This week's book giveaways are in the Cloud and AI/ML forums. We're giving away four copies each of Cloud Native Patterns and Natural Language Processing and have the authors on-line! See this thread and this one for details.
Your problem must be the other way around; old class files work just fine with new JDKs, but class files compiled on a new JDK won't, by default, work with some older ones. Try typing "java -version" to make sure of the Java version you're running when you run the application. Please don't tell me you already know what it will say -- you might be surprised!