How do I resolve the error "java.lang.NoClassDefFoundError" or "java.lang.ClassNotFoundException"?

These errors mean that a java class was not found by the JVM.

It can be returned at startup if the main program name has not been passed correctly in the command line (consider that Java is case-sensitive and the .class extension must be omitted) or if a typo caused Java to treat an option in the command line as if it was the program to launch.