This fails:

$ gcj -I . -C -d classes frysk/sys/Wait.java frysk/model/Signal.java
frysk/event/Signal.java frysk/event/SignalEvent.java frysk/proc/LinuxHost.java
frysk/event/SignalEvent.java:3: error: Can't access class ‘frysk.model.Signal’.
Only public classes and interfaces in other packages can be accessed.
       extends Signal
               ^
frysk/event/SignalEvent.java:0: confused by earlier errors, bailing out

(the order of the parameters is very important) given the files:

<< frysk/event/SignalEvent.java >>
package frysk.event;
public abstract class SignalEvent
    extends Signal
{
    public SignalEvent (int signal)
    {
        super (signal);
    }
}
<< frysk/event/Signal.java >>
package frysk.event;
public class Signal
{
    public Signal (int signal)
    {
    }
}
<< frysk/model/Signal.java >>
package frysk.model;
class Signal
{
}
<< frysk/proc/LinuxHost.java >>
package frysk.proc;
import frysk.event.SignalEvent;
import frysk.sys.Wait;
public class LinuxHost
{
    class PollWaitOnSigChld
        extends SignalEvent
    {
        PollWaitOnSigChld ()
        {
            super (0);
        }
        Wait.Observer waitObserver = new Wait.Observer ()
            {
            };
        public final void execute ()
        {
        }
    }
}
<< frysk/sys/Wait.java >>
package frysk.sys;
public final class Wait
{
    public interface Observer
    {
    }
}
cagney@localhost$