无法设置 TextView 的文本

Question

I wanted to have the Text of a TextView changed to whatever the username of the active user is, but the method updateUI() always returns this error:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.cocko.industries.einkaufshelfer.shopper, PID: 18704
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.cocko.industries.einkaufshelfer.shopper/com.cocko.industries.einkaufshelfer.shopper.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
    at android.app.ActivityThread.-wrap11(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
    at com.cocko.industries.einkaufshelfer.shopper.MainActivity.updateUI(MainActivity.java:203)
    at com.cocko.industries.einkaufshelfer.shopper.MainActivity.onCreate(MainActivity.java:76)
    at android.app.Activity.performCreate(Activity.java:6237)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
    at android.app.ActivityThread.-wrap11(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:148) 
    at android.app.ActivityThread.main(ActivityThread.java:5417) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

So what's the problem? It seems to me as if the TextView can't be found and is therefore null. But why? And how can I fix that? Below you can find the MainActivity class with the onCreate and updateUI method and the fragment that I used.

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
    private DrawerLayout drawer;
    private NavigationView nav_view;
    private FirebaseAuth mAuth;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        mAuth = FirebaseAuth.getInstance();

        nav_view = findViewById(R.id.nav_view);
        nav_view.setNavigationItemSelectedListener(this);

        drawer = findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.nav_drawer_open, R.string.nav_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();

        try {
            Intent intent = getIntent();
            if (intent.getStringExtra("activity").equals("register")) {
                getSupportFragmentManager().beginTransaction().replace(R.id.framgment_container, new UserprofileFragment()).commit();
            }
        } catch (NullPointerException noIntentExtra) {
            getSupportFragmentManager().beginTransaction().replace(R.id.framgment_container, new AssignmentsFragment()).commit();
        }

        if (mAuth.getCurrentUser() != null){
            updateUI();
        }
    }

And the updateUI function is as follows.

public void updateUI() {
    TextView textView = findViewById(R.id.textview_assignemntsheader);
    textView.setText("Test");
}

Here's the layout for my fragment, that is fragment_assignments.xml :

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/textview_assignemntsheader"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:layout_marginEnd="8dp"
        android:text="@string/textview_assigmentsHeader"
        android:textAlignment="center"
        android:textAppearance="@style/TextAppearance.AppCompat.Display1"
        android:textStyle="bold"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <ScrollView
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="16dp"
        android:layout_marginEnd="8dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textview_assignemntsheader">

        <LinearLayout
            android:id="@+id/linearlayout_assignments"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical">

        </LinearLayout>
    </ScrollView>


</androidx.constraintlayout.widget.ConstraintLayout>

And the layout for my activity is activity_main.xml :

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context=".MainActivity"
    tools:openDrawer="start">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <androidx.appcompat.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="@color/colorPrimary"
            android:elevation="4dp"
            android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
            app:popupTheme="@style/ThemeOverlay.AppCompat.Light" />

        <FrameLayout
            android:id="@+id/framgment_container"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

    </LinearLayout>

    <com.google.android.material.navigation.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        app:headerLayout="@layout/nav_header"
        app:menu="@menu/menu" />
</androidx.drawerlayout.widget.DrawerLayout>

Answer 1

you can't find your textView in MainActivity because it is not in the main_activity.xml, so updateUI should be in UserProfile Fragment.

in OnCreateView

try this :

View view = inflater.inflate(R.layout.fragment_assignments, container,false);

TextView text=view.findViewById(R.id.textview_assignemntsheader);

Answer 2

Where do you inflate fragment_assignments.xml? in this line, you inflate main_activity.xml

setContentView(R.layout.activity_main);

But, textview_assignemntsheader is in another XML file and can`t find it and returns null. findViewById will search for ID only in the inflated view. Move UpdateUI function to UserProfile (where fragment_assignments.xml is inflated)

Answer 3

In your MainActivity you are inflating the activity_main layout which does not have the TextView . Hence you are getting the NullPointerException while trying to find the TextView and assign some text to it.

In the onCreate method of your MainActivity class tries to set a text to a TextView which is not there or not inflated. The fragment_assignments.xml layout might be inflated in your fragment, and hence you will find the layout reference there in your fragment.

Looks like you want to pass the auth information to your fragment and set some value there. In that case, you can just pass the information using a bundle while doing the fragment transaction.

Hence, you might have the following function for your fragment transaction in your activity.

public void launchAssignmentFragment() {

    AssignmentFragment frag = new AssignmentFragment();
    Bundle bundle = new Bundle();

    if (mAuth.getCurrentUser() != null) {
        bundle.putBoolean("auth", true);
    } else {
        bundle.putBoolean("auth", false);
    }

    // set Arguments to be passed to the fragment 
    frag.setArguments(bundle);

    // Now load the fragment 
    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

    transaction.replace(R.id.fragment_container, frag);
    transaction.addToBackStack(null);

    transaction.commit();
}

Now from the onCreateView of your fragment, you can do something like this.

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout. fragment_assignments, parent, false);
    TextView tv = rootView.findViewById(R.id.textview_assignemntsheader);

    // Get the passed-in value here
    boolean auth = getArguments().getBoolean("auth");

    if (auth) tv.setText("Success");
    else tv.setText("Fail");

    return rootView;
}

And finally, update your onCreate in the MainActivity as follows.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    mAuth = FirebaseAuth.getInstance();

    nav_view = findViewById(R.id.nav_view);
    nav_view.setNavigationItemSelectedListener(this);

    drawer = findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.nav_drawer_open, R.string.nav_drawer_close);
    drawer.addDrawerListener(toggle);
    toggle.syncState();

    try {
        Intent intent = getIntent();
        if (intent.getStringExtra("activity").equals("register")) {
            getSupportFragmentManager().beginTransaction().replace(R.id.framgment_container, new UserprofileFragment()).commit();
        }
    } catch (NullPointerException noIntentExtra) {
        getSupportFragmentManager().beginTransaction().replace(R.id.framgment_container, new AssignmentsFragment()).commit();
    }

    if (mAuth.getCurrentUser() != null){
        launchAssignmentFragment(); // launch the fragment maybe?
    }
}

I hope that helps!