You can but do you really want to?

If you want to execute a file called Foo.class directly from the command
line, you automatically execute main() first (assuming Foo is an
application, not an applet). For example:

java Foo

will always execute the main() method first.

If you invoke Foo from a class called Bar, it will look like this:

=======================
public class Bar {

   Foo myFoo = new Foo();
}
========================

but the method called will be the appropriate Foo constructor (the one that
takes no parameters in this example) and not main().

It would be rather unusual to want to invoke the main() method of Foo from
Bar. Invoking some other method of Foo wouldn't be that unusual but main()
is typically only invoked by running Foo from the command line.

However, you could probably invoke main() via reflection if you really
wanted to. I believe it is somewhat expensive but it should be possible. I
don't have a link to a tutorial on Reflection handy but you should be able
to find one via Google if you really want to execute Foo's main from another
class.

Rhino

// Yes.

import java.io.PrintWriter;
import java.lang.reflect.Method;
import java.lang.reflect.InvocationTargetException;

public class ClassMain {

        static PrintWriter err = new PrintWriter(System.err, true);

        public static void main(String[] args) {
                callMain(Helper.class, args);
        }

        static void callMain(Class c, Object args) {
                try {
                        Method m = c.getDeclaredMethod(
                                        "main", String[].class);

                        m.invoke(null, args);

                } catch(NoSuchMethodException x) {
                        err.println(x);
                } catch(IllegalAccessException x) {
                        err.println(x);
                } catch(InvocationTargetException x) {
                        err.println(x);
                }
        }
}

class Helper {

        static PrintWriter out = new PrintWriter(System.out, true);

        public static void main(String[] args) {
                out.println("Hello from Helper.");
        }
}

of course.  Most of the time you don't get source from the authors.