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[WIP] Curated List of Practical Tips to migrate your Android App to Kotlin

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Kotlin Migration Guide

Practical Tips to migrate your Android App to Kotlin


Contents

Configure Kotlin

  1. In Android Studio, select Tools > Kotlin > Configure Kotlin in Project. If a window titled Choose Configurator appears, select Android with Gradle, make sure All modules is selected, and click OK.
  2. You will be prompted to sync the project with Gradle as the build.gradle files have changed. Once the sync is complete, move to the next step and write some Kotlin code.

Note: When using Android Studio 3.0 or higher and creating a new project, you can have Kotlin configured by default by selecting the Include Kotlin support checkbox on the Create Android Project dialog.


Steps to Convert

Once you learn basics syntax of Kotlin

  1. Convert files, one by one, via "⌥⇧⌘K", make sure tests still pass
  2. Go over the Kotlin files and make them more idiomatic.
  3. Repeat step 2 until you convert all the files.
  4. Ship it.

Comparing the type system between Java and Kotlin


Any v/s Object

All classes in Kotlin have a common superclass Any, that is a default super for a class with no supertypes declared:

class Example // Implicitly inherits from Any

Any is not java.lang.Object; in particular, it does not have any members other than equals(), hashCode() and toString(). Please consult the Java interoperability section for more details.

Further, from the section on mapped types we find:

Kotlin treats some Java types specially. Such types are not loaded from Java “as is”, but are mapped to corresponding Kotlin types. The mapping only matters at compile time, the runtime representation remains unchanged. Java’s primitive types are mapped to corresponding Kotlin types (keeping platform types in mind):

java.lang.Object kotlin.Any!

This says that at runtime java.lang.Object and kotlin.Any! are treated the same. But the !also means that the type is a platform type, which has implication with respect to disabling null checks etc.

Any reference in Java may be null, which makes Kotlin’s requirements of strict null-safety impractical for objects coming from Java. Types of Java declarations are treated specially in Kotlin and called platform types. Null-checks are relaxed for such types, so that safety guarantees for them are the same as in Java (see more below).

When we call methods on variables of platform types, Kotlin does not issue nullability errors at compile time, but the call may fail at runtime, because of a null-pointer exception or an assertion that Kotlin generates to prevent nulls from propagating:


Type inference

// val abc: Map<String,Int> = mapOf("a" to 1, "b" to 2, "c" to 3)
// the actual type of the Map is obvious
// thus we can skip Map<String,Int> and let kotlin compiler to make type inference
val abc = mapOf("a" to 1, "b" to 2, "c" to 3)

val abcd = abc + ("d" to 4) // ok!
val abce = abc + ("e" to "5") // compile error: Type mismatch

// val c: Int? = abc["c"]
var c = abc["c"] 

c = "2" // compile error: Type mismatch

// Same Question: 
// If you are asked to refactor from Map<String,Integer> to Map<String,String>,
// try to estimate the efforts!

What nullability means in Kotlin?

Source


Why Optionals exist?


How data classes are based on Value Types


class v/s data-class


Using Parcelize annotation for Parcelable

Source

Here is the Java Class:

public class MyParcelable implements Parcelable {
     private int mData;

     public int describeContents() {
         return 0;
     }

     public void writeToParcel(Parcel out, int flags) {
         out.writeInt(mData);
     }

     public static final Parcelable.Creator<MyParcelable> CREATOR
             = new Parcelable.Creator<MyParcelable>() {
         public MyParcelable createFromParcel(Parcel in) {
             return new MyParcelable(in);
         }

         public MyParcelable[] newArray(int size) {
             return new MyParcelable[size];
         }
     };
     
     private MyParcelable(Parcel in) {
         mData = in.readInt();
     }
 }

Converted Kotlin Class:

data class MyParcelable(var data: Int): Parcelable {

    override fun describeContents() = 1

    override fun writeToParcel(dest: Parcel, flags: Int) {
        dest.writeInt(data)
    }

    companion object {
        @JvmField 
        val CREATOR = object : Parcelable.Creator<MyParcelable> {
            override fun createFromParcel(source: Parcel): MyParcelable {
                val data = source.readInt()
                return MyParcelable(data)
            }

            override fun newArray(size: Int) = arrayOfNulls<MyParcelable>(size)
        }
    }
}

To use @Parcelize we need to set experimental flag in build.gradle

androidExtensions {
    experimental = true
}

Android Extensions plugin now includes an automatic Parcelable implementation generator. Declare the serialized properties in a primary constructor and add a @Parcelize annotation, and writeToParcel()/createFromParcel() methods will be created automatically:

@Parcelize
class MyParcelable(val data: Int): Parcelable

String templates

Kotlin has string templates, which is awesome. e.g. "$firstName $lastName" for simple variable name or "${person.name} is ${1 * 2}" for any expressions. You can still do the string concatenation if you like e.g. "hello " + "world", but that means being stupid.


Removing ButterKnife and findViewById to use Kotlin-Android-Extensions


apply, let, with and also


Common Migration Gotchas

  • annotationProcessor must be replaced by kapt in build.gradle
  • Configure tests to mock final classes
  • @JvmField to rescue while using ButterKnife @InjectView and Espresso @Rule

If you are using android data-binding, include:

kapt "com.android.databinding:compiler:${compiler_version}"

Fixes following the Gradle warning:

Warning:warning: The following options were not recognized by any processor: '[android.databinding.artifactType,
android.databinding.printEncodedErrors, android.databinding.minApi, android.databinding.isTestVariant,
android.databinding.enableDebugLogs, android.databinding.sdkDir, android.databinding.bindingBuildFolder,
android.databinding.enableForTests, android.databinding.modulePackage, kapt.kotlin.generated, 
android.databinding.generationalFileOutDir, android.databinding.xmlOutDir]'

Common Converter Gotchas

  • TypeCasting for the sake of Interoperability.
  • Companion will add extra layer.
  • If java method starting with getX(), converter looks for property with the name X.
  • Generics is 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:

public class DemoFragment extends BaseFragment implements DemoView {
    @Override public void displayMessageFromApi(String apiMessage) {
      ...
    }
}

// Kotlin class
class DemoResponse {
    @SerializedName("message") var message: String? = null
}

// Typecasting to String
mainView?.displayMessageFromApi(demoResponse.message as String)

Companion will add extra layer

Here is Java class:

public class DetailActivity extends BaseActivity implements DetailMvpView{
    public static final String EXTRA_POKEMON_NAME = "EXTRA_POKEMON_NAME";

    public static Intent getStartIntent(Context context, String pokemonName) {
        Intent intent = new Intent(context, DetailActivity.class);
        intent.putExtra(EXTRA_POKEMON_NAME, pokemonName);
        return intent;
    }
}

Converted Kotlin class:

class DetailActivity : BaseActivity(), DetailMvpView {
    companion object {
        val EXTRA_POKEMON_NAME = "EXTRA_POKEMON_NAME"

        fun getStartIntent(context: Context, pokemonName: String): Intent {
            val intent = Intent(context, DetailActivity::class.java)
            intent.putExtra(EXTRA_POKEMON_NAME, pokemonName)
            return intent
        }
    }
}

public class MainActivity extends BaseActivity implements MainMvpView {
  private void pokemonClicked(Pokemon pokemon) {
      startActivity(DetailActivity.Companion.getStartIntent(this, pokemon))    
  }
}

Method names starting with get

Here is the Java class:

public interface DemoService {
    @GET("posts")
    Observable<PostResponse> getDemoResponse();

    @GET("categories")
    Observable<CategoryResponse> getDemoResponse2();
}

Converted Kotlin class:

interface DemoService {
    @get:GET("posts")
    val demoResponse: Observable<PostResponse>
    
    @get:GET("categories")
    val demoResponse2: Observable<CategotyResponse>
}

Expecting methods demoResponse and demoResponse2, They are being interpreted as getter methods, this will cause lots of issues.


Eliminating !! from your Kotlin code

Source

  • Use val instead of var
  • Use lateinit
  • Use let function
  • User Elivis operator

Use val instead of var

  • Kotlin makes you think about immutability on the language level and that’s great.
  • var and val mean "writable" and "read-only"
  • If you use them as immutables, you don’t have to care about nullability.

No ArgumentCaptor

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

java.lang.IllegalStateException: classCaptor.capture() must not be null

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


Use lateinit

private var adapter: RecyclerAdapter<Droids>? = null

override fun onCreate(savedInstanceState: Bundle?) {
   super.onCreate(savedInstanceState)
   adapter = RecyclerAdapter(R.layout.item_droid)
}

fun updateTransactions() {
   adapter!!.notifyDataSetChanged()
}

private lateinit var adapter: RecyclerAdapter<Droids>

override fun onCreate(savedInstanceState: Bundle?) {
   super.onCreate(savedInstanceState)
   adapter = RecyclerAdapter(R.layout.item_droid)
}

fun updateTransactions() {
   adapter?.notifyDataSetChanged()
}

Use let function

private var photoUrl: String? = null

fun uploadClicked() {
    if (photoUrl != null) {
        uploadPhoto(photoUrl!!)
    }
}

private var photoUrl: String? = null

fun uploadClicked() {
    photoUrl?.let { uploadPhoto(it) }
}

Use elvis operator

Elvis operator is great when you have a fallback value for the null case. So you can replace this:

fun getUserName(): String {
   if (username != null) {
       return username!!
   } else {
       return "Anonymous"
   }
}

elvis operator is great when you have a fallback value for the null case. So you can replace this:

fun getUserName(): String {
   return username ?: "Anonymous"
}

Annotations to make Kotlin interoperable with Java.


Migrating Unit Tests to Use Mockito-Kotlin


Utils -> Kotlin-Extensions


Migrating Room to Kotlin


Idiomatic Kotlin


Acknowledgement

Arun Sasidharan for Initial Idea

Ritesh Gupta for More Ideas


References

https://medium.com/fueled-android/practical-tips-to-migrate-your-android-app-to-kotlin-4d331e5256dc https://codelabs.developers.google.com/codelabs/taking-advantage-of-kotlin/index.html https://android.jlelse.eu/how-to-remove-all-from-your-kotlin-code-87dc2c9767fb https://medium.com/google-developers/migrating-an-android-project-to-kotlin-f93ecaa329b7 https://blog.philipphauer.de/idiomatic-kotlin-best-practices/ https://stackoverflow.com/a/38761552/1257042 https://gist.github.com/gaplo917/c3b70249b660bff9cdc0b5453c34f98b


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[WIP] Curated List of Practical Tips to migrate your Android App to Kotlin

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