無法設置 TextView 的文本

Question

我想將TextView的文本更改為活動用戶的任何用戶名，但方法updateUI()始終返回此錯誤：

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)

所以有什么問題？ 在我看來好像找不到TextView ，因此為空。 但為什么？ 我該如何解決？ 您可以在下面找到帶有onCreate和updateUI方法的MainActivity類以及我使用的片段。

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();
        }
    }

而updateUI函數如下。

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

這是我的片段的布局，即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>

我的活動的布局是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

你在 MainActivity 中找不到你的 textView 因為它不在 main_activity.xml 中，所以 updateUI 應該在 UserProfile Fragment 中。

在OnCreateView

嘗試這個 ：

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

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

Answer 2

你在哪里膨脹 fragment_assignments.xml？ 在這一行中，您膨脹 main_activity.xml

setContentView(R.layout.activity_main);

但是，textview_assignemntsheader 位於另一個 XML 文件中，無法找到它並返回 null。 findViewById 將僅在膨脹視圖中搜索 ID。 將 UpdateUI 函數移動到 UserProfile（其中 fragment_assignments.xml 被膨脹）

Answer 3

在您的MainActivity您正在膨脹沒有TextView的activity_main布局。 因此，您在嘗試查找TextView並為其分配一些文本時遇到NullPointerException 。

在MainActivity類的onCreate方法中，嘗試將文本設置為不存在或未膨脹的TextView 。 fragment_assignments.xml布局可能會在您的片段中膨脹，因此您會在片段中找到布局引用。

看起來您想將身份驗證信息傳遞給您的片段並在那里設置一些值。 在這種情況下，您可以在執行片段事務時使用包傳遞信息。

因此，您的活動中的片段交易可能具有以下功能。

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();
}

現在從你的片段的onCreateView ，你可以做這樣的事情。

@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;
}

最后，按如下方式更新MainActivity的onCreate 。

@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?
    }
}

我希望這有幫助！