What does the error "Unable to initialize main class App" mean and why is it caused by "java.lang.NoClassDefFoundError: Stage"?

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The error "Unable to initialize main class App" suggests that the main class in the application could not be initialized properly.

The additional message "java.lang.NoClassDefFoundError: Stage" indicates that the problem may be caused by a missing class named "Stage". This class is a part of the JavaFX library and it is responsible for handling the application window and other UI components.

The error could be caused by one of the following reasons:

1. The JavaFX library is not included in the classpath of the application.
2. The version of the JavaFX library being used is incompatible with the version of Java being used.
3. The JavaFX library is not installed on the system.
4. The application is not being run from the correct directory.

To fix the error, the appropriate version of JavaFX library should be added to the classpath or installed correctly on the system. The application should also be run from the correct directory.