That Cryptic sbt Error? My JDK Was Too New

I was following Scala course on Coursera. I've downloaded the code for the given assignments, I've got the tests, and I was ready to go. I opened the terminal, navigate to the project directory, and type the magic words: sbt test.

Then, it happens. A wall of red text fills your screen.

error: bad constant pool index: 0 at pos: 48454 while compiling: <no file> ... Exception in thread "sbt-parser-init-thread" java.lang.ExceptionInInitializerError Caused by: scala.reflect.internal.FatalError: bad constant pool index: 0 at pos: 48454 ... 

My first thought was, "What did I break?" I scanned the code, but I haven't even changed anything yet. This wasn't a simple typo or a missing semicolon. It was a deep, scary-looking error from the internals of the compiler.

The Real Culprit: A Generational Gap

This error was a tell-tale sign that my Java version (JDK) is too new for your sbt version.

When I hit this problem, the log showed I was running sbt 1.8.2 with Java 21. Here’s what’s happening under the hood: The sbt build tool is itself a Scala program, and older versions of it use an older Scala compiler (version 2.12). This older compiler simply doesn’t know how to handle the modern bytecode produced by a very new JDK like Java 21.

Think of it like trying to play a vintage video game on a 4K TV. The TV is too advanced and doesn't understand the old signal format, so the screen just glitches out.

The Solution: SDKMAN! to the Rescue

I could hunt down old Java versions and manually manage my JAVA_HOME path for every project, but who has time for that? The best tool for this job was SDKMAN, a command-line tool for managing parallel versions of multiple Software Development Kits.

With SDKMAN!, you can install multiple Java versions and switch between them with a single command.

Here’s how I solved the "bad constant pool index" error in under five minutes:

Step 1: Install SDKMAN

If you don't have it, just follow the simple instructions on the official SDKMAN website. It's usually a single curl command.

Step 2: Find and Install a Compatible JDK

The sbt 1.x series works beautifully with JDK 11 or JDK 17. I opted for 17. You can list all available versions and then install your choice.

# List available Java versions sdk list java # Install a compatible version (I chose Temurin) sdk install java 17.0.12-tem 

Step 3: Switch Your Java Version

This is the magic step. For your current terminal session, just tell SDKMAN which Java version to use.

# Switch to the newly installed JDK sdk use java 17.0.12-tem 

SDKMAN will reconfigure your shell's PATH on the fly. You can verify the change with java -version.

Step 4: Run sbt Again

Now, back in your project folder, run the command that started all this trouble.

sbt test 

Success! The cryptic errors vanish. The build kicks off, dependencies download, tests run, and you see the beautiful green text of a successful test suite.