MainActivity.java

textName.setTypeface(TypefaceUtil.createLightFont(getApplicationContext()));

MyApp.java

public class MyApp extends Application {
  private static Context context;

  @Override
  public void onCreate() {
    super.onCreate();
    context = getApplicationContext();
    TypefaceUtil.init(context);
  }

  public static Context getContext() {
    return context;
  }
}

TypefaceUtil.java

public class TypefaceUtil {
  public static final String BLACK_FONT_NAME = "fonts/BlackFont.otf";
  public static final String MEDIUM_FONT_NAME = "fonts/MediumFont.otf";
  public static final String REGULAR_FONT_NAME = "fonts/RegularFont.otf";
  public static final String LIGHT_FONT_NAME = "fonts/LightFont.otf";
  public static final String THIN_FONT_NAME = "fonts/ThinFont.otf";

  public static final String SANS_SERIF_BLACK = "sans-serif-black";
  public static final String SANS_SERIF_MEDIUM = "sans-serif-medium";
  public static final String SANS_SERIF = "sans-serif";
  public static final String SANS_SERIF_LIGHT = "sans-serif-light";
  public static final String SANS_SERIF_THIN = "sans-serif-thin";

  public static void init(Context context) {
    Typeface black = createBlackFont(context);
    Typeface medium = createMediumFont(context);
    Typeface regular = createRegularFont(context);
    Typeface light = createLightFont(context);
    Typeface thin = createThinFont(context);

    Map<String, Typeface> fonts = new HashMap<>();
    fonts.put(SANS_SERIF_BLACK, black);
    fonts.put(SANS_SERIF_MEDIUM, medium);
    fonts.put(SANS_SERIF, regular);
    fonts.put(SANS_SERIF_LIGHT, light);
    fonts.put(SANS_SERIF_THIN, thin);
    overrideFonts(fonts);

    overrideFont("DEFAULT", regular);
    overrideFont("DEFAULT_BOLD", black);
  }

  public static Typeface createBlackFont(Context context) {
    return Typeface.createFromAsset(context.getAssets(), BLACK_FONT_NAME);
  }

  public static Typeface createMediumFont(Context context) {
    return Typeface.createFromAsset(context.getAssets(), MEDIUM_FONT_NAME);
  }

  public static Typeface createRegularFont(Context context) {
    return Typeface.createFromAsset(context.getAssets(), REGULAR_FONT_NAME);
  }

  public static Typeface createLightFont(Context context) {
    return Typeface.createFromAsset(context.getAssets(), LIGHT_FONT_NAME);
  }

  public static Typeface createThinFont(Context context) {
    return Typeface.createFromAsset(context.getAssets(), THIN_FONT_NAME);
  }

  public static void overrideFonts(Map<String, Typeface> typefaces) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
      try {
        final Field field = Typeface.class.getDeclaredField("sSystemFontMap");
        field.setAccessible(true);

        Map<?, ?> tempMap = (Map<?, ?>) field.get(null);
        Map<String, Typeface> oldFonts = new HashMap<>();

        for (Map.Entry<?, ?> entry : tempMap.entrySet()) {
          oldFonts.put((String) entry.getKey(), (Typeface) entry.getValue());
        }

        oldFonts.putAll(typefaces);
        field.set(null, oldFonts);
        field.setAccessible(false);
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
  }

  public static void overrideFont(String fontName, Typeface typeface) {
    try {
      final Field field = Typeface.class.getDeclaredField(fontName);
      field.setAccessible(true);
      field.set(null, typeface);
      field.setAccessible(false);
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}