Kotlin decompiled

Who am I?
• Klymenko Ruslan
• Software engineer at ‘zorallabs’
• Passionate about JVM
• Ternopil JUG co-founder
• Founder of ‘Dobroe IT’

Who use kotlin in everyday
practice?

So… What is Kotlin?
• Modern
• Jvm-based (not only! =) )
• Statically-typed

Interoperability. Kotlin is same as java. It also
produces bytecode.

But… Wait a minute. What is bytecode?

What is decompilation?
• The reverse process of compilation.
• Takes executable code as input.
• Produces high-level code as output.

What is this talk about?
• We’re going to decompile Kotlin code to Java analogue.
• It will give a better understanding of what’s going on under the hood.

What is this talk about?
• It will give a feeling when Kotlin is an appropriate solution which
minimizes boilerplate.
• It will show Kotlin’s weak points(if there any)

So… Let’s start from the beginning!
fun main(args: Array<String>) {
println("Hey, folks!")
} ????

Hello, world!
@Metadata(
mv = {1, 1, 9},
bv = {1, 0, 2},
k = 2,
d1 = {"u0000u0014nu0000n...."},
d2 = {"main", "", "args", "", "", …}
)
public final class Test1Kt {
public static final void main(@NotNull String[] args) {
Intrinsics.checkParameterIsNotNull(args, "args");
String var1 = "Hey, folks!";
System.out.println(var1);
}
}

What’s wrong here?
object Test {
}
}

public final class Test {
public static final Test INSTANCE;
public final void main(@NotNull String[] args) {
Intrinsics.checkParameterIsNotNull(args, "args");
}
static {
Test var0 = new Test();
INSTANCE = var0;
}
}

@JvmStatic
object Test {
@JvmStatic
}
}

Extension methods or how to add functionality
even to final types in a natural way.
fun String.isTest() =
this == "test"
????

Extension methods or how to add functionality
even to final types in a natural way.
public static final boolean isTest(@NotNull String
$receiver) {
Intrinsics.checkParameterIsNotNull($receiver,
"$receiver");
return Intrinsics.areEqual($receiver, "test");
}
}

Extension methods from client’s point of
view
fun testExtension() {
"".isTest()
}

Extension methods from client’s point of
view
public static final void testExtension() {
isTest("");
}

The simplest class ever. But wait.
class Test3(test: String)
????

The simplest class ever.
public final class Test3 {
public Test3(@NotNull String test) {
Intrinsics.checkParameterIsNotNull(test, "test");
}
}

The simplest bean ever. The correct one
class Test4(val test: String)

@NotNull
private final String test;
@NotNull
public final String getTest() {
return this.test;
}
Intrinsics.checkParameterIsNotNull(test, "test");
this.test = test;
}
}

class Test4(var test: String) ????

The simplest bean ever + DEFAULT arg
class Test4(val test: String = "")

The simplest bean ever + DEFAULT arg
…
// $FF: synthetic method
public Test4(String var1, int var2,DefaultConstructorMarker var3) {
if ((var2 & 1) != 0) {
var1 = "";
}
this(var1);
}
...
this.test = test;
}
public Test4() {
this((String)null, 1, (DefaultConstructorMarker)null);
}
}

Default constructor args. Call from java code.
class Test4(val test: String = "")
void test() {
new Test4("test");
new Test4();
}

Default method args. Call from java code.
class Test5 {
fun x(x: Int = 5) { }
}

class Test5 {
fun x(x: Int = 5) { }
}
void test() {
new Test5().x();
}

public final void x(int x) {
}
public static void x$default(Test5 var0,
int var1,
int var2,
Object var3) {
if ((var2 & 1) != 0) {
var1 = 5;
}
var0.x(var1);
}
}

Default method args. Correct call from java
code.
void test() {
new Test5().x();
}
class Test5 {
@JvmOverloads
fun x(x: Int = 5) { }
}

Give me more DATA!!!
data class Test6(val x: Int = 0, val y: Int = 0)

Give me more DATA!!!
private final int x; private final int y;
// + getX(), getY(), constructor with both parameters
// + toString(), hashCode(), equals(Object obj)
public final int component1() {
return this.x;
}
public final int component2() {
return this.y;
}
@NotNull
public final Test6 copy(int x, int y) {
return new Test6(x, y);
}
}

Destructurization
fun test() {
val (a,b,c) = Test6()
}
public final void test() {
Test6 var4 = new Test6(0, 0, (Test4)null, 7,
DefaultConstructorMarker)null);
int var1 = var4.component1();
int var2 = var4.component2();
Test4 c = var4.component3();
}

Let my Kotlin go
fun test7() = Any().let {
it.toString()
this.toString()
}

Let my Kotlin go
public final String test7() {
Object var1 = new Object();
var1.toString();
return this.toString();
}

Let’s think about it
????fun test() = null.let { println(it) }

Let’s think about it
public static final void test() {
Object var0 = null;
Void it = (Void)var0;
System.out.println(it);
}

The power of primitives
fun test() = 1
public final int test () {
return 1;
}

fun test(): Int {
val x: Int = 1
return x
}
public final int test() {
int x = 1;
return x;
}

fun test(): Int {
val x: Int = 1
x.toString()
x.compareTo(2)
x.hashCode()
return x
}
????

fun test(): Int {
val x: Int = 1
x.toString()
x.compareTo(2)
x.hashCode()
return x
}
public final int test() {
int x = 1;
String.valueOf(x);
Intrinsics.compare(x, 2);
Integer.valueOf(x).hashCode();
return x;
}

Nullable types
fun test(value: Int?) {
val test1 = value ?: 0
}
????

Nullable types
public static final void test(@Nullable Integer value)
{
int test1 = value != null ? value.intValue() : 0;
}

Nullable types
fun test(value: Int?) {
val test1 = value!!
}
????

Nullable types
public static final void test(@Nullable Integer
value) {
if (value == null) {
Intrinsics.throwNpe();
}
int test1 = value.intValue();
}

Cast me babe
val x = Any() is String
val y: Any = 1
val z: Int = y as Int

Cast me babe
private static final boolean x =
new Object() instanceof String;
@NotNull
private static final Object y = Integer.valueOf(1);
private static final int z = 1;

Cast me babe. And please, be smart!
fun test() {
val any = "test" as Any
if(any is String) {
any.trim()
any.substringBefore("test")
}
}

Cast me babe. And please, be smart!
public static final void test() {
Object any = (Object)"test";
String var1 = (String)any;
StringsKt.trim((CharSequence)var1).toString();
StringsKt.substringBefore$default((String)any, "test",
(String)null, 2,
(Object)null);
}

When
fun test(x: Any) {
when(x) {
0 -> println(0)
is Int -> println("Int")
is Number -> println("Number")
else -> println("Not an int")
}
}
????

When
public static final void test(@NotNull Object x) {
Intrinsics.checkParameterIsNotNull(x, "x");
if (Intrinsics.areEqual(x, Integer.valueOf(0))) {
byte var2 = 0;
} else {
String var3;
if (x instanceof Integer) {
var3 = "Int";
} else if (x instanceof Number) {
var3 = "Number";
} else {
var3 = "Not an int";
}
}
}

Where is my ternary?
fun test (value: Int) = value == 1 ? 1 : 0
public static final int test (int value) {
return value == 1 ? 1 : 0;
}

Where is my ternary?
fun test (value: Int) = if(value == 1) 1 else 0

Range
@NotNull
private static final IntRange range;
@NotNull
public static final IntRange
getRange() {
return range;
}
static {
byte var0 = 1;
range = new IntRange(var0, 100);
}
}
val range = 1 .. 100

Range
????val result = 50 in 1..100

Range
private static final boolean result = true;
public static final boolean getResult() {
return result;
}
}

Range
fun test14(){
for(x in 1 .. 100) { }
}

Range
public static final void test14() {
int var0 = 1;
for(byte var1 = 101; var0 < var1; ++var0) {
;
}
}

Range
fun test14(){
for(x in 1 .. 100 step 1) { }
}

Range
byte var3 = 1;
IntProgression var10000 =
RangesKt.step(
(IntProgression)(new IntRange(var3, 100)), 1);
int x = var10000.getFirst();
int var1 = var10000.getLast();
int var2 = var10000.getStep();
if (var2 > 0) {
if (x > var1) {
return;
}
} else if (x < var1) {
return;
}
while(x != var1) {
x += var2;
}
}

Inlines
fun test15() {
println(test15_1 { it * 2 })
}
fun test15_1(num: (Int) -> Int) = num(2)

Inlines
int var0 = test15_1((Function1)null.INSTANCE);
}
public static final int test15_1(@NotNull Function1 num)
{
Intrinsics.checkParameterIsNotNull(num, "num");
return ((Number)num.invoke(2)).intValue();
}

Inlines
fun test15() {
println(test15_1 { it * 2 })
}
inline fun test15_1(num: (Int) -> Int) = num(2)

Inlines
int it = 2;
int var3 = it * 2;
}
public static final int test15_1(@NotNull Function1 num)
{
return ((Number)num.invoke(2)).intValue();
}

Inlines. Noinline
fun test15() {
println(test15_1({ it + 1 }, { it * 2 }))
}
inline fun test15_1(num: (Int) -> Int, noinline num2:
(Int) -> Int)
= num(1) + num2(1)

Inlines. Noinline
Function1 num2$iv = (Function1)null.INSTANCE;
int it = 1;
int var4 = it + 1 +
((Number)num2$iv.invoke(1)).intValue();
}
public static final int test15_1(@NotNull Function1 num,
@NotNull Function1 num2) {
Intrinsics.checkParameterIsNotNull(num2, "num2");
return ((Number)num.invoke(1)).intValue() +
((Number)num2.invoke(1)).intValue();
}

Overloading with generics
public void sort(List<String> x) {}
public void sort(List<Integer> x) {}

fun sort(x: List<Any>) {}
fun sort(x: List<Int>) {}

fun sort(x: List<Any>) {}
@JvmName("sortInt")
fun sort(x: List<Int>) {}

Overloading of operators
class IntArray(val data: Array<Int>) {
operator fun plus(value: Int) = data + value
fun test() {
val intArr = IntArray(arrayOf(1))
val newArr = intArr + 2
}
}

Overloading of operators
public final class IntArray {
@NotNull
private final Integer[] data;
@NotNull
public final Integer[] plus(int value) {
return (Integer[])ArraysKt.plus((Object[])this.data, value);
}
public final void test() {
IntArray intArr = new IntArray(new Integer[]{Integer.valueOf(1)});
Integer[] newArr = intArr.plus(2);
}
@NotNull
public final Integer[] getData() {
return this.data;
}
public IntArray(@NotNull Integer[] data) {
Intrinsics.checkParameterIsNotNull(data, "data");
super();
this.data = data;
}
}

Delegation using ‘by’
interface Interf {
fun xx(): Int
}
class A: Interf {
override fun xx() = 1
}
class B(a: A): Interf by a
????

Delegation using ‘by’
public interface Interf { int xx();}
public final class A implements Interf { public int xx() { return 1; }}
public final class B implements Interf {
private final A $$delegate_0;
public B(@NotNull A a) {
Intrinsics.checkParameterIsNotNull(a, "a");
super();
this.$$delegate_0 = a;
}
public int xx() {
return this.$$delegate_0.xx();
}

Recursion
fun factorial(value: Int): Int =
if(value <= 1) 1
else value * factorial(value -1)

Recursion
public static final int factorial(int value) {
return value <= 1 ?
1 :
value * factorial(value - 1);
}

Tail recursion
????tailrec fun factorial(value: Int)
: Int =
if(value <= 1) 1
else value * factorial(value -1)

Tail recursion
public static final int factorial(int value) {
return value <= 1 ?
1 :
value * factorial(value - 1);
}

Tail recursion
tailrec fun factorial(value: Int, acc: Int): Int =
if(value <= 1) acc * value
else factorial(value - 1, acc * value)

Tail recursion
public static final int factorial(int value, int acc) {
while(value > 1) {
int var10000 = value - 1;
acc *= value;
value = var10000;
}
return acc * value;
}

Cons of Kotlin
• Learning curve
• Slow compilation speed
• Smaller community with
comparison to java

Pros of Kotlin
• Reduces boilerplate
• Compatible with jvm
• Competitive performance
• Elegant
• Best IDE integration
• Choice №1 for android

Kotlin decompiled

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