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.
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Answer
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.