Example to override the default font of an Application.

OverrideTypefaceApplication.java

public class OverrideTypefaceApplication extends Application {
  
  @Override
  public void onCreate() {
    TypefaceUtil.overrideFont(getApplicationContext(), "SERIF", "fonts/Roboto-Regular.ttf"); // font from assets: "assets/fonts/Roboto-Regular.ttf
  }
}

themes.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <style name="CustomTheme" parent="@android:style/Theme.Holo.Light">
    <!-- you should set typeface which you want to override with TypefaceUtil -->
    <item name="android:typeface">serif</item>
  </style>
</resources>

TypefaceUtil.java

import android.content.Context;
import android.graphics.Typeface;
import android.util.Log;

import java.lang.reflect.Field;

public class TypefaceUtil {

    /**
     * Using reflection to override default typeface
     * NOTICE: DO NOT FORGET TO SET TYPEFACE FOR APP THEME AS DEFAULT TYPEFACE WHICH WILL BE OVERRIDDEN
     * @param context to work with assets
     * @param defaultFontNameToOverride for example "monospace"
     * @param customFontFileNameInAssets file name of the font from assets
     */
    public static void overrideFont(Context context, String defaultFontNameToOverride, String customFontFileNameInAssets) {
        try {
            final Typeface customFontTypeface = Typeface.createFromAsset(context.getAssets(), customFontFileNameInAssets);

            final Field defaultFontTypefaceField = Typeface.class.getDeclaredField(defaultFontNameToOverride);
            defaultFontTypefaceField.setAccessible(true);
            defaultFontTypefaceField.set(null, customFontTypeface);
        } catch (Exception e) {
            Log.e("Can not set custom font " + customFontFileNameInAssets + " instead of " + defaultFontNameToOverride);
        }
    }
}

Hi,
Thanks for this, it is working well for my application when the targetSdkVersion was API level 24, I upgraded to level 26 today and it is not working anymore. Do you have idea what is going wrong please?

#tjohnn I am facing same issue. Any solutions?

This solution fails for API level 26 and above.

Solution is to follow these steps https://developer.android.com/guide/topics/ui/look-and-feel/fonts-in-xml , instead of overriding fonts.