java8新特性stream流示例

一、简介
java.util.stream表示能应用在一组元素上一次执行的操作序列。stream操作分为中间操作和最终操作，最终操作返回一特定类型的计算结果，而中间操作返回stream本身，这样就可以将多个操作串起来。stream操作需要指定一个数据源，比如java.util.collection的子类、list或者set，map类型不支持。stream可以串行stream()执行或者并行parallelStream()执行。

二、Stream



三、示例
1、foreach循环

    @Test
    public void Foreach(){
        //你不鸟我，我也不鸟你
        List<String> list = Arrays.asList("you","don't","bird","me",",",
                "i","don't","bird","you");

        //方式一：jdk1.8之前的循环遍历方式
        for(String item:list){
            System.out.println(item);
        }

        //方式二：使用stream流
        // void forEach(Consumer<? super T> action)
        list.stream().forEach(item -> System.out.println(item));

        //方式三：使用stream流+lambda表达式
        list.stream().forEach(System.out::println);
    }

2、filter过滤

    public class User{
        private long id;
        private String name;
        private Integer age;

        public User(){}

        public User(long id, String name, Integer age){
            this.id = id;
            this.name = name;
            this.age = age;
        }

        public Integer getAge(){
            return this.age;
        }
    }

    @Test
    public void filter(){
        List<User> users = Arrays.asList(
                new User(1, "wanwei", 23),
                new User(2, "zhangsan", 25),
                new User(3, "lisi", 29)
        );
        users.stream().filter(user -> user.getAge()>24).forEach(user -> System.out.println(user.name));
    }

3、map映射

    @Test
    public void map(){
        List<String> list = Arrays.asList("djfh", "sdf", "sdfg");
        list.stream().map(item -> item.toUpperCase()).forEach(System.out::println);
    }

4、flatmap

    @Test
    public void flatmap(){
        List<Integer> a = Arrays.asList(1,2,3);
        List<Integer> b = Arrays.asList(4,5,6);

        List<List<Integer>> collect = Stream.of(a,b).collect(Collectors.toList());
        System.out.println(collect);

        //将多个集合中的元素合并为一个集合
        List<Integer> mergelist = Stream.of(a,b).flatMap(list -> list.stream()).collect(Collectors.toList());
        System.out.println(mergelist);

        //通过Builder方式来构建
        Stream<Object> stream = Stream.builder().add("hello").add("hi").add("HI").build();
        stream.forEach(System.out::println);
    }

5、sorted排序

    @Test
    public void sorted(){
        List<String> list = Arrays.asList("c","b","e","a");
        list.stream().sorted((s1,s2) -> s1.compareTo(s2)).forEach(System.out::println);
    }

6、distinct去重

    @Test
    public void distinct(){
        Stream<String> stream = Stream.of("hello","hello","hi","hi","i");
        stream.distinct().forEach(System.out::println);
    }

7、count总数量

    @Test
    public void count(){
        Stream<String> stream = Stream.of("known","is","known","nok","is","nok");
        long count = stream.count();
        System.out.println(count);
    }

8、skip、limit

@Test
public void skip(){
    List<String> list = Arrays.asList("a", "b", "c", "d", "e");
    // Stream<T> skip(long n)
    list.stream().skip(2).forEach(System.out::println);  // c、d、e
}

@Test
public void limit(){
    List<String> list = Arrays.asList("a", "b", "c", "d", "e");
    list.stream().skip(2).limit(2).forEach(System.out::println);    // c、d
}

9、concat

@Test
public void concat(){
    List<String> list = Arrays.asList("a", "b");
    List<String> list2 = Arrays.asList("c", "d");
    Stream<String> concatStream = Stream.concat(list.stream(), list2.stream());
    concatStream.forEach(System.out::println);
}

10、anyMatch、allMatch

@Test
public void match(){
    // 你给我站住
    List<String> list = Arrays.asList("you", "give", "me", "stop");
    // boolean anyMatch(Predicate<? super T> predicate);
    // parallelStream可以并行计算，速度比stream更快
    boolean result = list.parallelStream().anyMatch(item -> item.equals("me"));
    System.out.println(result);
}

11、reduce归纳

@Test
public void reduce(){
    Stream<String> stream = Stream.of("you", "give", "me", "stop");
    // Optional<T> reduce(BinaryOperator<T> accumulator);
    Optional<String> optional = stream.reduce((before, after) -> before + "," + after);
    optional.ifPresent(System.out::println);    // you,give,me,stop
}

public static void main(String[] args) {
    List<BigDecimal> list = Arrays.asList(
            new BigDecimal("11.11"),
            new BigDecimal("22.22"),
            new BigDecimal("33.33")
    );
    // 66.66
    BigDecimal sum = list.stream().reduce(BigDecimal.ZERO, BigDecimal::add);
    System.out.println(sum);
}

12、findFirst、findAny

    @Test
    public void findFirst(){
        Stream<String> stream = Stream.of("you", "give", "me", "stop");
        String value = stream.findFirst().get();
        System.out.println(value);
    }

    @Test
    public void findAny(){
        Stream<String> stream = Stream.of("you", "give", "me", "stop");
        String value2 = stream.findAny().get();
        System.out.println(value2);
    }