import java.util.jar.*;

static LS classNamesInJarOrDir(File dir, S prefixInJar default "") {
  new LS classes;
  if (dir == null) {}
  else if (dir.isDirectory()) {
    for (File f : listFiles(dir)) { // TODO: subdirectories
      S s = f.getName();
      if (s.endsWith(".class"))
        classes.add(dropSuffix(".class", s));
    }
  } else if (dir.isFile()) pcall {
    JarFile jarFile = new JarFile(dir);
    try {
      Enumeration<JarEntry> e = jarFile.entries();
      while (e.hasMoreElements()) {
        JarEntry je = e.nextElement();
        if (je.isDirectory()
          || je.getName().startsWith("META-INF/")
          || !je.getName().endsWith(".class"))
          continue;
        S className = dropSuffix(".class", je.getName());
        className = dropPrefixOrNull(prefixInJar, className);
        if (className == null) continue;
        if (className.contains("-")) continue;
        className = className.replace('/', '.');
        classes.add(className);
      }
    } finally {
      jarFile.close();
    }
  }
  ret classes;
}

Began life as a copy of #1006696