Manually migrating an Android project to Kotlin

Kotlin is now officialy Kotlin and Android, and as such I wanted to give it another try. There are of course tutorials out there, but they all aim users of Android Studio. Let's see how we can do without.

First things first, let's add Kotlin to our build script. We need to add to the build.gradle of our application three things:

buildscript {
+    repositories {
+        jcenter()
+        mavenCentral()
+    }
+    dependencies {
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.1.2"
+    }
 apply plugin: ''
+apply plugin: 'kotlin-android'
+apply plugin: 'kotlin-android-extensions'
 dependencies {
     compile fileTree(dir: 'libs', include: ['*.jar'])
     compile ''
+    compile 'org.jetbrains.kotlin:kotlin-stdlib:1.1.2'
+    testCompile 'org.jetbrains.kotlin:kotlin-stdlib:1.1.2'

If we run our gradlew script, we can see that it is installing the Kotlkin toolchain, but that no Kotlin code is built. Of course, because there are none yet.

Kotlin developers provide an online Java-to-Kotlkin transpiler. It is far from perfect, but it is enough for beginers with an existing Java project. In our application source directory, we create the same architecture where we replace src/main/java/<package> with src/main/kotlin/<package>. We transpile all the .java files to .kt and we run again our gradlew script.