Android 传感器返回 null？

Question

The Android Java app that I am writing has a simple principle of operation: it reads data from sensors and displays it in a TextView. The first version of the application worked without any problems, but I wanted to improve it and work on the design. I decided to add a slideout side menu and that's when the problems started. When trying to start in the emulator, an error was thrown that the application was stopped. There was a bug in the logcat (below) above related to TexView that wasn't there.

MainActivity.java

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener, SensorEventListener {


    private static final String TAG = "MainActivity";
    private SensorManager sensorManager;
    private Sensor accelerometer, mGyro, mMagno, mLight, mPressure, mTemp, mHumi;
    TextView xAccValue, yAccValue, zAccValue, xGyroValue, yGyroValue, zGyroValue, xMagnoValue, yMagnoValue, zMagnoValue, light, pressure, temp, humi;

    private DrawerLayout drawer;
    private Object savedInstanceState;


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

        SensorHandler();
        MenuHandler();
    }


    public void SensorHandler() {


        xAccValue = (TextView) findViewById(R.id.AccValueX);
        yAccValue = (TextView) findViewById(R.id.AccValueY);
        zAccValue = (TextView) findViewById(R.id.AccValueZ);

        xGyroValue = (TextView) findViewById(R.id.GyroValueX);
        yGyroValue = (TextView) findViewById(R.id.GyroValueY);
        zGyroValue = (TextView) findViewById(R.id.GyroValueZ);

        xMagnoValue = (TextView) findViewById(R.id.MagnoValueX);
        yMagnoValue = (TextView) findViewById(R.id.MagnoValueY);
        zMagnoValue = (TextView) findViewById(R.id.MagnoValueZ);

        light = (TextView) findViewById(R.id.LightValue);
        pressure = (TextView) findViewById(R.id.PressureValue);
        temp = (TextView) findViewById(R.id.TempSensor);
        humi = (TextView) findViewById(R.id.HumiValue);

        Log.d(TAG, "onCreate: Initializing Sensor Services");
        sensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);

        accelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        if (accelerometer != null) {

            sensorManager.registerListener(MainActivity.this, accelerometer, SensorManager.SENSOR_DELAY_NORMAL);
            Log.d(TAG, "onCreate: Registered accelerometer listener");
        } else {
            xAccValue.setText("Accelerometer not supported");
            yAccValue.setText("Accelerometer not supported");
            zAccValue.setText("Accelerometer not supported");
        }

        mGyro = sensorManager.getDefaultSensor(Sensor.TYPE_GYROSCOPE);
        if (mGyro != null) {

            sensorManager.registerListener(MainActivity.this, mGyro, SensorManager.SENSOR_DELAY_NORMAL);
            Log.d(TAG, "onCreate: Registered Gyro listener");
        } else {
            xGyroValue.setText("Gyroscope not supported");
            yGyroValue.setText("Gyroscope not supported");
            zGyroValue.setText("Gyroscope not supported");
        }

        mMagno = sensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD);
        if (mMagno != null) {

            sensorManager.registerListener(MainActivity.this, mMagno, SensorManager.SENSOR_DELAY_NORMAL);
            Log.d(TAG, "onCreate: Registered Magno listener");
        } else {
            xMagnoValue.setText("Magno not supported");
            yMagnoValue.setText("Magno not supported");
            zMagnoValue.setText("Magno not supported");
        }

        mLight = sensorManager.getDefaultSensor(Sensor.TYPE_LIGHT);
        if (mLight != null) {

            sensorManager.registerListener(MainActivity.this, mLight, SensorManager.SENSOR_DELAY_NORMAL);
            Log.d(TAG, "onCreate: Registered Light listener");
        } else {
            light.setText("Light not supported");
        }

        mPressure = sensorManager.getDefaultSensor(Sensor.TYPE_PRESSURE);
        if (mPressure != null) {

            sensorManager.registerListener(MainActivity.this, mPressure, SensorManager.SENSOR_DELAY_NORMAL);
            Log.d(TAG, "onCreate: Registered Pressure listener");
        } else {
            pressure.setText("Pressure not supported");
        }

        mTemp = sensorManager.getDefaultSensor(Sensor.TYPE_AMBIENT_TEMPERATURE);
        if (mTemp != null) {
            sensorManager.registerListener(MainActivity.this, mTemp, SensorManager.SENSOR_DELAY_NORMAL);
            Log.d(TAG, "onCreate: Registered Temp listener");
        } else {
            temp.setText("Pressure not supported");
        }

        mHumi = sensorManager.getDefaultSensor(Sensor.TYPE_RELATIVE_HUMIDITY);
        if (mHumi != null) {
            sensorManager.registerListener(MainActivity.this, mHumi, SensorManager.SENSOR_DELAY_NORMAL);
            Log.d(TAG, "onCreate: Registered Humi listener");
        } else {
            humi.setText("Humi not supported");
        }
    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int i) {

    }

    @Override
    public void onSensorChanged(@NotNull SensorEvent sensorEvent) {
        Sensor sensor = sensorEvent.sensor;
        if (sensor.getType() == Sensor.TYPE_ACCELEROMETER) {
            Log.d(TAG, "onSensorChanged: X: " + sensorEvent.values[0] + "Y: " + sensorEvent.values[1] + "Z: " + sensorEvent.values[2]);

            xAccValue.setText("xAccValue: " + sensorEvent.values[0]);
            yAccValue.setText("yAccValue: " + sensorEvent.values[1]);
            zAccValue.setText("zAccValue: " + sensorEvent.values[2]);

        } else if (sensor.getType() == Sensor.TYPE_GYROSCOPE) {
            xGyroValue.setText("xGyroValue: " + sensorEvent.values[0]);
            yGyroValue.setText("yGyroValue: " + sensorEvent.values[1]);
            zGyroValue.setText("zGyroValue: " + sensorEvent.values[2]);

        } else if (sensor.getType() == Sensor.TYPE_MAGNETIC_FIELD) {
            xMagnoValue.setText("xMagnoValue: " + sensorEvent.values[0]);
            yMagnoValue.setText("yMagnoValue: " + sensorEvent.values[1]);
            zMagnoValue.setText("zMagnoValue: " + sensorEvent.values[2]);
        } else if (sensor.getType() == Sensor.TYPE_LIGHT) {
            light.setText("Light: " + sensorEvent.values[0]);
        } else if (sensor.getType() == Sensor.TYPE_PRESSURE) {
            pressure.setText("Pressure: " + sensorEvent.values[0]);
        } else if (sensor.getType() == Sensor.TYPE_AMBIENT_TEMPERATURE) {
            temp.setText("Temp: " + sensorEvent.values[0]);
        } else if (sensor.getType() == Sensor.TYPE_RELATIVE_HUMIDITY) {
            humi.setText("Humi: " + sensorEvent.values[0]);
        }
    }

    public void MenuHandler() {
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        drawer = findViewById(R.id.drawer_layout);
        NavigationView navigationView = findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);

        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();

        if (savedInstanceState == null) {

            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new AccFragment()).commit();
            navigationView.setCheckedItem(R.id.nav_view);
        }
    }

    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
        switch (menuItem.getItemId()) {
            case R.id.nav_acc:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new AccFragment()).commit();
                break;
            case R.id.nav_gyro:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new GyroFragment()).commit();
                break;
            case R.id.nav_magno:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new MagnoFragment()).commit();
                break;
            case R.id.nav_light:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new LightFragment()).commit();
                break;
            case R.id.nav_pressure:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new PressureFragment()).commit();
                break;
            case R.id.nav_temp:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new TempFragment()).commit();
                break;
            case R.id.nav_humi:
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new HumiFragment()).commit();
                break;
        }

        drawer.closeDrawer(GravityCompat.START);

        return true;
    }

    @Override
    public void onBackPressed() {
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }

    }

}

fragment_acc.xml where I try pass value

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#ffe135">



        <TextView
            android:id="@+id/AccValueX"
            android:layout_width="200sp"
            android:layout_height="wrap_content"
            android:text="@string/accelerometer"
            android:layout_centerHorizontal="true"
            android:textSize="30sp" />

        <TextView
            android:id="@+id/AccValueY"
            android:layout_width="200sp"
            android:layout_height="wrap_content"
            android:layout_below="@id/AccValueX"
            android:text="@string/accelerometer"
            android:layout_centerInParent="true"
            android:textSize="30sp" />

        <TextView
            android:id="@+id/AccValueZ"
            android:layout_width="200sp"
            android:layout_height="wrap_content"
            android:layout_below="@id/AccValueY"
            android:text="@string/accelerometer"
            android:layout_centerInParent="true"
            android:textSize="30sp" />


</RelativeLayout>

LogCat Error

2020-08-16 11:13:54.853 24668-24668/com.example.sensapp_v11 E/SensorManager: Exception dispatching input event.
2020-08-16 11:13:54.854 24668-24668/com.example.sensapp_v11 E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.sensapp_v11, PID: 24668
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
        at com.example.sensapp_v11.MainActivity.onSensorChanged(MainActivity.java:155)
        at android.hardware.SystemSensorManager$SensorEventQueue.dispatchSensorEvent(SystemSensorManager.java:699)
        at android.os.MessageQueue.nativePollOnce(Native Method)
        at android.os.MessageQueue.next(MessageQueue.java:323)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:6077)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)

I see this is the problem in line 155:

    xAccValue.setText("xAccValue: " + sensorEvent.values[0]);

It seems sensorEvent return null. The question is why? The first version of the app without additional fragments work perfectly.

Answer 1

First, you need to look at the error.

attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference

It means that you called the method ON a null thing, not WITH a null parameter. Therefore the problem is the TextView xAccValue, which is null.

From what I've seen, it looks like you put the TextView in a Fragment , in that case you need to reference (and set its text) from the Fragment 's class.

You cannot reference Views in your Fragment from MainActivity.