java–流已经启动或关闭

超、凢脫俗 2022-06-06 09:17 256阅读 0赞

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.

[Stream]: https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html