Fresh Scala project: Cannot create any new class

[ppp7032]

IntelliJ version:

IntelliJ IDEA 2026.1 EAP
Build #IU-261.20362.25, built on February 4, 2026
Source revision: 4dbe47e0fe1f5
Runtime version: 25.0.2+10-b300.57 aarch64137.0.17-261-b56
VM: OpenJDK 64-Bit Server VM by JetBrains s.r.o.
Toolkit: sun.awt.wl.WLToolkit
Linux 6.18.9+1-asahi_1
Void Linux; glibc: 2.41; desktop: KDE (plasmashell 6.5.5)
Exception reporter ID: 12022639b820c4f-10e2-43c3-a3bc-ac25f88b551d
GC: G1 Young Generation, G1 Concurrent GC, G1 Old Generation
Memory: 2048M
Cores: 10
Non-Bundled Plugins:
  com.intellij.classic.ui (261.20362.8)
  org.intellij.scala (2026.1.3)
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Kotlin: 261.20362.25-IJ
Current Desktop: KDE

Scala version: 3.3.7LTS
Java source compatability: 8
JDK version: 25
gdx-liftoff: gdx-liftoff-1.14.0.6-linux.jar

After making a fresh project, adding a single Scala class to core/main/scala/g/d/f:

Execution failed for task ':core:jar'.
> Entry g/d/f/ScalaMain.class is a duplicate but no duplicate handling strategy has been set. Please refer to https://docs.gradle.org/9.3.1/dsl/org.gradle.api.tasks.Copy.html#org.gradle.api.tasks.Copy:duplicatesStrategy for details.

Result of running the following command:

$ find ./ -name ScalaMain\*
./core/src/main/scala/g/d/f/ScalaMain.scala
./core/build/classes/scala/main/io/github/some_example_name/ScalaMain.tasty
./core/build/classes/scala/main/g/d/f/ScalaMain.class
./core/build/classes/scala/main/g/d/f/ScalaMain.tasty
./core/build/classes/java/main/g/d/f/ScalaMain.class
./core/build/classes/java/main/g/d/f/ScalaMain.tasty

[tommyettinger]

This is happening because the template given was probably the default (Classic), which creates a Java ApplicationAdapter class called the name given early in the project menu (which was ScalaMain here). That Java code is still there, so creating a Scala class called ScalaMain is a duplicate. In your core project, you should be able to safely delete the core/src/main/java/ folder, since you have a core/src/main/scala/ folder with ScalaMain in it that will be found instead. You don't need to delete the other src/main/java/ source folders in other platforms, since the launchers there are Java by default, and should be able to run the Scala code in core just fine.

There isn't any template with Scala launchers (or Groovy) because I don't really know Scala anymore, and it looks like a lot has changed since I last used it (about a decade ago). If anyone wants to contribute a KotlinLogo-style launcher that acts like the templates with Kotlin launchers (but uses Scala), that would be welcome.

[ppp7032]

Unfortunately, this is very much not the case. As you can see by my find command, there is no other source file called ScalaMain. I called ScalaMain that to contrast it to Main (which was still Java) in this test.

I first ran into this issue by deleting the default Java ApplicationAdapter and rewriting it in Scala. I drove myself crazy trying to figure out how the deleted Java code was still being compiled or at least its build was still being used, only to rename the Scala ApplicationAdapter and to experience the same issue. In fact, as I said in my initial post, trying to create any new Scala class in core will result in this issue, even if the name is unique, and even if it's not even being called by any Java code (yet).

[tommyettinger]

I'm wondering if you just need to run gradlew clean to get rid of the old .class file, but if any new Scala classes are resulting in the issue, maybe Scala is compiling two .class files per .scala file, which seems very strange. I'll create a new Scala project and try to follow along the same steps you took.

[ppp7032]

I don't mean to be rude, but I don't feel like I'm being listened to. I am telling you that creating any new Scala source code fails. Even when there is no old .class file. Please do let me know how your test goes.

[tommyettinger]

I mean, I'm trying to follow along, but I really don't use Scala, and have never encountered this before. If you can paste your ScalaMain.scala file, that would help me out a lot.

[tommyettinger]

It works here.

Scala262.zip

Here's my project, generated by the current Liftoff (which hasn't had any Scala changes in a while). I was able to build a jar using gradlew lwjgl3:jar, and it runs, but I removed it from the .zip file because it was large (20+ MB). I don't have any .tasty files and I don't have a clue what those are. I deleted the generated Java file in core before I built anything, but I copied its contents into scala/com/libgdx/liftoff/Main.scala, which to my surprise got IDEA to prompt a "Convert to Scala?" dialog, which I confirmed. I renamed Main.scala to ScalaMain.scala . Running the Gradle task lwjgl3:run worked at this point, as did lwjgl3:jar . I actually didn't make any changes to Gradle files, other than adding a commented block of code I thought I might need to core/build.gradle . I didn't actually need it.

So, at this point I haven't been able to reproduce your reported issue. I'll keep trying things, like creating additional Scala source files, but so far, everything works with Gradle just fine.

[tommyettinger]

Huh, I can create and extend a Scala Trait just fine, also. Here's that source, in case it helps.

Scala262.zip

You ran the gradlew clean task, right? The error only mentioned ScalaMain.class, not any newer classes, so I'm wondering if Gradle's clean task might need extra configuration to remove the duplicate .class file that the generated Java file produced early on for you. The newer classes might be fine, and only the older .class file persisting might be the last problem.

If the error is different for the newer classes you've added, it would be good to see that error.

[ppp7032]

Yes, I had tried running the clean task. I had also tried deleting the build folders.

This is crucial to understanding my issue: the duplicate .class files are not just leftovers from past builds. Any new Scala file I write immediately produces two such .class files upon being built.

I now believe this may be a bug with (my version of) IntelliJ. I'll try to reproduce the issue with a stable version of IntelliJ later, but it looks like this issue occurs consistently when the new Scala file you're creating is being made in the IntelliJ GUI. Deleting build and core/build doesn't fix it, neither does deleting .idea. Deleting all three does fix it, however.

I do apologise if it does turn out that this issue only exists because I was using an EAP version of IntelliJ. 2026.1 fixed a pretty annoying bug for me.

[tommyettinger]

Ah, that might make sense! I got a notification from IDEA that Nightly Builds "work better with Scala 3" but I try to avoid EAPs if I can. The one piece of code I was thinking might help would be added in core/build.gradle, where there isn't any "duplicates strategy" assigned to its jar task, even though other projects do handle duplicates for their jar tasks. What might help with the multiple .class files getting compiled, depending on the order they appear in, would be:

jar {
  duplicatesStrategy = DuplicatesStrategy.EXCLUDE
}

This is used in lwjgl3/build.gradle's jar task, but hadn't yet been needed in the core module. There's a remote chance that EXCLUDE might need to be INCLUDE, but that seems like it is the opposite of what we want here. I think that configuration can go at the end of core/build.gradle without any problems, I just don't know if it will help, either. It definitely seems like there's a caching bug with IDEA or some tool it uses, if you've completely eliminated the ScalaMain.java file and it's still creating .class files compiled from it. IDEA is definitely known to have caching bugs and at least provides a way to conveniently clear the caches (File -> Invalidate Caches).

[ppp7032]

I made another discovery. It seems running ./gradlew lwjgl3:run in the terminal works fine. It's only when running the gradle task from the GUI that this issue occurs.

The duplicate .class file is not from cached Java code. It is the scala code itself that is being made into two identical .class files. This is clear because, as I've tried to make clear, any new Scala source code file even if it's name is completely unique and there was never a Java file with the same name is made into two .class files.

I had previously added this DuplicateStrategy line and it did fix the issue for me, but I'm not 100% sure what it does so it seems like a hacky solution to include this. Does it stop two .class files being made from the one Scala source file, or does it just pick one of them at random (they should be identical) when producing the JAR file?

Here are reproduction instructions, being as clear as possible. Start with this project as a base:

gdxtest_original.tar.gz

Extract the tarball then make two copies of the folder/project, one gdxtest_idea and another gdxtest_cmdline.

Testing without IntelliJ:

$ cd ./gdx_cmdline
$ echo -e 'class ScalaTest:\n\tdef test(): Unit = {}' > core/src/main/scala/ScalaTest.scala
$ ./gradlew :core:jar

Testing with IntelliJ:

1. Open the project with IntelliJ. (2026.1 EAP may be needed).
2. Wait for project to finish importing.
3. Navigate to core/src/main/scala.
4. Right click and select "New Scala file/class".
5. Name it ScalaTest and input the same code as in the previous step.
6. Navigate to gradle tab on the right and double-click lwjgl3->Tasks->application->run.
7. Observe the error.

Now go back to the terminal and do this:

$ # Ensure you are in the directory with both gdxtest projects.
$ find . -name 'ScalaTest*' # Here are my results from this:
./gdxtest_cmdline/core/src/main/scala/ScalaTest.scala
./gdxtest_cmdline/core/build/classes/scala/main/ScalaTest.class
./gdxtest_cmdline/core/build/classes/scala/main/ScalaTest.tasty
./gdxtest_idea/core/src/main/scala/ScalaTest.scala
./gdxtest_idea/core/build/classes/java/main/ScalaTest.class
./gdxtest_idea/core/build/classes/java/main/ScalaTest.tasty
./gdxtest_idea/core/build/classes/scala/main/ScalaTest.tasty

These are all the instructions I did. I never touched any Java code. I never deleted any Java code.

[ppp7032]

Unable to reproduce when using latest stable IntelliJ 2025.3.2.

[ppp7032]

Filed a bug report on IntelliJ here.

It might still be a good idea to add the duplicate handling strategy, though. I'm not sure.

[tommyettinger]

Yeah, I don't think the duplicate handling can hurt, since it's done anyway in the other jar tasks. I'll add it since it seems to counteract the bug in IDEA for now... I still don't know what a .tasty file is, though, and I am wondering if that's a new thing in the latest IDEA EAP.

[ppp7032]

.tasty files are intermediate files made by the Scala 3 compiler, I believe.