Stream学习过程中遇到的一个问题记录

Stream学习过程中遇到的一个问题记录

@Test
public void stream_modifiedV2(){
    List<String> strings = Arrays.asList("aaa","bb","cc","dddddddddddddddddddddddddddddddd");
    Stream<String> stringStream = strings.stream();
    stringStream.filter(s -> s.length()<=12);
    stringStream.forEach(System.out::println);
}

上面的代码会报错：
java.lang.IllegalStateException: stream has already been operated upon or closed

查找了相关资料，总结一下：

Stream 就如同一个迭代器（Iterator），单向，不可往复，数据只能遍历一次，遍历过一次后即用尽了，就好比流水从面前流过，一去不复返。

stream.filter会根据传入的Predicate进行过滤，但是过滤后返回了一个新的Stream。原来的Stream已经被操作过了，新的Stream没有被操作过。

JavaDoc里面的额

A stream should be operated on (invoking an intermediate or terminal stream operation) only once.

因此正确的写法是:

@Test
public void stream_modifiedV2(){
    List<String> strings = Arrays.asList("aaa","bb","cc","dddddddddddddddddddddddddddddddd");
    Stream<String> stringStream = strings.stream();
    stringStream = stringStream.filter(s -> s.length()<=12);  //注意这个
    stringStream.forEach(System.out::println);
}

官方推荐更简洁的做法：

Stream<String> stringStream = strings.stream.filter(s-> s.length()<=12);

参考回答

http://stackoverflow.com/questions/34677708/why-is-this-java-stream-operat ed-upon-twice