These errors happend because some interface/class in the hierarchy cannot be resolve. For example: the error is occur in your class - class x, x inherits y, and in turn, y inherits z. However, the compiler cannot resolve z (in above error is EventManager), because z is belong to a library that is not included. Therefore, you have to add package containing z to the classpath/ or project's Java Build Path (if you are using eclipse).
For example: the error is occur in your class - class x, x inherits y, and in turn, y inherits z. However, the compiler cannot resolve z (in above error is EventManager), because z is belong to a library that is not included.
Therefore, you have to add package containing z to the classpath/ or project's Java Build Path (if you are using eclipse).