Why Using Kotlin instead of Java?

Kotlin offers a unique advantage over standard Java, especially in the case of Android:

• Kotlin is fully interoperable with Java.
  • Java and Kotlin can be used in the same project seamless.
  • No need to wait for 3rd party libraries to be ported to Kotlin.
• Kotlin offers advanced features which are fully compatible with older Android versions:
  • Reified Generics 
  • Lambdas
  • Function as first class citizens.
  • Advance Collection library.
  • Immutability.
• Kotlin runtime is small (latest is about 900 KB).
• Nullability builtin as a language feature –  instead as a constant source of Null Pointer Exceptions. 
• It is officially supported by Google, (alongside Java, and C/C++):
  • Google now has Kotlin specific documentation.
  • Google supports Kotlin friendly android extensions. 
• The language design favours developer productivity 
• The language is backed by JetBrains, which means Android Studio offers excellent tooling.

How to start

You need Android Studio 3.3.x, or later. You may either start with a new project or an existing project.  

To start with an existing 

1. Goto Tools|Kotlin|Configure Kotin in Project. 

   

2. You will present with a dialog with two options. Choose "Android with Gradle":
    
3. Choose the "app" to enable Kotlin integration (choose the latest version corresponding to the plugin on your IDE):
   

This will add Kotlin Gradle plugin to your project and apply the following plugins to your app project:

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android-extensions'
apply plugin: 'kotlin-android'

// omitted... 

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
}

New Project

1. Start with the new project wizard on Android studio.
2. Choose "Kotlin" language:
   

Typical Integration Steps

1. Implement the GreenIDListener interface on your activity, for example:

   GreenIDActivity.kt

   class GreenIDActivity :  AppCompatActity(), GreenIDListener {
   	
   	override fun onGreenIDSuccess(resultCode: Int, verificatationToken: String?, verificationStatus: String?) {
       	// Your business logic here 
   	}	
   
   	override fun onGreenIDFailed(resultCode: Int, message: String?, description: String?) {
   		// Handle all failures
   		when (result) {
   			ResultCode.NO_NETWORK -> noNetwork()
   			ResultCode.BACK -> userPressedBack()
   			else -> handleGeneralGreenIDError(resultCode,message)
   		}
   	}
   }
   

   • Override is a required keyword in Kotlin.
   • When overriding a function, you do not have specify the access scope:
     • Public stays public,
     • Protected stays protected!

2. Write a function to start the GreenID™ SDK:

   GreenActivity.kt

   // In GreenActivity.kt
   
   private fun startGreenID() {
   
   	val configBundle = Bundle().apply {
   		putString("accountId", "account-id-as-provided")
   		putString("baseUrl", "https://simpleui-uat1.vixverify.com/") // Change this
   		putString("apiCode", "api-code-as-provided")          
   		putString("countryCode", "XX")
   		putString("mode", "onboarding")         
   	}
   
       try {
       	GreenID.getInstance(applicationContext).start(configBundle, this)	
       catch(e: GreenIDConfigException) {
   		// You have to handle this!
   	}
   }

3. Start the GreenID™, for example on a button/view click:

   // onCreate: 
   
   startGreenIdBtn.setOnClickListener { startGreenID() }

Kotlin Resource for Java/Android Developers