Getting Started on Kotlin

Kotlin is now an official language on Android. Kotlin is expressive, concise, extensible, powerful, and a joy to read and write. It has wonderful safety features in terms of nullability and immutability.

If you're interested in using Kotlin, it's easy to get started because it works side by side with Java and C++ on Android. So you can keep your existing code, continue to use the various Android libraries, and incrementally add Kotlin code to your project. Unlike almost any other language, Kotlin is a drop-in replacement you can use bi-directionally—you can call into the Java language from Kotlin, and you can call into Kotlin from the Java language.

Of course, IDE support is also crucial, and we have it. Android Studio is built upon IntelliJ IDEA, an IDE built by JetBrains—the same company that created the Kotlin language. The JetBrains team has been working for years to make sure Kotlin works great with IntelliJ IDEA. So we're inheriting all their hard work. Starting with Android Studio 3.0, tooling support for Kotlin is bundled directly into Android Studio.

Create a new project with Kotlin

Using Kotlin with a new project requires just one extra click in the New Project wizard:

1. In Android Studio, click File > New > New Project. Or if you've just opened Android Studio and see the Welcome to Android Studio window, click Start a new Android Studio project.
2. On the first screen, check Include Kotlin support. That's the only difference.
3. Click Next and continue through the wizard until you're done.

Add Kotlin to an existing project

If you want to add Kotlin code to an existing project, simply click File > New and select one of the various Android templates. If you don't see the list of templates in this menu, first open the Project window and select your app module.

Convert existing Java code to Kotlin code

In Android Studio 3.0, open a Java file and select Code > Convert Java File to Kotlin File.

Or, create a new Kotlin file (File > New > Kotlin File/Class), and then paste your Java code into that file—when prompted, click Yes to convert the code to Kotlin. You can check Don't show this dialog next time, which makes it easy to dump Java code snippets into your Kotlin files.

Use Android APIs with Kotlin

Kotlin provides complete interoperability with the Java language, so calling the Android APIs often looks exactly like the matching Java code. Except now you can combine those method calls with Kotlin's syntax features.

Here are a few examples of what it looks like to call Android APIs in Kotlin, compared to the same code in Java language:

Declare Activity in Java

public class MyActivity extends AppCompatActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity);
  }
}

Declare Activity in Kotlin

class MyActivity : AppCompatActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity)
  }
}

On-click listener in Java

FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
  @Override
  public void onClick(View view) {
    //TODO: Do your code
  }
});

On-click listener in Kotlin

val fab = findViewById(R.id.fab) as FloatingActionButton
fab.setOnClickListener {
  //TODO: Do your code
}

Item click listener in Java

private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
    = new BottomNavigationView.OnNavigationItemSelectedListener() {
  @Override
  public boolean onNavigationItemSelected(@NonNull MenuItem item) {
    switch (item.getItemId()) {
      case R.id.navigation_home:
        mTextMessage.setText(R.string.title_home);
        return true;
      case R.id.navigation_dashboard:
        mTextMessage.setText(R.string.title_dashboard);
        return true;
    }
    return false;
  }
};

Item click listener in Kotlin

private val mOnNavigationItemSelectedListener
    = BottomNavigationView.OnNavigationItemSelectedListener { item ->
  when (item.itemId) {
    R.id.navigation_home -> {
      mTextMessage.setText(R.string.title_home)
      return@OnNavigationItemSelectedListener true
    }
    R.id.navigation_dashboard -> {
      mTextMessage.setText(R.string.title_dashboard)
      return@OnNavigationItemSelectedListener true
    }
 }
 false
}

Here is the Get Started with Kotlin video.

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