Kotlin: The Good, The Bad, and The Ugly

AJ Alt
AJ Alt
Aug 19, 2016 · 14 min read

The Good

Automatic conversion of Java to Kotlin

lateinit, Delegates.notNull and lazy

val name: String
var name: String? = null
lateinit var name: String
var age: Int by Delegates.notNull<Int>()
val imm: InputMethodManager by lazy { 
getSystemService(INPUT_METHOD_SERVICE) as InputMethodManager

Functional collection extensions

Named and default function arguments

class Presenter(
val okhttp: OkHttp = productionOkHttp(),
val picasso: Picasso = productionPicassoInstance()
) {...}

The Bad

No namespaces

fun foo() {...}
object FooActions {
fun foo() {...}

No static modifier

public class View {
public static final int VISIBLE = 0x00000000;
public static final int INVISIBLE = 0x00000004;
public static View inflate(Context context, int resource) {...}
class View {
companion object {
val VISIBLE: Int = 0x00000000
val INVISIBLE: Int = 0x00000004
fun inflate(context: Context, resource: Int) {...}
// With annotations:
//Without annotations:

Automatic conversion of Java to Kotlin

Required property accessor syntax

Method count

The Ugly

SAM conversion and Unit returning lambdas

public void registerCallback(View.OnClickListener r)
// Java
registerCallback(() -> { /** do stuff */ })
registerCallback { /** do stuff */ }
fun registerCallback(r: View.OnClickListener)// Kotlin. Note that parenthesis are required now.
registerCallback(View.OnClickListener { /** do stuff */ })
fun registerCallback(r: () -> Unit)
registerCallback(() -> {
/** do stuff */
return Unit.INSTANCE;

Closed by default

“Best practices say that you should not allow these hacks anyway”

“People successfully use other languages (C++, C#) that have similar approach”

“If people really want to hack, there still are ways: you can always write your hack in Java and call it from Kotlin (see Java Interop), and Aspect frameworks always work for these purposes”


Keepsafe Engineering

Engineering problems at Keepsafe.

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