When does findViewById() return null?

I have this seemingly innocent findViewById() in my onCreate() which keeps returning null:

mCheckLicenseButton = (Button) findViewById(R.id.btn_checkout);

My res/layout/main.xml clearnly contains this button:

<Button android:id="@+id/btn_checkout" 
        android:orientation="vertical"
        android:layout_width="wrap_content"
        android:layout_height="w开发者_开发百科rap_content" 
        android:layout_below="@+id/top_textview"
    android:layout_alignParentRight="true"
    android:text="Check License" />

So I don't understand why findViewById() returns null.

In what circumstances does findViewById() return null?

Obviously, findViewById() doesn't succeed in finding the layout resource but what could possibly explain this?

---

I'm going to go out on a limb and say that you did not call setContentView(R.layout.main) before calling that, which you must do.

---

You need to call setContentView before:

setContentView(R.layout.main);