code: String[] toArray() {
      String[] arr = new String[super.size()];
      Enumeration it = super.keys();
      int i = -1;
      while (it.hasMoreElements()) {
        String key = (String) it.nextElement();
        String val = (String) get(key);
        i++;
        arr[i] = key + "=" + val;
      }
      return arr;
    }
docs: to be used with Runtime.exec(String[] cmdarray, String[] envp)