public class Sensor {

  // 使用靜態(tài)變量記錄唯一的實(shí)例
  private static Sensor sensorInstance;

  /**
   * 私有的構(gòu)造方法
   */
  private Sensor(){}

  /**
   * 實(shí)例化方法
   * @return Sersor
   * synchronized包住不會(huì)有兩個(gè)線程同時(shí)進(jìn)入
   */
  public static synchronized Sensor getSersorInstance(){
    if(sensorInstance == null) {
       sensorInstance = new Sensor();
    }
    // 返回Sensor唯一實(shí)例
    return sensorInstance;
  }

}

public class Sensor {

  private static Sensor sensorInstance = new Sensor();

  private Sensor(){}

  public static Sensor getSersorInstance(){
    return sensorInstance;
  }
}

public class Sensor {

  // volatile 保證 sensorInstance 被初始化 多個(gè)線程正確的處理
  private volatile static Sensor sensorInstance;
  
  private Sensor(){}
  
  public static Sensor getSersorInstance(){
    // 檢查 sensorInstance是否存在 如果不存在就進(jìn)入同步區(qū)塊
    
    if(sensorInstance == null) {
      // 同步區(qū)塊里面的代碼只有在第一次才會(huì)執(zhí)行
      synchronized(Sensor.class) {
        if(sensorInstance == null) {
          sensorInstance = new Sensor();
        }
      }
    }
    return sensorInstance;
  }

}

Sensor sensor = Sensor.getSersorInstance(MainActivity.this);

public class Sensor {

    public static final Sensor SENSOR_INSTANCE = new Sensor();
    private List<MyListener> mListenerList;

    private Sensor() {
      mListenerList = new ArrayList<MyListener>();
    }

    public static Sensor getInstance() {
      return SENSOR_INSTANCE;
    }

    public void registerListener(MyListener listener) {
      if (!mListenerList.contains(listener)) {
        mListenerList.add(listener);
      }
    }
    public void unregisterListener(MyListener listener) {
      mListenerList.remove(listener);
    }
  }

  interface MyListener {
    public void onSomeThingHappen();
  }

public class MainActivity extends Activity {

    private MyListener mMyListener=new MyListener() {
      @Override
      public void onSomeThingHappen() {
      }
    };

    private Sensor sensor = Sensor.getInstance();
  
    @Override
    protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      sensor.registerListener(mMyListener);
    }
}

@Override
  protected void onDestroy() {
    Sensor.unregisterListener(mMyListener);
    super.onDestroy();
  }