public class StereoController {
  private Volume volume;
  private Equalizer equalizer;
  public StereoController(Volume volume,
                          Equalizer equalizer) {
    this.volume = volume;
    this.equalizer = equalizer;
  }
  public void changeVolume(int ticks) {
    volume.adjust(ticks);
  }
  public void changeChannel(int channel,
                            int ticks) {
    if ( equalizer != null ) {
      equalizer.getChannel(channel).adjust(ticks);
    }
  }
}