Android - 传感器

Question

我正在使用传感器，我在网站上找到了这段代码。我理解大部分代码，但我不了解浮动速度= Math.abs的计算。有人可以解释一下吗？此外，我对last_x，last_y和last_z有点困惑。

public class MainActivity extends AppCompatActivity implements 
SensorEventListener {

//ACCELEROMETER
private Sensor mAccelerometer;
private SensorManager mSensorManager;
private SensorEventListener mSensorListener = this;
private long lastUpdate = 0;
private float last_x, last_y, last_z;
private static final int shakeSensitivity = 600;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mSensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
    mAccelerometer = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
    mSensorManager.registerListener(this, mAccelerometer, SensorManager.SENSOR_DELAY_NORMAL);

}

@Override
public void onSensorChanged(SensorEvent e) {
    float x = e.values[0];
    float y = e.values[1];
    float z = e.values[2];

    //Find current time
    long currentTime = System.currentTimeMillis();

    //Check if more than 100 milliseconds (0,1 sec) has passed since last invokment of this onSensorChanged method
    if ((currentTime - lastUpdate) > 100) {
        long diffTime = (currentTime - lastUpdate);
        lastUpdate = currentTime;

        float speed = Math.abs(x + y + z - last_x - last_y - last_z) / diffTime * 10000;

        if (speed > shakeSensitivity) {
           //do something
        }

        last_x = x;
        last_y = y;
        last_z = z;
    }

}