Java – Stream has already been operated upon or closed 修改教程

In Java 8, Stream cannot be reused, once it is consumed or used, the stream will be closed.

1. Example – Stream is closed!

Review the following example, it will throw an IllegalStateException, saying “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 – Reuse a stream

For whatever reason, you really want to reuse a Stream, try the following Supplier solution :

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

Each get() will return a new stream.

Why?
May I know why you need to reuse a stream, for testing?

References

  1. Supplier JavaDoc
  2. Stream summary JavaDoc

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