Java8 Stream 已經被操作或關閉

引言

在 java8 中，Stream 不能被重用，一旦它被使用或使用，流將被關閉。

1. 流關閉

查看下面的示例，它將拋出一個 IllegalStateException，表示“ stream is closed”。

public static void main(String[] args) {
    String[] array = {"a", "b", "c", "d", "e"};
    Stream<String> stream = Arrays.stream(array);

    // loop a stream
    stream.forEach(x-> System.out.println(x));

    // 拒絕過濾，拋出 throws IllegalStateException
    long count = stream.filter(x -> "b".equals(x)).count();
    System.out.println(count);
  }

輸出：

a
b
c
d
e
Exception in thread "main" java.lang.IllegalStateException: stream has already been operated upon or closed
	at java.util.stream.AbstractPipeline.<init>(AbstractPipeline.java:203)
	at java.util.stream.ReferencePipeline.<init>(ReferencePipeline.java:94)
	at java.util.stream.ReferencePipeline$StatelessOp.<init>(ReferencePipeline.java:618)
	at java.util.stream.ReferencePipeline$2.<init>(ReferencePipeline.java:163)
	at java.util.stream.ReferencePipeline.filter(ReferencePipeline.java:162)
	at com.jimzhang.stream.ClosedTest.main(ClosedTest.java:22)

2. 重用流

不管出於什么原因，你真的想重用一個數據流，試試下面的 Supplier 解決方案:

public static void main(String[] args) {

    String[] array = {"a", "b", "c", "d", "e"};
    Supplier<Stream<String>> streamSupplier = () -> Stream.of(array);

    //get new stream
    streamSupplier.get().forEach(x-> System.out.println(x));

    //get another new stream

    long count = streamSupplier.get().filter(x -> "b".equals(x)).count();
    System.out.println(count);
  }

輸出：

a
b
c
d
e
1

每個 get ()將返回一個新的流。

源碼見：java-8-demo

系列文章詳見：Java 8 教程