java8stream流(一) - 高大北博客

什么是stream流

Stream 是JDK1.8 中处理集合的关键抽象概念，Lambda 和 Stream 是JDK1.8新增的函数式编程最有亮点的特性了，它可以指定你希望对集合进行的操作，可以执行非常复杂的查找、过滤和映射数据等操作。使用Stream API 对集合数据进行操作，就类似于使用SQL执行的数据库查询。Stream 使用一种类似用 SQL 语句从数据库查询数据的直观方式来提供一种对 Java 集合运算和表达的高阶抽象。Stream API可以极大提高Java程序员的生产力，让程序员写出高效率、干净、简洁的代码。

这种风格将要处理的元素集合看作一种流，流在管道中传输，并且可以在管道的节点上进行处理，比如筛选，排序，聚合等。

元素流在管道中经过中间操作（intermediate operation）的处理，最后由最终操作(terminal operation)得到前面处理的结果。

Stream ：非常方便精简的形式遍历集合实现过滤、排序等。
Mysql：select userName from mayikt where userName =‘mayikt’
Order by age limt(0,2)
1652061750252

Stream创建方式

parallelStream为并行流采用多线程执行
Stream采用单线程执行 
parallelStream效率比Stream要高。

Stream将list转换为Set

   ArrayList<UserEntity> userEntities = new ArrayList<>();
        userEntities.add(new UserEntity("11", 20));
        userEntities.add(new UserEntity("11", 20));
        userEntities.add(new UserEntity("22", 35));
        userEntities.add(new UserEntity("223", 16));
        userEntities.add(new UserEntity("33", 16));
        //串形流
        Stream<UserEntity> stream = userEntities.stream();
        //转set集合
        Set<UserEntity> collect = stream.collect(Collectors.toSet());
        //打印集合，但是没去重复，需要重写eques方法
        collect.forEach(collect1 -> System.out.println(collect1));

Stream将list转换为Map

    public static void main(String[] args) {
        ArrayList<UserEntity> userEntities = new ArrayList<>();
        userEntities.add(new UserEntity("11", 20));
        userEntities.add(new UserEntity("111", 21));
        userEntities.add(new UserEntity("22", 35));
        userEntities.add(new UserEntity("223", 16));
        userEntities.add(new UserEntity("33", 16));
        //串形流
        Stream<UserEntity> stream = userEntities.stream();
        //转map集合
        Map<String, Integer> collect = stream.collect(Collectors.toMap(UserEntity::getName, UserEntity::getAge));
        //打印集合，但是没去重复，需要重写eques方法
        collect.forEach((k, v) -> System.out.println(k + ":" + v));

//        //并行流
//        userEntities.parallelStream();
    }

Stream将Reduce 求和

    public static void main(String[] args) {
        ArrayList<UserEntity> userEntities = new ArrayList<>();
        userEntities.add(new UserEntity("11", 20));
        userEntities.add(new UserEntity("111", 21));
        userEntities.add(new UserEntity("22", 35));
        userEntities.add(new UserEntity("223", 16));
        userEntities.add(new UserEntity("33", 16));
        Stream<UserEntity> stream = userEntities.stream();
        Optional<UserEntity> reduce = stream.reduce((user1, user2) -> {
            user1.setAge(user1.getAge() + user2.getAge());
            return user1;
        });
        System.out.println(reduce.get());
    }

StreamMatch 匹配

anyMatch表示，判断的条件里，任意一个元素成功，返回true
allMatch表示，判断条件里的元素，所有的都是，返回true
noneMatch跟allMatch相反，判断条件里的元素，所有的都不是，返回true

   public static void main(String[] args) {
        ArrayList<UserEntity> userEntities = new ArrayList<>();
        userEntities.add(new UserEntity("11", 20));
        userEntities.add(new UserEntity("11", 20));
        userEntities.add(new UserEntity("22", 35));
        userEntities.add(new UserEntity("223", 16));
        userEntities.add(new UserEntity("33", 16));
        Stream<UserEntity> stream = userEntities.stream();
        boolean b = stream.anyMatch(userEntity -> userEntity.getName() .equals("11"));
        boolean b = stream.noneMatch(userEntity -> userEntity.getName() .equals("11"));
        boolean b = stream.allMatch(userEntity -> userEntity.getName() .equals("11"));
        System.out.println(b);
    }

Stream过滤器

   public static void main(String[] args) {
        ArrayList<UserEntity> userEntities = new ArrayList<>();
        userEntities.add(new UserEntity("11", 20));
        userEntities.add(new UserEntity("11", 23));
        userEntities.add(new UserEntity("22", 35));
        userEntities.add(new UserEntity("223", 16));
        userEntities.add(new UserEntity("33", 16));
        Stream<UserEntity> stream = userEntities.stream();
        stream.filter(userEntity -> userEntity.getName().equals("11") && userEntity.getAge() > 20).forEach(System.out::println);
    }

Stream limit和skip

Limit 从头开始获取
Skip 就是跳过

    public static void main(String[] args) {
        ArrayList<UserEntity> userEntities = new ArrayList<>();
        userEntities.add(new UserEntity("11", 20));
        userEntities.add(new UserEntity("11", 23));
        userEntities.add(new UserEntity("2211", 35));
        userEntities.add(new UserEntity("2213", 16));
        userEntities.add(new UserEntity("311113", 16));
        userEntities.add(new UserEntity("1112213211", 20));
        userEntities.add(new UserEntity("13213121", 23));
        userEntities.add(new UserEntity("224431312", 35));
        Stream<UserEntity> stream = userEntities.stream();
        stream.skip(1).limit(5).forEach(System.out::println);
   }

Stream排序 sorted

    public static void main(String[] args) {
        ArrayList<UserEntity> userEntities = new ArrayList<>();
        userEntities.add(new UserEntity("11", 20));
        userEntities.add(new UserEntity("11", 23));
        userEntities.add(new UserEntity("2211", 35));
        userEntities.add(new UserEntity("2213", 16));
        userEntities.add(new UserEntity("311113", 16));
        userEntities.add(new UserEntity("1112213211", 20));
        userEntities.add(new UserEntity("13213121", 23));
        userEntities.add(new UserEntity("224431312", 35));
        Stream<UserEntity> stream = userEntities.stream();
        //升序
        stream.sorted((o1, o2) -> o1.getAge() - o2.getAge()).forEach(System.out::println);
        //降序
        stream.sorted((o1, o2) -> o2.getAge() - o1.getAge()).forEach(System.out::println);
        
   }

Stream 综合案例

    public static void main(String[] args) {
        ArrayList<UserEntity> userEntities = new ArrayList<>();
        userEntities.add(new UserEntity("11", 20));
        userEntities.add(new UserEntity("11", 23));
        userEntities.add(new UserEntity("2211", 35));
        userEntities.add(new UserEntity("2213", 16));
        userEntities.add(new UserEntity("311113", 16));
        userEntities.add(new UserEntity("1112213211", 20));
        userEntities.add(new UserEntity("13213121", 23));
        userEntities.add(new UserEntity("224431312", 35));
        Stream<UserEntity> stream = userEntities.stream();
        //对数据流进行降序排序，且名称要有包含11 并且获取1位
        stream.sorted((o1, o2) -> o2.getAge() - o1.getAge()).filter(userEntity -> userEntity.getName().contains("11")).limit(1).forEach(System.out::println);

   }

并行流与串行流区别

串行流：单线程的方式操作；数据量比较少的时候。
并行流：多线程方式操作；数据量比较大的时候，原理：
Fork join 将一个大的任务拆分n多个小的子任务并行执行，
最后在统计结果，有可能会非常消耗cpu的资源，确实可以
提高效率。

注意：数据量比较少的情况下，不要使用并行流。