A "factorial" module

build.sh

mkdir -p sources/fact/fact
mkdir modules
# edit the files!
javac -d modules -modulepath modules \
      -sourcepath sources \
      sources/fact/module-info.java \
      sources/fact/fact/Factorial.java

Fact.java

package fact;

public class Factorial {
  public static int factorial(int n) {
    if (n <= 0) { return 1; }
    else        { return n * factorial(n - 1); }
  }
}

module-info.java

module fact @1.0 { }