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Kotlin and Android

Kotlin and Android

You can write Android apps in Kotlin. Kotlin is expressive, concise, and powerful. Best of all, it's interoperable with the Java programming language and the existing Android runtime.

Modern. Expressive. Safe.

Kotlin is concise while being expressive. It contains safety features for nullability and immutability, to make your Android apps healthy and performant by default.

Safer code

Write safer code and avoid NullPointerExceptions in your app.

    var output: String
    output = null   // Compilation error

    ==================================

    val name: String? = null    // Nullable type
    println(name.length())      // Compilation error

Readable and concise

Data classes

Focus on expressing your ideas and write less boilerplate code.

  // Create a POJO with getters, setters, equals(), hashCode(),
  // toString(), and copy() with a single line:
  data class User(val name: String, val email: String)

Lambdas

Use lambdas to simplify your code.

  button.setOnClickListener(new View.OnClickListener(){
    @Override
    public void onClick(View v){
      doSomething();
    }
  });
  button.setOnClickListener { doSomething() }

Default and named arguments

Reduce the number of overloaded functions by using default arguments. Call functions using named arguments to make your code more readable.

  fun format(str: String,
    normalizeCase: Boolean = true,
    upperCaseFirstLetter: Boolean = true,
    divideByCamelHumps: Boolean = false,
    wordSeparator: Char = ' ') {
    …
  }

  ==================================

  // Call function with named arguments.
  format(str, normalizeCase = true, upperCaseFirstLetter = true)

Say goodbye to findViewById

Avoid findViewById() calls in your code. Focus on writing your logic with less verbosity.

  import kotlinx.android.synthetic.main.content_main.*

  class MainActivity : AppCompatActivity() {

      override fun onCreate(savedInstanceState: Bundle?) {
          super.onCreate(savedInstanceState)
          setContentView(R.layout.activity_main)
          // No need to call findViewById(R.id.textView) as TextView
          textView.text = "Kotlin for Android rocks!"

      }
  }

Extend functionality without inheritance

Extension functions and properties let you easily extend functionality of classes without inheriting from them. Calling code is readable and natural.

  // Extend ViewGroup class with inflate function
  fun ViewGroup.inflate(layoutRes: Int): View {
      return LayoutInflater.from(context).inflate(layoutRes, this, false)
  }

  ==================================

  // Call inflate directly on the ViewGroup instance
  override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
      val v = parent.inflate(R.layout.view_item)
      return ViewHolder(v)
  }

100% interoperable with Java

Add as little or as much of Kotlin as you want. Kotlin is a JVM language that's completely interoperable with the Java programming language.

  // Calling Java code from Kotlin
  class KotlinClass {
      fun kotlinDoSomething() {
          val javaClass = JavaClass()
          javaClass.javaDoSomething()
          println(JavaClass().prop)
      }
  }

  ==================================

  // Calling Kotlin code from Java
  public class JavaClass {
      public String getProp() { return "Hello"; }
      public void javaDoSomething() {
          new KotlinClass().kotlinDoSomething();
      }
  }

Great tooling support

Android Studio 3.0 provides tools to help you start using Kotlin. Convert entire Java files or convert code snippets on the fly when you paste Java code into a Kotlin file.

Kotlin is open

Just like Android, Kotlin is an open source project under Apache 2.0. Our choice of Kotlin reaffirms our commitment to an open developer ecosystem as we evolve and grow the Android platform, and we are excited to see the language evolve.

See Kotlin on Github

Apps built with Kotlin

Many apps are already built with Kotlin—from the hottest startups to Fortune 500 companies.

  • Slack
  • Lyft
  • Robinhood
  • Evernote
  • Square
  • Deliveroo
  • Busuu
  • Udacity
  • Instacart
  • Pinterest
  • Basecamp
  • Pocket
  • Reddit
  • Jira Cloud
  • Trello
  • Foursquare
  • Stride
  • Amex
  • Expedia
  • Netflix
  • App Lock
  • Keepsafe
  • NYTimes
  • Nova Launcher
  • Wordpress
  • Simple
  • Keepsafe VPN
  • Asos
  • Pigment
  • Foursquare Swarm
  • TeamSnap
  • GO-JEK
  • Confluence Cloud
  • Zola Sales
  • Juno

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