Attempt to invoke virtual method null object reference - nullpointerexception

Question

Main Activty:
TextView tvDescription;
ArrayList<String> descriptions;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tvDescription = (TextView)findViewById(R.id.tvDescription);
descriptions = new ArrayList<>();
descriptions.add("Description for item 1");
descriptions.add("Description for item 2");
descriptions.add("Description for item 3");
}
#Override
public void onItemSelected(int index) {
tvDescription.setText(descriptions.get(index));
}
}
activity_main.xml:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
tools:context=".MainActivity">
<androidx.fragment.app.FragmentContainerView
android:id="#+id/listFrag"
android:name="com.example.fragmentsvideo2018.ListFrag"
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="1"
tools:layout="#layout/fragment_list" />
<androidx.fragment.app.FragmentContainerView
android:id="#+id/detailFrag"
android:name="com.example.fragmentsvideo2018.DetailFrag"
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="2"
tools:layout="#layout/fragment_detail" />
</LinearLayout>
ListFrag:
ItemSelected activity;
public interface ItemSelected
{
void onItemSelected (int index);
}
public ListFrag() {
// Required empty public constructor
}
#Override
public void onAttach(#NonNull Context context) {
super.onAttach(context);
activity = (ItemSelected) context;
}
#Override
public void onActivityCreated(#Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
ArrayList<String> data = new ArrayList<String>();
data.add("1.This is item 1");
data.add("2.This is item 2");
data.add("3.This is item 3");
setListAdapter(new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, data));
activity.onItemSelected(0);
}
#Override
public void onListItemClick(#NonNull ListView l, #NonNull View v, int position, long id) {
activity.onItemSelected(position);
}
}
fragment_list.xml:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
tools:context=".ListFrag">
<ListView
android:id="#+id/lvLIst"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
Detail Frag:
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_detail, container, false);
}
}
fragment_deal.xml:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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="#color/design_default_color_primary"
android:orientation="vertical"
tools:context=".DetailFrag">
<TextView
android:id="#+id/tvDescription"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginLeft="20dp"
android:layout_marginTop="20dp"
android:layout_marginRight="20dp"
android:text="#string/textview"
android:textColor="#FFFFFF"
android:textSize="18sp" />
</LinearLayout>
Error:
2021-09-11 16:27:03.357 29251-29251/com.example.fragmentsvideo2018 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.fragmentsvideo2018, PID: 29251
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at com.example.fragmentsvideo2018.MainActivity.onItemSelected(MainActivity.java:30)
at com.example.fragmentsvideo2018.ListFrag.onActivityCreated(ListFrag.java:52)
at androidx.fragment.app.Fragment.performActivityCreated(Fragment.java:2996)
at androidx.fragment.app.FragmentStateManager.activityCreated(FragmentStateManager.java:580)
at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:285)
at androidx.fragment.app.FragmentStore.moveToExpectedState(FragmentStore.java:112)
at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1647)
at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:3128)
at androidx.fragment.app.FragmentManager.dispatchActivityCreated(FragmentManager.java:3072)
at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:251)
at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:502)
at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:246)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1435)
at android.app.Activity.performStart(Activity.java:8024)
at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3475)
at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:221)
at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:201)
at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:173)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
I tried to search in this site and other but I didn't find the solution.
Maybe I'm doing something wrong because I'm only a beginner.
Please help me!!

Answer 1

As far as i can tell from the code you provided, the issue lies in the xml files. You try to get the TextView via findViewById() from within ActivityMain.java, but the TextView is declared in fragment_deal.xlm. As far as i know findViewById only finds components declared in the xml file associated with the java file calling.