Google recommend Kotlin is the first to approach for  Android

If you’re looking to build an Android app, Google recommend starting with Kotlin to take advantage of its best-in-class features. At Google I/O 2019, Google announced that Android development will be increasingly Kotlin-first

To learn more about Android's commitment to being Kotlin-first, see Android's commitment to Kotlin.

Even google App Development team also use Kotlin to develop app ex. Maps, Home, Play, Pay, and Drive.

Start with Kotlin+android is following two way

  • Add Kotlin to an existing app
  • Get Started with Kotlin on Android

Add Kotlin to an existing app

Android Studio full support Kotlin development, enabling you to add Kotlin files to your existing project and convert Java language code to Kotlin.

Click on File > New or package folder , and choose one of the various Android templates, such as a new blank Login Fragment,

In Wizard select File Source Kotlin and finish.

Alternatively in your application you can click File > New > Kotlin File/Class to create a basic Kotlin file.

The New Kotlin File/Class window lets you define the file name and provides several choices for the file type: File, Class, Interface, Enum Class, or Object.

If this is the first time you have added a new Kotlin class or file to your project directly (not using the Android templates), Android Studio displays a warning that Kotlin is not configured in the project, as shown in figure 3. Configure Kotlin by clicking Configure either in the upper right corner of the editor or in the event log alert that pops up in the lower-right corner.

Choose the option to configure Kotlin for All modules containing Kotlin files when prompted,

Once you click OK, Android Studio adds Kotlin to your project class path and applies the Kotlin and Kotlin Android Extensions plugins to each module that contains Kotlin files. Your build.gradle files should look similar to the examples below:

buildscript {
    ext.kotlin_version = '1.3.72'

    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath "com.android.tools.build:gradle:4.0.0"
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'

android {
    ....
    ....
}

dependencies {

    implementation "androidx.core:core-ktx:+"
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

}

Source organisation

android {  

	sourceSets {     
    	main.java.srcDirs += 'src/main/kotlin'  
    }
}

By default, new Kotlin files are saved in src/main/java/, which makes it easy to see both Kotlin and Java files in one location. best practice is you can put Kotlin files under src/main/kotlin/ instead. If you do this, then you also need to include this directory in your sourceSets configuration, as shown above

Convert existing Java code to Kotlin code

To convert Java code to Kotlin, open the Java file in Android Studio, and select Code > Convert Java File to Kotlin File. Alternatively, create a new Kotlin file (File > New > Kotlin File/Class), and then paste your Java code into that file.

Get Started with Kotlin on Android

Android Studio fully supports Kotlin, enabling you to create new projects with Kotlin files, add Kotlin files to your existing project, and convert Java language code to Kotlin.

Create pure Kotlin Project - Hello World

Declare an Activity

class MainActivity : AppCompatActivity() {

Using Kotlin OnCreate method.

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setSupportActionBar(findViewById(R.id.toolbar))

Ref.