How to set default font family for entire Android App?

This example demonstrate about How to set default font family for entire Android App.
Step 1 − Create a new project in Android Studio, go to File ⇒ New Project and fill all required details to create a new project.
Step 2 − Add the following code to res/layout/activity_main.xml.
<? xml version= "1.0" encoding= "utf-8" ?>
<RelativeLayout xmlns: android = "http://schemas.android.com/apk/res/android"
   xmlns: tools = "http://schemas.android.com/tools"
   android :layout_width= "match_parent"
   android :layout_height= "match_parent"
   android :layout_margin= "16dp"
   tools :context= ".MainActivity" >
   <TextView
      android :layout_width= "match_parent"
      android :layout_height= "wrap_content"
      android :layout_centerInParent= "true"
      android :text= "@string/lorem"
      android :textSize= "18sp" />
</RelativeLayout>
Step 3 − Add the following code to src/MainActivity.java
package app.tutorialspoint.com.sample ;
import android.support.v7.app.AppCompatActivity ;
import android.os.Bundle ;
public class MainActivity extends AppCompatActivity {
   @Override
   protected void onCreate (Bundle savedInstanceState) {
      super .onCreate(savedInstanceState) ;
      setContentView(R.layout. activity_main ) ;
   }
}
Step 4 − Add the following code to res/values/styles.xml
<resources>
   <!-- Base application theme. -->
   <style name= "AppBaseTheme" parent= "Theme.AppCompat.Light.DarkActionBar" >
      <!-- Customize your theme here. -->
      <item name= "colorPrimary" > @color/colorPrimary </item>
      <item name= "colorPrimaryDark" > @color/colorPrimaryDark </item>
      <item name= "colorAccent" > @color/colorAccent </item>
   </style>
   <style name= "AppTheme" parent= "AppBaseTheme" >
      <item name= "android:textViewStyle" > @style/RobotoTextViewStyle </item>
      <item name= "android:buttonStyle" > @style/RobotoButtonStyle </item>
   </style>
   <style name= "RobotoTextViewStyle" parent= "android:Widget.TextView" >
      <item name= "android:fontFamily" > sans-serif-light </item>
   </style>
   <style name= "RobotoButtonStyle" parent= "android:Widget.Holo.Button" >
      <item name= "android:fontFamily" > sans-serif-light </item>
   </style>
</resources>
Step 5 − Add the following code to androidManifest.xml
<? xml version= "1.0" encoding= "utf-8" ?>
<manifest xmlns: android = "http://schemas.android.com/apk/res/android"
   package= "app.tutorialspoint.com.sample" >
   <application
      android :allowBackup= "true"
      android :icon= "@mipmap/ic_launcher"
      android :label= "@string/app_name"
      android :roundIcon= "@mipmap/ic_launcher_round"
      android :supportsRtl= "true"
      android :theme= "@style/AppTheme" >
      <activity android :name= ".MainActivity" >
         <intent-filter>
            <action android :name= "android.intent.action.MAIN" />
            <category android :name= "android.intent.category.LAUNCHER" />
         </intent-filter>
      </activity>
   </application>
</manifest>
Let's try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from android studio, open one of your project's activity files and click Run  icon from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen –