如何使用SharedPreferences更改字体

Question

通过使用此代码，我想更改textview的字体，但是该程序无法正常工作。
我该做什么？

public class MainActivity extends Activity {
SharedPreferences shared = getSharedPreferences("Prefs", MODE_PRIVATE);
SharedPreferences.Editor editor = shared.edit();
public String fonts = shared.getString("fonts", "BHOMA.ttf");
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    setFace();
}
protected void setFace()
{
    TextView txt1 = (TextView)findViewById(R.id.textView1);
     Typeface face = Typeface.createFromAsset(getAssets(), "font/"+fonts+"");
     txt1.setTypeface(face);
}

}

和在prefs.class

公共类Prefs扩展了PreferenceActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);

        String fonts = prefs.getString("fonts", "Tahoma.ttf");
        Boolean b = prefs.getBoolean("FIRSTRUN", true);      
}

}