Java8的Stream

在Java8中，一个比较大的变化就是流（Stream），具体可以看之前的一篇文章：

Java9的Stream

Java9中Stream增加了4个方法，分别是：

• takeWhile：在有序的Stream中，takeWhile返回从开头开始的尽量多的元素

default Stream
    
      takeWhile(Predicate
      predicate) {    Objects.requireNonNull(predicate);    // Reuses the unordered spliterator, which, when encounter is present,    // is safe to use as long as it configured not to split    return StreamSupport.stream(        new WhileOps.UnorderedWhileSpliterator.OfRef.Taking<>(spliterator(), true, predicate),            isParallel()).onClose(this::close);}复制代码
    

• dropWhile：与takeWhile相反

default Stream
    
      dropWhile(Predicate
      predicate) {    Objects.requireNonNull(predicate);    // Reuses the unordered spliterator, which, when encounter is present,    // is safe to use as long as it configured not to split    return StreamSupport.stream(        new WhileOps.UnorderedWhileSpliterator.OfRef.Dropping<>(spliterator(), true, predicate),            isParallel()).onClose(this::close);}复制代码
    

• ofNullable：可以创建一个单元素Stream，可以为null

public static
    
      Stream
     
       ofNullable(T t) {    return t == null ? Stream.empty()         : StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);}复制代码
     
    

• iterate

public static
    
      Stream
     
       iterate(final T seed, final UnaryOperator
      
        f) {    Objects.requireNonNull(f);    Spliterator
       
         spliterator = new Spliterators.AbstractSpliterator<>(Long.MAX_VALUE,    Spliterator.ORDERED | Spliterator.IMMUTABLE) {        T prev;        boolean started;        @Override        public boolean tryAdvance(Consumer
         action) {            Objects.requireNonNull(action);            T t;            if (started)                t = f.apply(prev);            else {                t = seed;                started = true;            }            action.accept(prev = t);            return true;        }    };    return StreamSupport.stream(spliterator, false);}复制代码
       
      
     
    

除了对 Stream 本身的扩展，Optional和Stream之间的结合也得到了改进，可以将optional对象转化为stream对象：

List
    
      list = new ArrayList<>() {
   {    add("a");add("b");add("c");}};Optional
     
      
       > optional = Optional.ofNullable(list);Stream
       
        
         > stream = optional.stream();stream.flatMap(x -> x.stream()).forEach(System.out::println);复制代码
        
       
      
     
    

使用举例

• takeWhile

List
    
      list = Arrays.asList(1, 4, 5, 2, 3, 6, 7, 8, 9, 10);list.stream().takeWhile(x -> x < 5).forEach(System.out::println);复制代码
    

输出：

1

4

• dropWhile

List
    
      list = Arrays.asList(1, 4, 5, 2, 3, 6, 7, 8, 9, 10);list.stream().dropWhile(x -> x < 5).forEach(System.out::println);复制代码
    

输出：

5

2

3

6

7

8

9

10

• ofNullable

Stream
    
      stream = Stream.of("", null);System.out.println(stream.count());Stream
     
       stream = Stream.of(null);System.out.println(stream.count());//会抛空指针异常Stream
      
        stream = Stream.ofNullable(null);System.out.println(stream.count());复制代码
      
     
    

输出：

2

0

• iterate

//java8Stream.iterate(1, i -> i + 1).limit(5).forEach(System.out::println);//java9Stream.iterate(1, i -> i < 6, i -> i + 1).forEach(System.out::println);复制代码

输出：

1

2

3

4

5