I don't know if any of this will help, but if you look back at previous posts there are a few issues which have been addressed like the following for example.
Your using another program to write your assignments rather than just the JDK - this might mean there are more issues to look into ? https://coderanch.com/t/4893/Cattle-Drive/DaysOld-assigment-classpath
I know it sounds obvious but you have double checked your spelling etc ? ( I know I have typed something wrong and not noticed it ! ).