java - 如何在执行一次操作后重用 Stream

Question

我想使用相同的 Stream 找出以特定字符开头的 String 两次，但是第二个 Stream 操作抛出一个异常(exception)。如何重新使用 Stream？

public static void main(String[] args) {
    Stream<String> stream = Stream.of("abc", "aef", "bcd", "bef", "crf");
    stream.filter(s -> s.startsWith("a")).forEach(System.out::println);;
    stream.filter(s -> s.startsWith("b")).forEach(System.out::println);;
}

异常

Exception in thread "main" java.lang.IllegalStateException: stream has already been operated upon or closed
    at java.util.stream.AbstractPipeline.<init>(Unknown Source)
    at java.util.stream.ReferencePipeline.<init>(Unknown Source)
    at java.util.stream.ReferencePipeline$StatelessOp.<init>(Unknown Source)
    at java.util.stream.ReferencePipeline$2.<init>(Unknown Source)
    at java.util.stream.ReferencePipeline.filter(Unknown Source)
    at StreamExp.main(StreamExp.java:13)

Answer 1

您不能执行终端操作，例如 forEach两次相同 Stream .

您还有其他选择。例如，您可以生成 Map<Character,List<String>>其中输入String s 按第一个字符分组:

Map<Character,List<String>> groups = 
    Stream.of("abc", "aef", "bcd", "bef", "crf")
          .collect(Collectors.groupingBy(s->s.charAt(0)));

这将导致以下 Map :

{a=[abc, aef], b=[bcd, bef], c=[crf]}