I've been on a project where I build tetris that has multiplayer, and it's been working for a couple of days with no problem, but now suddenly there's this exception: "Exception in thread "main" java.lang.NoClassDefFoundError: Game/Piece". This basically means it can't find the .class of my class called Piece, but whenever I do a "clean and build" the .class file appears, then instantly disappears when I run the project. Is there a reason for this exception? I found someone that asked this before on stackoverflow with no answers.
CodePudding user response：
While it's possible that this is due to a classpath mismatch between compile-time and run-time, it's not necessarily true.
It is important to keep two or three different exceptions straight in our head in this case:
java.lang.ClassNotFoundException This exception indicates that the class was not found on the classpath. This indicates that we were trying to load the class definition, and the class did not exist on the classpath.
java.lang.NoClassDefFoundError This exception indicates that the JVM looked in its internal class definition data structure for the definition of a class and did not find it. This is different than saying that it could not be loaded from the classpath. Usually this indicates that we previously attempted to load a class from the classpath, but it failed for some reason - now we're trying to use the class again (and thus need to load it, since it failed last time), but we're not even going to try to load it, because we failed loading it earlier (and reasonably suspect that we would fail again). The earlier failure could be a ClassNotFoundException or an ExceptionInInitializerError (indicating a failure in the static initialization block) or any number of other problems. The point is, a NoClassDefFoundError is not necessarily a classpath problem.
And this question has been asked before.
CodePudding user response：
This type of error appear because the compiler is too different then the JVM.
Personally, I had this type of error by running Java 16 code with Java 8 JRE, and I fix it by removing the package with the Java 16 code.
So, to fix your issue, there is multiple ways:
- Use same Java in Netbeans and in your project config
- Upgrade your Java JRE installed (instead of old Java 8 version)
- Downgrade Netbeans JDK to Java 8