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Android Hello World with Kotlin

This project is a POC for showing how to integrate the Kotlin language in an Android project. It includes a mix of Java and Kotlin source code, with a working Maven configuration and Android Studio / IntelliJ project files.

What is Kotlin?

Kotlin is a programming language developed by JetBrains. It can be compiled to JVM bytecode (and even Javascript), so it can be used in any Java program.

See: http://kotlin.jetbrains.org/

Why using Kotlin with Android?

Mainly for functional programming.

Currently there is no official plan to support Java 8 and its lambdas on Android. Scala is a good candidate, but at the time of writing, its integration with Android is a pain due to its very large runtime. That's why I tried Kotlin.

Sample code

package org.example.hakm.activity

import android.os.Bundle
import android.app.Activity
import org.example.hakm.R
import android.widget.TextView
import org.example.hakm.SomeJavaClass
import android.widget.Button

class HelloKotlinActivity() : Activity() {

    var clicks = 0

    protected override fun onCreate(savedInstanceState : Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.hello_kotlin_activity)

        val textView = findViewById(R.id.textView) as TextView
        textView.setText(getString(R.string.hello_message, SomeJavaClass.getItWorksMessage()));

        val button = findViewById(R.id.button) as Button
        button.setOnClickListener { textView.setText(getMessage()) }
    }

    fun getMessage(): String {
        when(++clicks) {
            1    -> return "You clicked once"
            2    -> return "You clicked twice"
            else -> return "You clicked $clicks times"
        }
    }
}

How to enable Kotlin in a Maven Android project?

It's just a matter of getting the right Maven configuration: see the pom.xml file if you need details. In a few words, here are the requirements:

  • Enable the Kotlin build plugin for Maven
  • Add the Kotlin library dependency
  • Adding repositories is required as the Kotlin dependencies are still snapshots
  • We need build-helper-maven-plugin to include src/main/kotlin as a source folder
  • As a workaround to a known interoperability problem between the Kotlin and the Android maven plugins, we need maven-antrun-plugin to create a missing target folder automatically (see http://youtrack.jetbrains.com/issue/KT-3679).

Prerequisites to run this project

  • Maven 3.1.1
  • Android SDK 19

Using Android Studio

All Android Studio project files are already under source control: just open the project with Android Studio. By default you will have a run configuration named "Launch Kotlin activity" that will build and launch the application on your device or emulator.

Build with Maven

Run the following command to build the APK with Maven:

$ mvn package

Limitations

There may be some.