What to do about NullPointerException when trying to grab RelativeLayout and add views to it?

[Eric Sweeten]

I'm trying to grab the RelativeLayout that's there by default and add one single view to it - an AdView. I'm making a game and otherwise don't deal with Views. This is my code:

RelativeLayout rLayout = (RelativeLayout) app.findViewById(R.id.relativeLayout);
adView = new AdView(app);
adView.setAdSize(AdSize.SMART_BANNER);
String test = "ca-app-pub-3940256099942544/6300978111";
adView.setAdUnitId(test);
RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(RelativeLayout.LayoutParams.WRAP_CONTENT, RelativeLayout.LayoutParams.WRAP_CONTENT);
params.addRule(RelativeLayout.BELOW);
rLayout.setLayoutParams(params);
adView.setLayoutParams(params);
rLayout.addView(adView);

It's throwing a NullPointerException as soon as I attempt to access the rLayout. It says:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.setLayoutParams(android.view.ViewGroup$LayoutParams)' on a null object reference

[Paul Clapham]

The message is even clearer than NullPointerException messages given by Java: It clearly says that rLayout is null.

And your code assigns the result of the findViewById method to rLayout, so clearly that result is null. So you should look into why app.findViewById() is returning null. Without knowing anything about your application I would guess that the method is looking up views based on an ID, and the ID you passed to the method doesn't have any corresponding view.

[Eric Sweeten]

The R.id.relativeLayout is referring to the only RelativeLayout in the xml file. Here is my xml file:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".main.GameActivity"
    android:id="@+id/relativeLayout">

</RelativeLayout>

I don't understand why it's returning null.