NullPointerException当使用一个片段从另一个类中获取值

Question

我是一名Android初学者，在处理NullPointerException时遇到了一些问题。我需要将数组中的磁力计读取值从一个类传递到一个片段。 这里是磁力类：

public class Magnetometer implements SensorEventListener { 
private Sensor mag; 
private SensorManager magman; 
private boolean magAvailable; 
private float[] magValue; 

public Magnetometer(MagFragment context){ 
    magman=(SensorManager)context.getActivity().getSystemService(Context.SENSOR_SERVICE); 
    magAvailable=magman.getDefaultSensor(Sensor.TYPE_ACCELEROMETER)!=null; 
    if(isMagAvailable()){ 
     magValue=new float[3]; 
     mag=magman.getDefaultSensor(Sensor.TYPE_ACCELEROMETER); 
     magman.registerListener(this,mag,SensorManager.SENSOR_DELAY_NORMAL); 
    } 
} 

@Override 
public void onSensorChanged(SensorEvent event) { 
    for(int i=0;i<3;i++) 
     magValue[i]=event.values[i]; 
} 
public float[] getLastReading(){ 
    return this.magValue; 
} 
@Override 
public void onAccuracyChanged(Sensor sensor, int i) { 

} 
public boolean isMagAvailable(){ 
    return this.magAvailable; 
} 
public void unregister(){ 
    if(isMagAvailable()) 
     magman.unregisterListener(this,mag); 
} 
} 

这里的片段：

ublic class MagFragment extends Fragment { 
private Magnetometer mag; 
private TextView x,y,z; 
private float[] magValues; 
public MagFragment() { 
    // Required empty public constructor 
} 
@Nullable 
@Override 
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { 
    //returning our layout file 
    //change R.layout.yourlayoutfilename for each of your fragments 
    View rootView = inflater.inflate(R.layout.fragment_mag, container, false); 
    x=(TextView) rootView.findViewById(R.id.mx); 
    y=(TextView) rootView.findViewById(R.id.my); 
    z=(TextView) rootView.findViewById(R.id.mz); 
    magValues=new float[3]; 
    getData(this); 
    return rootView; 
} 


@Override 
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) { 
    super.onViewCreated(view, savedInstanceState); 
    //you can set the title for your toolbar here for different fragments different titles 
    getActivity().setTitle("Magnetometer"); 
} 
public void getData(MagFragment context) 
{ 
    magValues=mag.getLastReading(); 
    displayData(magValues); 
} 
public void displayData(float [] magV) 
{ 
    x.setText(String.valueOf(magV[0])); 
    y.setText(String.valueOf(magV[1])); 
    z.setText(String.valueOf(magV[2])); 
} 
} 

我明白了，该函数返回一些空值。 我得到的错误是“试图调用虚拟方法'浮动[] com.example.ark.ark.Sensors.Magnetometer.getLastReading（）'在一个空对象引用”

Answer 1

你从来没有创建一个Magnetometer的实例你的MagFragment。

ex：Magnetometer mag = new Magnetometer(this);