Android How to set custom Font for entire application

In the styles.xml, define a custom style 

     <style name="CustomTextViewStyle" parent="android:Widget.TextView">

       <item name="android:fontFamily">@font/fonts</item>

     </style>

now inside the fonts folder, create a fonts.xml file and define the below

 <?xml version="1.0" encoding="utf-8"?>

 <font-family xmlns:android="http://schemas.android.com/apk/res/android">

   <font

        android:fontStyle="normal"

        android:fontWeight="400"

        android:font="@font/montserrat" />

    <font

        android:fontStyle="normal"

        android:fontWeight="700"

        android:font="@font/montserrat_bold" />

 </font-family>

and place the font files in the fonts folder itself. 

references:

https://stackoverflow.com/questions/2711858/is-it-possible-to-set-a-custom-font-for-entire-of-application