Kotlin and Android

Kotlin is now an official language on Android. It's expressive, concise, and powerful. Best of all, it's interoperable with our existing Android languages and runtime.

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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 Java.


// 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 helpful 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