code: @Override
    public synchronized Integer next() {
      if (availablePorts.isEmpty()) {
        return null;
      }
      Collections.sort(availablePorts);
      Integer port = availablePorts.get(0);
      portsInUse.add(port);
      availablePorts.remove(port);
      return port;
    }
docs: /* (non-Javadoc)
  @see io.selendroid.server.model.impl.EmulatorPortFinder#next()