How to change a TextView's style at runtime in Android using Kotlin?

This example demonstrates how to change a TextView's style at runtime in Android using Kotlin.
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"
   tools:context=".MainActivity">
   <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_centerHorizontal="true"
      android:layout_marginTop="70dp"
      android:background="#008080"
      android:padding="5dp"
      android:text="TutorialsPoint"
      android:textColor="#fff"
      android:textSize="24sp"
      android:textStyle="bold" />
   <TextView
      android:id="@+id/textView"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_centerInParent="true"
      android:text="Have a nice day!"
      android:textColor="@android:color/background_dark"
      android:textSize="36sp"
      android:textStyle="bold" />
</RelativeLayout>
Step 3 − Add the following code in res/values/styles.xml
<resources>
   <style name="boldText">
      <item name="android:textStyle">bold|italic</item>
      <item name="android:textColor">#FFFFFF</item>
   </style>
   <style name="normalText">
      <item name="android:textStyle">normal</item>
      <item name="android:textColor">#C0C0C0</item>
   </style>
</resources>
Step 4 − Add the following code in res/values/colors.xml
<resources>
   <color name="highlightedTextViewColor">@android:color/holo_green_light</color>
   <color name="normalTextViewColor">@android:color/holo_red_dark</color>
</resources>
Step 5 − Add the following code to src/MainActivity.kt
import android.os.Build
import android.os.Bundle
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity
class MainActivity : AppCompatActivity() {
   lateinit var textView: TextView
   override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_main)
      title = "KotlinApp"
      textView = findViewById(R.id.textView)
      textView.setOnClickListener {
         if (Build.VERSION.SDK_INT < 23) {
            textView.setTextAppearance(applicationContext, R.style.boldText)
         }
         else {
            textView.setTextAppearance(R.style.boldText)
         }
         textView.setBackgroundResource(R.color.highlightedTextViewColor)
      }
   }
}
Step 6 − 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="com.example.q11">
   <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 the Run icon  from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen.
Click here to download the project code.