Class Not Found java/lang/ClassNotFoundException Error

Good day,
I am new to J2ME. I have created an app using LWUIT. There are 5 classes I made and the main class will be calling the 4 next classes. I have used buttons to connect them altogether. Now, while compiling, I have encountered an error which is MIDlet manager error. So, I deleted the MIDlet extensions to the 4 classes and remain the MIDlet only on the main class. Now, when I try it to Nokia Series 40 phone, the error is Class Not Found java/lang/ClassNotFoundException". What's that supposed to mean?
Thank you guys

How many classes are in the JAR you install?

This exception means that the code executing refers to a class that does not exist, either on the device or in the application.

Possibly, the LWUIT classes have not been copied into your JAR by the build process.