circular reference : Java Glossary

You need to compile A before B and B before A so because the compiler needs to know the structure of a class before it can generate code to invoke its methods.

Javac gives you a way out of this. You compile both classes at once on the command line and in some unspecified amazing way, the compiler sorts the problem out for itself, e. g.

rem compiling a circular reference
javac.exe A.java B.java