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Hello, David,</div>
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I don't think this is necessary - the cause of the exception is already quite clear from the stack trace. I checked a few use sites of the no-arg constructor of IllegalStateException - many of them are in the same situation, that they are easy to understand
once we see their location in code. I assume such a use of no-arg ISE constructor is widely accepted.</div>
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For error messages, sometimes it is not feasible to be too detailed - I would think if we do add any, something like "closed" would be sufficient. Printing too many details in exceptions and log messages can sometimes constitute security risks.</div>
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For the stream part, I believe both Files.walk and Stream.flatMap do their job - walk specifies you should close after use, and flatMap specifies the returned streams are closed once their elements are all taken. However, I do agree that there are possible
documentation enhancements, that:</div>
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1. Stream factories should specify if their elements are still usable after the streams are closed, and</div>
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2. Stream should note that they should be closed after a terminal operation (which is the fault you made here)</div>
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Regards,</div>
<div style="font-family: "Calibri Light", "Helvetica Light", sans-serif; font-size: 12pt; color: rgb(0, 0, 0);" class="elementToProof">
Chen Liang</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> core-libs-dev <core-libs-dev-retn@openjdk.org> on behalf of David Alayachew <davidalayachew@gmail.com><br>
<b>Sent:</b> Monday, November 24, 2025 1:38 PM<br>
<b>To:</b> core-libs-dev <core-libs-dev@openjdk.org><br>
<b>Subject:</b> Add better exception message to FileTreeIterator.hasNext()?</font>
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<div class="x_gmail_default" style="font-family:monospace">Hello <a class="x_gmail_plusreply" id="x_plusReplyChip-0" href="mailto:core-libs-dev@openjdk.org" tabindex="-1">@core-libs-dev</a>,</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
</div>
<div class="x_gmail_default" style="font-family:monospace">I was writing a simple script today -- find how many Java files are in each of my top level folders of my current directory.</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
</div>
<div class="x_gmail_default" style="font-family:monospace">So, I ran the following code.</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
</div>
<div class="x_gmail_default" style="font-family:monospace">Files<br>
list(Path.of("."))<br>
filter(Files::isDirectory)<br>
collect<br>
<br>
Collectors.groupingBy(<br>
Function.<Path>identity(),<br>
Collectors<br>
.flatMapping<br>
(<br>
(final Path root) -><br>
{<br>
try (final Stream<Path> streamWalker = Files.walk(root))<br>
{<br>
return<br>
streamWalker<br>
.filter(Files::isRegularFile)<br>
.filter(file -> file.toString().endsWith(".java"))<br>
;<br>
}<br>
catch (final Exception exception)<br>
{<br>
throw new RuntimeException(exception);<br>
}<br>
},<br>
Collectors.counting()<br>
)<br>
)<br>
;</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
</div>
<div class="x_gmail_default" style="font-family:monospace">Now, there is a not-so-obvious bug -- when using a flatMapping Collector, no need for try-with-resources.</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
</div>
<div class="x_gmail_default" style="font-family:monospace">The above code threw the below stack trace.</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
</div>
<div class="x_gmail_default" style="font-family:monospace">| Exception java.lang.IllegalStateException<br>
| at FileTreeIterator.hasNext (FileTreeIterator.java:100)<br>
| at Iterator.forEachRemaining (Iterator.java:132)<br>
| at Spliterators$IteratorSpliterator.forEachRemaining (Spliterators.java:1939)<br>
| at AbstractPipeline.copyInto (AbstractPipeline.java:570)<br>
| at AbstractPipeline.wrapAndCopyInto (AbstractPipeline.java:560)<br>
| at ForEachOps$ForEachOp.evaluateSequential (ForEachOps.java:153)<br>
| at ForEachOps$ForEachOp$OfRef.evaluateSequential (ForEachOps.java:176)<br>
| at AbstractPipeline.evaluate (AbstractPipeline.java:265)<br>
| at ReferencePipeline.forEach (ReferencePipeline.java:632)<br>
| at Collectors.lambda$flatMapping$0 (Collectors.java:483)<br>
| at Collectors.lambda$groupingBy$0 (Collectors.java:1113)<br>
| at ReduceOps$3ReducingSink.accept (ReduceOps.java:169)<br>
| at ReferencePipeline$2$1.accept (ReferencePipeline.java:197)<br>
| at Iterator.forEachRemaining (Iterator.java:133)<br>
| at Spliterators$IteratorSpliterator.forEachRemaining (Spliterators.java:1939)<br>
| at AbstractPipeline.copyInto (AbstractPipeline.java:570)<br>
| at AbstractPipeline.wrapAndCopyInto (AbstractPipeline.java:560)<br>
| at ReduceOps$ReduceOp.evaluateSequential (ReduceOps.java:921)<br>
| at AbstractPipeline.evaluate (AbstractPipeline.java:265)<br>
| at ReferencePipeline.collect (ReferencePipeline.java:723)<br>
| at (#4:5)<br>
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<div class="x_gmail_default" style="font-family:monospace"><br>
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<div class="x_gmail_default" style="font-family:monospace">Not obvious what is wrong here.</div>
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<div class="x_gmail_default" style="font-family:monospace">Now, the exception message is being thrown by the hasNext() method of the FileTreeIterator class. After looking at the source code of that class, it's clear what the problem is.</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
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<div class="x_gmail_default" style="font-family:monospace"><a href="https://github.com/openjdk/jdk/blob/jdk-26%2B25/src/java.base/share/classes/java/nio/file/FileTreeIterator.java#L98">https://github.com/openjdk/jdk/blob/jdk-26%2B25/src/java.base/share/classes/java/nio/file/FileTreeIterator.java#L98</a></div>
<div class="x_gmail_default" style="font-family:monospace"><br>
</div>
<div class="x_gmail_default" style="font-family:monospace">It throws an exception because the FileTreeWalker contained within the FileTreeIterator is closed (presumably by the try-with-resources). However, the thrown exception message is empty. This exact same
check-then-throw logic is duplicated all over FileTreeIterator.</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
</div>
<div class="x_gmail_default" style="font-family:monospace">I propose that all such instances of check-then-throw be provided with exception messages along the lines of "Cannot check for more elements because the file walker is already closed!". I am not picky
on the wording. Just something to inform me that the issue is something is closed, as opposed to just a generic IllegalStateException. I would even accept an empty message body if the exception type communicated the issue.</div>
<div class="x_gmail_default" style="font-family:monospace"><br>
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<div class="x_gmail_default" style="font-family:monospace">Thank you for your time.</div>
<div class="x_gmail_default" style="font-family:monospace">David Alayachew</div>
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