I have seen lots of people converting their tests to Kotlin and then converting other module, But there is no such rule, pick any random file and convert it, check if everything is working as before.

Steps to Convert

Once you know basics of Kotlin

Convert files, one by one, via “⌥⇧⌘K”, make sure tests still pass. Go over the Kotlin files and make them more idiomatic. Repeat step 2 until you convert all the files.

Common issues while converting Android App to Kotlin

TypeCasting for the sake of Interoperability.

Companion will add extra layer.

will add extra layer. If java method starting with getX() , converter looks for property with the name X .

, converter looks for property with the name . Generics are hard to get it right on the first go.

No argument captor.

git diff If two developers are working on same java file and one guy converts it to Kotlin, it will be rework.

TypeCasting for the sake of Interoperability

Kotlin is not Interoperable right away, but you need to do a lot of work around to make it Interoperable

Here is the Java class:

We need to typecast message as String to make it Interoperable with Java.

Companion will add extra layer

Here is Java class:

Converted Kotlin class:

When MainActivity method needs to call DetailsActivity we need to call through Companion which looks ugly in Java code.

Remember: you do not need to stress about migrating the entire codebase. Kotlin and Java can seamlessly interact, and there is no need now to have a 100% Kotlin code. Do it until it feels comfortable.

Method names starting with get

Here is the Java class

Converted Kotlin class is expecting parameters demoResponse and demoResponse2 , They are being interpreted as getter methods, this will cause lots of issues.

No ArgumentCaptor

If you are using Mockito’s ArgumentCaptor you will most probably get following error

The return value of classCaptor.capture() is null, but the signature of SomeClass#someMethod(Class, Boolean) does not allow a null argument.

mockito-kotlin library provides supporting functions to solve this problem

Key Takeaways

annotationProcessor must be replaced by kapt in build.gradle

must be replaced by in Configure tests to mock final classes

classes If you are using data-binding, include:

kapt com.android.databinding:compiler:3.0.0-alpha8

Use @JvmField annotation to use ButterKnife InjectView and any testRule field with @Rule annotation.

annotation to use ButterKnife and any testRule field with annotation. @JvmField to rescue while using ButterKnife @InjectView and Espresso @Rule

Eliminate all !! from your Kotlin code

Here is the Kotlin file with full of !! :

Here is the updated Kotlin class with removal of !! :

Don’t try to learn the whole language at once

Migrating you existing codebase to Kotlin will be fun and needs lot of patience but it will be worth the effort. Make sure you have some unit tests in your existing codebase to migrate with less hassle.

Source Code

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