Referenced class file 'filename' may be older than 'filename'
The compiler detected a class file, which is referenced by the source file being compiled, that is out of date or not found. This warning usually occurs when automatic re-compilation of referenced classes has been disabled (using the ref- compiler option). If you are compiling within Visual J++, the ref- option is passed to the compiler by default. If any of your source files reference a class file that is not in your project and the file is out of date with its source, you will need to update the referenced class files. You can either recompile the external class files to make them up to date, add the external files to your project, or add the ref compiler option. You can add the ref compiler option by entering it in the Additional Compiler Options text box in the Compile tab of the Project Settings dialog box.