There is a code:

MainActivity.java:

package asus.example.com.fitnessapp; import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.view.Menu; import android.view.MenuInflater; import android.view.MenuItem; import android.widget.TextView; public class MainActivity extends AppCompatActivity { final TextView textView = findViewById(R.id.textView); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); } @Override public boolean onCreateOptionsMenu(Menu menu){ MenuInflater inflater = getMenuInflater(); inflater.inflate(R.menu.nav_items,menu); return true; } @Override public boolean onOptionsItemSelected(MenuItem item){ switch (item.getItemId()){ case R.id.home: textView.setText("Home"); return true; case R.id.notification: textView.setText("Notification"); return true; case R.id.profile: textView.setText("Profile"); return true; default: return super.onOptionsItemSelected(item); } } }

activity_main.xml:

 <?xml version="1.0" encoding="utf-8"?> <android.support.constraint.ConstraintLayout 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:layout_width="match_parent" android:layout_height="match_parent" tools:context=".MainActivity"> <android.support.design.widget.BottomNavigationView android:layout_width="match_parent" android:layout_height="52dp" app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintStart_toStartOf="parent" app:menu="@menu/nav_items"/> <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/textView"/> </android.support.constraint.ConstraintLayout>

nav_items.xml:

 <?xml version="1.0" encoding="utf-8"?> <menu xmlns:android="http://schemas.android.com/apk/res/android"> <item android:id="@+id/home" android:title="@string/home" android:icon="@drawable/home" /> <item android:id="@+id/notification" android:title="@string/notifications" android:icon="@drawable/notification" /> <item android:id="@+id/profile" android:icon="@drawable/person" android:title="@string/profile" /> </menu>

UPDATE

AndroidManifest.xml:

 <?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="asus.example.com.fitnessapp"> <application android:allowBackup="true" android:icon="@mipmap/ic_launcher" android:label="@string/app_name" android:roundIcon="@mipmap/ic_launcher_round" android:supportsRtl="true" android:theme="@style/AppTheme"> <activity android:name=".MainActivity"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> </application> </manifest>

When executed, it gives an error:

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo {asus.example.com.fitnessapp / asus.example.com.fitnessapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method android.view.Window $ Callback android.view.Window.getCallback () 'on a null object reference at android.app.ActivityThread.performLaunchActivity (ActivityThread.java:2843) at android.app.ActivityThread.handleLaunchActivity (ActivityThread.java:3048) at android.app. servertransaction.LaunchActivityItem.execute (LaunchActivityItem.java:78) at android.app.servertransaction.TransactionExecutor.executeCallbacks (TransactionExecutor.java:108) at android.app.servertransaction.TransactionExecutor.execute (TransactionExecutor.r.apprn.servicetransaction.TransactionExecutor.execute (TransactionExecutor.ologa.appaserser.java:108) .ActivityThread $ H.handleMessage (ActivityThread.java:1808) at android.os.Handler.dispatchMessage (Handler.java:106) at android.os.Looper.loop (Looper.java:193) at android.app.ActivityThread. main (ActivityThread.java:6669) at java.lang.reflect.Method.invoke (Native Method) at com.android. .NullPointerException: Attempt to invoke virtual method "android.view.Window $ Callback android.view.Window.getCallback ()" android.support. AppCompatActivity.java:191) at asus.example.com.fitnessapp.MainActivity. (MainActivity.java:11) at java.lang. ) at android.support.v4.app.CoreComponentFactory.instantiateActivity (CoreComponentFactory.java:43) at android.app.Instrumentation.newActivity (I nstrumentation.java:1215) at android.app.ActivityThread.performLaunchActivity (ActivityThread.java:2831)

What is the problem and how to solve?

    2 answers 2

    Get a link to the view in the onCreate method

     final TextView textView; @Override protected void onCreate(Bundle savedInstanceState { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); textView = findViewById(R.id.textView); }
    • Does not help. You have at least an error that final is not initialized. And what's the difference where to get the link in the method or globally in the class? - Sergei Mikhailovskii
    • Then show the manifest code - McDaggen 6:55 pm
    • Added manifesto - Sergei Mikhailovskii
    • Hmm, it seems everything is in order in the code, replace final with private, the reason for the reason could be only in an attempt to initialize the view before initializing the markup. If final> private replacement does not help, try disabling InstantRun and also do Build> Rebuild & Clean - McDaggen
    • Yes, apparently this is some kind of lag in the studio. After Clean, everything started fine. Only for some reason does not respond to pressing the menu. According to the logic, the text should be added to the textview, but in fact there is no - Sergei Mikhailovskii

    documentation

    It says that findViewById after onCreate () should be activated.

     private TextView textView; @Override protected void onCreate(Bundle savedInstanceState { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); textView = findViewById(R.id.textView); }