.get() + "," + sum3);
        System.out.println("list求积:" + product.get());
        System.out.println("list求和:" + max.get() + "," + max2);
    }
}

案例二:求所有员工的工资之和和最高工资。

public class StreamTest {
    public static void main(String[] args) {
        List personList = new ArrayList();
        personList.add(new Person("Tom", 8900, 23, "male", "New York"));
        personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
        personList.add(new Person("Anni", 8200, 24, "female", "New York"));
        personList.add(new Person("Owen", 9500, 25, "male", "New York"));
        personList.add(new Person("Alisa", 7900, 26, "female", "New York"));
​
        Optional sumSalary = personList.stream().map(Person::getSalary).reduce(Integer::sum);
​
        Integer sumSalary2 = personList.stream().reduce(0, (sum, p) -> sum += p.getSalary(),
                (sum1, sum2) -> sum1 + sum2);
​
        Integer sumSalary3 = personList.stream().reduce(0, (sum, p) -> sum += p.getSalary(), Integer::sum);
​
        Integer maxSalary = personList.stream().reduce(0, (max, p) -> max > p.getSalary() ? max : p.getSalary(),
                Integer::max);
​
        Integer maxSalary2 = personList.stream().reduce(0, (max, p) -> max > p.getSalary() ? max : p.getSalary(),
                (max1, max2) -> max1 > max2 ? max1 : max2);
​
        System.out.println("工资之和:" + sumSalary.get() + "," + sumSalary2 + "," + sumSalary3);
        System.out.println("最高工资:" + maxSalary + "," + maxSalary2);
    }
}

3.6 收集(collect)

collect,收集,可以说是内容最繁多、功能最丰富的部分了。从字面上去理解,就是把一个流收集起来,最终可以是收集成一个值也可以收集成一个新的集合。

collect主要依赖 java.util.stream.Collectors类内置的静态方法。

3.6.1 归集(toList/toSet/toMap)

因为流不存储数据,那么在流中的数据完成处理后,需要将流中的数据重新归集到新的集合里。 toListtoSettoMap比较常用,另外还有 toCollectiontoConcurrentMap等复杂一些的用法。

下面用一个案例演示 toListtoSettoMap

public class StreamTest {
    public static void main(String[] args) {
        List list = Arrays.asList(1, 6, 3, 4, 6, 7, 9, 6, 20);
        List listNew = list.stream().filter(x -> x % 2 == 0).collect(Collectors.toList());
        Set set = list.stream().filter(x -> x % 2 == 0).collect(Collectors.toSet());
​
        List personList = new ArrayList();
        personList.add(new Person("Tom", 8900, 23, "male", "New York"));
        personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
        personList.add(new Person("Anni", 8200, 24, "female", "New York"));
​
        Map map = personList.stream().filter(p -> p.getSalary() > 8000)
                .collect(Collectors.toMap(Person::getName, p -> p));
        System.out.println("toList:" + listNew);
        System.out.println("toSet:" + set);
        System.out.println("toMap:" + map);
    }
}

运行结果:

3.6.2 统计(count/averaging)

Collectors提供了一系列用于数据统计的静态方法:

案例:统计员工人数、平均工资、工资总额、最高工资。

public class StreamTest {
    public static void main(String[] args) {
        List personList = new ArrayList();
        personList.add(new Person("Tom", 8900, 23, "male", "New York"));
        personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
​
        Long count = personList.stream().collect(Collectors.counting());
​
        Double average = personList.stream().collect(Collectors.averagingDouble(Person::getSalary));
​
        Optional max = personList.stream().map(Person::getSalary).collect(Collectors.maxBy(Integer::compare));
​
        Integer sum = personList.stream().collect(Collectors.summingInt(Person::getSalary));
​
        DoubleSummaryStatistics collect = personList.stream().collect(Collectors.summarizingDouble(Person::getSalary));
​
        System.out.println("员工总数:" + count);
        System.out.println("员工平均工资:" + average);
        System.out.println("员工工资总和:" + sum);
        System.out.println("员工工资所有统计:" + collect);
    }
}

3.6.3 分组(partitioningBy/groupingBy)

案例:将员工按薪资是否高于8000分为两部分;将员工按性别和地区分组

public class StreamTest {
    public static void main(String[] args) {
        List personList = new ArrayList();
        personList.add(new Person("Tom", 8900, "male", "New York"));
        personList.add(new Person("Jack", 7000, "male", "Washington"));
        personList.add(new Person("Lily", 7800, "female", "Washington"));
        personList.add(new Person("Anni", 8200, "female", "New York"));
        personList.add(new Person("Owen", 9500, "male", "New York"));
        personList.add(new Person("Alisa", 7900, "female", "New York"));
​
        Map> part = personList.stream().collect(Collectors.partitioningBy(x -> x.getSalary() > 8000));
​
        Map> group = personList.stream().collect(Collectors.groupingBy(Person::getSex));
​
        Map>> group2 = personList.stream().collect(Collectors.groupingBy(Person::getSex, Collectors.groupingBy(Person::getArea)));
        System.out.println("员工按薪资是否大于8000分组情况:" + part);
        System.out.println("员工按性别分组情况:" + group);
        System.out.println("员工按性别、地区:" + group2);
    }
}

3.6.4 接合(joining)

joining可以将stream中的元素用特定的连接符(没有的话,则直接连接)连接成一个字符串。

public class StreamTest {
    public static void main(String[] args) {
        List personList = new ArrayList();
        personList.add(new Person("Tom", 8900, 23, "male", "New York"));
        personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
​
        String names = personList.stream().map(p -> p.getName()).collect(Collectors.joining(","));
        System.out.println("所有员工的姓名:" + names);
        List list = Arrays.asList("A", "B", "C");
        String string = list.stream().collect(Collectors.joining("-"));
        System.out.println("拼接后的字符串:" + string);
    }
}

3.6.5 归约(reducing)

Collectors类提供的 reducing方法,相比于 stream本身的 reduce方法,增加了对自定义归约的支持。

public class StreamTest {
    public static void main(String[] args) {
        List personList = new ArrayList();
        personList.add(new Person("Tom", 8900, 23, "male", "New York"));
        personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
​
        Integer sum = personList.stream().collect(Collectors.reducing(0, Person::getSalary, (i, j) -> (i + j - 5000)));
        System.out.println("员工扣税薪资总和:" + sum);
​
        Optional sum2 = personList.stream().map(Person::getSalary).reduce(Integer::sum);
        System.out.println("员工薪资总和:" + sum2.get());
    }
}

3.7 排序(sorted)

sorted,中间操作。有两种排序:

案例:将员工按工资由高到低(工资一样则按年龄由大到小)排序

public class StreamTest {
    public static void main(String[] args) {
        List personList = new ArrayList();
​
        personList.add(new Person("Sherry", 9000, 24, "female", "New York"));
        personList.add(new Person("Tom", 8900, 22, "male", "Washington"));
        personList.add(new Person("Jack", 9000, 25, "male", "Washington"));
        personList.add(new Person("Lily", 8800, 26, "male", "New York"));
        personList.add(new Person("Alisa", 9000, 26, "female", "New York"));
​
        List newList = personList.stream().sorted(Comparator.comparing(Person::getSalary)).map(Person::getName)
                .collect(Collectors.toList());
​
        List newList2 = personList.stream().sorted(Comparator.comparing(Person::getSalary).reversed())
                .map(Person::getName).collect(Collectors.toList());
​
        List newList3 = personList.stream()
                .sorted(Comparator.comparing(Person::getSalary).thenComparing(Person::getAge)).map(Person::getName)
                .collect(Collectors.toList());
​
        List newList4 = personList.stream().sorted((p1, p2) -> {
            if (p1.getSalary() == p2.getSalary()) {
                return p2.getAge() - p1.getAge();
            } else {
                return p2.getSalary() - p1.getSalary();
            }
        }).map(Person::getName).collect(Collectors.toList());
​
        System.out.println("按工资升序排序:" + newList);
        System.out.println("按工资降序排序:" + newList2);
        System.out.println("先按工资再按年龄升序排序:" + newList3);
        System.out.println("先按工资再按年龄自定义降序排序:" + newList4);
    }
}

3.8 提取/组合

流也可以进行合并、去重、限制、跳过等操作。

public class StreamTest {
    public static void main(String[] args) {
        String[] arr1 = { "a", "b", "c", "d" };
        String[] arr2 = { "d", "e", "f", "g" };
​
        Stream stream1 = Stream.of(arr1);
        Stream stream2 = Stream.of(arr2);
​
        List newList = Stream.concat(stream1, stream2).distinct().collect(Collectors.toList());
​
        List collect = Stream.iterate(1, x -> x + 2).limit(10).collect(Collectors.toList());
​
        List collect2 = Stream.iterate(1, x -> x + 2).skip(1).limit(5).collect(Collectors.toList());
​
        System.out.println("流合并:" + newList);
        System.out.println("limit:" + collect);
        System.out.println("skip:" + collect2);
    }
}

  1. stream不存储数据,而是按照特定的规则对数据进行计算,一般会输出结果。

  2. stream不会改变数据源,通常情况下会产生一个新的集合或一个值。

  3. stream具有延迟执行特性,只有调用终端操作时,中间操作才会执行。

  4. 2 Stream的创建

    Stream可以通过集合数组创建。

    1、通过 java.util.Collection.stream() 方法用集合创建流

    List list = Arrays.asList("a", "b", "c");
    ​
    Stream stream = list.stream();
    ​
    Stream parallelStream = list.parallelStream();

    2、使用 java.util.Arrays.stream(T[] array)方法用数组创建流

    int[] array={1,3,5,6,8};
    IntStream stream = Arrays.stream(array);

    3、使用 Stream的静态方法: of()、iterate()、generate()

    Stream stream = Stream.of(1, 2, 3, 4, 5, 6);
    ​
    Stream stream2 = Stream.iterate(0, (x) -> x + 3).limit(4);
    stream2.forEach(System.out::println);
    ​
    Stream stream3 = Stream.generate(Math::random).limit(3);
    stream3.forEach(System.out::println);

    streamparallelStream 的简单区分: stream是顺序流,由主线程按顺序对流执行操作,而 parallelStream是并行流,内部以多线程并行执行的方式对流进行操作,但前提是流中的数据处理没有顺序要求。例如筛选集合中的奇数,两者的处理不同之处: 如果流中的数据量足够大,并行流可以加快处速度。 除了直接创建并行流,还可以通过 parallel()把顺序流转换成并行流:

    Optional findFirst = list.stream().parallel().filter(x->x>6).findFirst();

    3 Stream的使用

    在使用stream之前,先理解一个概念: Optional

    Optional类是一个可以为 null的容器对象。如果值存在则 isPresent()方法会返回 true,调用 get()方法会返回该对象。 更详细说明请见:菜鸟教程Java 8 Optional类

    接下来,大批代码向你袭来!我将用20个案例将Stream的使用整得明明白白,只要跟着敲一遍代码,就能很好地掌握。

    ; 案例使用的员工类

    这是后面案例中使用的员工类:

    List personList = new ArrayList();
    personList.add(new Person("Tom", 8900, "male", "New York"));
    personList.add(new Person("Jack", 7000, "male", "Washington"));
    personList.add(new Person("Lily", 7800, "female", "Washington"));
    personList.add(new Person("Anni", 8200, "female", "New York"));
    personList.add(new Person("Owen", 9500, "male", "New York"));
    personList.add(new Person("Alisa", 7900, "female", "New York"));
    ​
    class Person {
        private String name;
        private int salary;
        private int age;
        private String sex;
        private String area;
    ​
        public Person(String name, int salary, int age,String sex,String area) {
            this.name = name;
            this.salary = salary;
            this.age = age;
            this.sex = sex;
            this.area = area;
        }
    ​
    }

    3.1 遍历/匹配(foreach/find/match)

    Stream也是支持类似集合的遍历和匹配元素的,只是 Stream中的元素是以 Optional类型存在的。 Stream的遍历、匹配非常简单。

    ​
    public class StreamTest {
        public static void main(String[] args) {
            List list = Arrays.asList(7, 6, 9, 3, 8, 2, 1);
    ​
            list.stream().filter(x -> x > 6).forEach(System.out::println);
    ​
            Optional findFirst = list.stream().filter(x -> x > 6).findFirst();
    ​
            Optional findAny = list.parallelStream().filter(x -> x > 6).findAny();
    ​
            boolean anyMatch = list.stream().anyMatch(x -> x < 6);
            System.out.println("匹配第一个值:" + findFirst.get());
            System.out.println("匹配任意一个值:" + findAny.get());
            System.out.println("是否存在大于6的值:" + anyMatch);
        }
    }

    3.2 筛选(filter)

    筛选,是按照一定的规则校验流中的元素,将符合条件的元素提取到新的流中的操作。

    案例一:筛选出 Integer 集合中大于7的元素,并打印出来

    public class StreamTest {
        public static void main(String[] args) {
            List list = Arrays.asList(6, 7, 3, 8, 1, 2, 9);
            Stream stream = list.stream();
            stream.filter(x -> x > 7).forEach(System.out::println);
        }
    }

    案例二: 筛选员工中工资高于8000的人,并形成新的集合。 形成新集合依赖 collect(收集),后文有详细介绍。

    public class StreamTest {
        public static void main(String[] args) {
            List personList = new ArrayList();
            personList.add(new Person("Tom", 8900, 23, "male", "New York"));
            personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
            personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
            personList.add(new Person("Anni", 8200, 24, "female", "New York"));
            personList.add(new Person("Owen", 9500, 25, "male", "New York"));
            personList.add(new Person("Alisa", 7900, 26, "female", "New York"));
    ​
            List fiterList = personList.stream().filter(x -> x.getSalary() > 8000).map(Person::getName)
                    .collect(Collectors.toList());
            System.out.print("高于8000的员工姓名:" + fiterList);
        }
    }

    3.3 聚合(max/min/count)

    maxmincount这些字眼你一定不陌生,没错,在mysql中我们常用它们进行数据统计。Java stream中也引入了这些概念和用法,极大地方便了我们对集合、数组的数据统计工作。

    案例一:获取 String 集合中最长的元素。

    public class StreamTest {
        public static void main(String[] args) {
            List list = Arrays.asList("adnm", "admmt", "pot", "xbangd", "weoujgsd");
    ​
            Optional max = list.stream().max(Comparator.comparing(String::length));
            System.out.println("最长的字符串:" + max.get());
        }
    }

    案例二:获取 Integer 集合中的最大值。

    public class StreamTest {
        public static void main(String[] args) {
            List list = Arrays.asList(7, 6, 9, 4, 11, 6);
    ​
            Optional max = list.stream().max(Integer::compareTo);
    ​
            Optional max2 = list.stream().max(new Comparator() {
                @Override
                public int compare(Integer o1, Integer o2) {
                    return o1.compareTo(o2);
                }
            });
            System.out.println("自然排序的最大值:" + max.get());
            System.out.println("自定义排序的最大值:" + max2.get());
        }
    }

    案例三:获取员工工资最高的人。

    public class StreamTest {
        public static void main(String[] args) {
            List personList = new ArrayList();
            personList.add(new Person("Tom", 8900, 23, "male", "New York"));
            personList.add(new Person("Jack", 7000, 25, "male", "Washington"));
            personList.add(new Person("Lily", 7800, 21, "female", "Washington"));
            personList.add(new Person("Anni", 8200, 24, "female", "New York"));
            personList.add(new Person("Owen", 9500, 25, "male", "New York"));
            personList.add(new Person("Alisa", 7900, 26, "female", "New York"));
    ​
            Optional max = personList.stream().max(Comparator.comparingInt(Person::getSalary));
            System.out.println("员工工资最大值:" + max.get().getSalary());
        }
    }

    案例四:计算 Integer 集合中大于6的元素的个数。

    import java.util.Arrays;
    import java.util.List;
    ​
    public class StreamTest {
        public static void main(String[] args) {
            List list = Arrays.asList(7, 6, 4, 8, 2, 11, 9);
    ​
            long count = list.stream().filter(x -> x > 6).count();
            System.out.println("list中大于6的元素个数:" + count);
        }
    }

    3.4 映射(map/flatMap)

    映射,可以将一个流的元素按照一定的映射规则映射到另一个流中。分为 mapflatMap

  5. map:接收一个函数作为参数,该函数会被应用到每个元素上,并将其映射成一个新的元素。

  6. flatMap:接收一个函数作为参数,将流中的每个值都换成另一个流,然后把所有流连接成一个流。

  7. 计数: count

  8. 平均值: averagingIntaveragingLongaveragingDouble

  9. 最值: maxByminBy

  10. 求和: summingIntsummingLongsummingDouble

  11. 统计以上所有: summarizingIntsummarizingLongsummarizingDouble

  12. 分区:将 stream按条件分为两个 Map,比如员工按薪资是否高于8000分为两部分。

  13. 分组:将集合分为多个Map,比如员工按性别分组。有单级分组和多级分组。

  14. sorted():自然排序,流中元素需实现Comparable接口

  15. sorted(Comparator com):Comparator排序器自定义排序

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