-have following groovy script:

class Car {
    public horsePower
    public color
}

class Shirt {
    public color
    public size
}

def printColor(item) {
    println("item color is: " + item.color)
}

car = new Car(color:"blue")
shirt = new Shirt(color:"black")
printColor(car)
printColor(shirt)

-save it

=>
 SEVERE [global]
     [exec] java.lang.AssertionError: Virtual source
/home/lukas/NetBeansProjects/JavaApplication2/src/javaapplication2/newGroovyScript.groovy provided more java files!
     [exec] 	at
org.netbeans.modules.java.source.usages.RepositoryUpdater$CompileWorker.updateFile(RepositoryUpdater.java:2251)
     [exec] 	at
org.netbeans.modules.java.source.usages.RepositoryUpdater$CompileWorker.compileWithDeps(RepositoryUpdater.java:2505)
     [exec] 	at
org.netbeans.modules.java.source.usages.RepositoryUpdater$CompileWorker.access$3300(RepositoryUpdater.java:1232)
     [exec] 	at org.netbeans.modules.java.source.usages.RepositoryUpdater$CompileWorker$1.run(RepositoryUpdater.java:1505)
     [exec] 	at org.netbeans.modules.java.source.usages.RepositoryUpdater$CompileWorker$1.run(RepositoryUpdater.java:1274)
     [exec] 	at org.netbeans.modules.java.source.usages.ClassIndexManager.writeLock(ClassIndexManager.java:105)
     [exec] 	at org.netbeans.modules.java.source.usages.RepositoryUpdater$CompileWorker.run(RepositoryUpdater.java:1271)
     [exec] 	at org.netbeans.modules.java.source.usages.RepositoryUpdater$CompileWorker.run(RepositoryUpdater.java:1232)
     [exec] [catch] at org.netbeans.api.java.source.JavaSource$CompilationJob.run(JavaSource.java:1559)
     [exec] 	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:417)
     [exec] 	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:269)
     [exec] 	at java.util.concurrent.FutureTask.run(FutureTask.java:123)
     [exec] 	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:650)
     [exec] 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:675)
     [exec] 	at java.lang.Thread.run(Thread.java:595)