NullPointerrException when calling addView(). How to fix?

Question

Android

XML:

<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"
    tools:context=".GameActivity">

    <FrameLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:layout_centerHorizontal="true"
        android:layout_alignParentTop="true"
        android:id="@+id/container">


    </FrameLayout>
</RelativeLayout>

game activity:

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.game);
        Field field = new Field(this);
    }

field:

public class Field extends View {
    private Context context;
    public Field(Context context) {
        super(context);
        this.context = context;
        createButton();
    }

    public void createButton()
    {
        FrameLayout container = (FrameLayout) findViewById(R.id.container);
        Button b = new Button(context);
        b.setText("123");
        b.setHeight(25);
        container.addView(b); //NullPointerrException
    }
}

What am I doing wrong?

Answer 1

Everything is not so done.
1 - findViewById searches within the local layout and not at the ancestor. And the field has no layout at all.
2. The createButton code is generally heresy. The child view should not add itself to the container itself. As well as keep track of your sizes. The layout type is responsible for this.
developer.android.com/training/custom-views/index.html read this until you are enlightened.