package myoven.powertube;public class PowerTube {  private static PowerTube instance;  private boolean isOn;//標志其工作狀態isOn==true為處于工作加熱狀態，isOn=false為不工作狀態  public static PowerTube getInstance(){    if(instance==null)      instance=new PowerTube();    return instance;//返回唯一實例  }  public boolean getIsOn(){    return isOn;  }  public PowerTube() {    isOn=false;//初始默認為非工作狀態  }  public void turnOn(){    isOn=true;  }  public void turnOff(){    isOn=false;  }}