NullPointerException when using setText to set TextView (defined after setContentView(); object exits in .xml file)

Question

I have checked the posts. And here is the checklist of what I have tried

1. Make sure that a TextView by that name exists
2. That all functions are called after setContentView() in onCreate()

It still continues to baffle me. Your help your be appreciated

Please note that this activity (for which the code is given) is trigger from a parent (as given in the Android Manifest snippet below).

The java code for the activity is: (This code has been changed to simplify the question)

public class MainActivity extends ActionBarActivity {

private TextView tDummy;

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    tDummy=(TextView)findViewById(R.id.textViewDummy);
    tDummy.setText("Testing.");

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
}
...
    public void changeMsg(View view){
    TextView dummy=(TextView)findViewById(R.id.textViewDummy);
    dummy.setText("Changed!");
}

Why am I still getting a null pointer exception?! It works fine when I click the button (which invokes the changeMsg function), but that just circumvents the problem.

I am using android studio.

XML file (There are two: activity_main.xml and fragment_main.xml) fragment_main.xml is as follows:

<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:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="mk.app.MainActivity$PlaceholderFragment">

<TextView
    android:text="@string/hello_world"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/textViewDummy" />
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textViewDummy"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true"
    android:layout_marginRight="42dp"
    android:layout_marginTop="26dp"
    android:onClick="changeMsg"/>

</RelativeLayout>

main_activity.xml is:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="mk.app.MainActivity"
tools:ignore="MergeRootFrame" />

Could it be an issue with having using the fragment layout? As the content is set to activity_main, but the definitions are in fragment_main.xml??

Answer 1

Just so this question has an answer, you need to make sure that your variables get defined in the layout that you have set in your onCreate method, in this case setContentView(R.layout.activity_main.xml); Since you have set your activity_main.xml as your content view for your Activity, you need to make sure that layout actually contains the views and id's you are trying to use. All you need to do is move the code from your fragment layout to your main layout and vice versa. Reason being, if a particular layout is not set as your content view, the content from that layout hasn't actually been loaded into your Activity to be referenced to.

If you are really intent on using the fragment, simply change this:

tDummy=(TextView)findViewById(R.id.textViewDummy);
tDummy.setText("Testing.");

if (savedInstanceState == null) {
    getSupportFragmentManager().beginTransaction()
            .add(R.id.container, new PlaceholderFragment())
            .commit();
}

to

if (savedInstanceState == null) {
    getSupportFragmentManager().beginTransaction()
            .add(R.id.container, new PlaceholderFragment())
            .commit();
}

tDummy=(TextView)findViewById(R.id.textViewDummy);
tDummy.setText("Testing.");

And it should work, since now your fragment has been loaded into your container