java–流已经启动或关闭

最新推荐文章于 2025-09-18 18:10:56 发布

翻译最新推荐文章于 2025-09-18 18:10:56 发布 · 2.4k 阅读

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Java 8 Tutorials（新特性）

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本文介绍 Java8 中 Stream 的一次性特性，并演示如何通过 Supplier 方法解决 Stream 重复使用的问题，避免 IllegalStateException 异常。

在 Java 8, Stream 不能被重新使用, 一旦它被使用或者消费了, stream 将被关闭.

1. Example – Stream 已经被关闭!

查看下面的例子 , 它将被抛出一个 IllegalStateException, 说 “stream is closed”.

  TestJava8.java

package com.mkyong.java8;

import java.util.Arrays;
import java.util.stream.Stream;

public class TestJava8 {

    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));

        // reuse it to filter again! throws IllegalStateException
        long count = stream.filter(x -> "b".equals(x)).count();
        System.out.println(count);

    }

}

Output

java.lang.IllegalStateException: stream has already been operated upon or closed
	at java.util.stream.AbstractPipeline.(AbstractPipeline.java:203)
	at java.util.stream.ReferencePipeline.(ReferencePipeline.java:94)
	at java.util.stream.ReferencePipeline$StatelessOp.(ReferencePipeline.java:618)
	at java.util.stream.ReferencePipeline$2.(ReferencePipeline.java:163)
	at java.util.stream.ReferencePipeline.filter(ReferencePipeline.java:162)
	at com.hostingcompass.whois.range.run.TestJava8.main(TestJava8.java:25)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

2. Example – 重新运用一个 stream

无论因为什么原因, 你真的想重新运用一个Stream, 可以使用下面的方式解决:

  TestJava8.java

package com.mkyong.java8;

import java.util.function.Supplier;
import java.util.stream.Stream;

public class TestJava8 {

    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);

    }

}

Output

a
b
c
d
e
1

每一个get() 将返回一个新的 stream.