Android, change fonts on a final text view-Collection of common programming errors

I want to change the font on a text view But the tv is modified at the pression of a button, and i define “final” the tv, and the app crashed… what i can do it?

Typeface tf = Typeface.createFromAsset(getAssets(), "fonts/HeroQuest.ttf");
final TextView tv = (TextView) findViewById(R.id.textView1);
    tv.setTypeface(tf);

This code work if tv is not final