public double[] toArray(double[] dest, int source_pos, int dest_pos, int len) {
   synchronized (mutex) {
     return list.toArray(dest, source_pos, dest_pos, len);
   }
 }
 public double[] toArray(double[] dest, int offset, int len) {
   synchronized (mutex) {
     return list.toArray(dest, offset, len);
   }
 }