Wsrtjtyk

I am trying to understand few of the Kotlin features by checking how it looks in Java side.

So as an experiment, I tried with this:

val printKotlin = fun () {

    print("Hello Kotlin")

}

So the output for the above snippet is:

public final class FunAsVariableKt {

    private static final Function0 printKotlin;



    public static final Function0 getPrintKotlin() {

        return printKotlin;

    }



    static {

        printKotlin = (Function0)null.INSTANCE;

    }

}

How do I understand the static block of the above decompiled code? Why it is generating this non working code?

Use the Kotlin bytecode inspector to see what is being generated as JVM bytecode rather than trying to decompile bytecode to Java, which likely won't work for code generated by a different language compiler because it might not follow the expected patterns. A bytecode decompiler does not always generate working code.

JVM bytecode != Java language specific

Kotlin generated JVM bytecode != Java generated JVM bytecode

The NULL instance you are seeing is a misinterpretation by your decompiler, and you can see all references to INSTANCE in the bytecode are setting values correctly.

This is the actual output:

// ================Stackoverflow_53384931Kt.class =================

// class version 52.0 (52)

// access flags 0x31

public final class Stackoverflow_53384931Kt {





  // access flags 0x1A

  // signature Lkotlin/jvm/functions/Function0<Lkotlin/Unit;>;

  // declaration: kotlin.jvm.functions.Function0<kotlin.Unit>

  private final static Lkotlin/jvm/functions/Function0; printKotlin

  @Lorg/jetbrains/annotations/NotNull;() // invisible



  // access flags 0x19

  // signature ()Lkotlin/jvm/functions/Function0<Lkotlin/Unit;>;

  // declaration: kotlin.jvm.functions.Function0<kotlin.Unit> getPrintKotlin()

  public final static getPrintKotlin()Lkotlin/jvm/functions/Function0;

  @Lorg/jetbrains/annotations/NotNull;() // invisible

   L0

    LINENUMBER 3 L0

    GETSTATIC Stackoverflow_53384931Kt.printKotlin : Lkotlin/jvm/functions/Function0;

    ARETURN

   L1

    MAXSTACK = 1

    MAXLOCALS = 0



  // access flags 0x8

  static <clinit>()V

   L0

    LINENUMBER 3 L0

    GETSTATIC Stackoverflow_53384931Kt$printKotlin$1.INSTANCE : LStackoverflow_53384931Kt$printKotlin$1;

    CHECKCAST kotlin/jvm/functions/Function0

    PUTSTATIC Stackoverflow_53384931Kt.printKotlin : Lkotlin/jvm/functions/Function0;

    RETURN

    MAXSTACK = 1

    MAXLOCALS = 0



  @Lkotlin/Metadata; ... 

  // access flags 0x18

  final static INNERCLASS Stackoverflow_53384931Kt$printKotlin$1 null null

  // compiled from: stackoverflow-53384931.kt

}





// ================Stackoverflow_53384931Kt$printKotlin$1.class =================

// class version 52.0 (52)

// access flags 0x30

// signature Lkotlin/jvm/internal/Lambda;Lkotlin/jvm/functions/Function0<Lkotlin/Unit;>;

// declaration: Stackoverflow_53384931Kt$printKotlin$1 extends kotlin.jvm.internal.Lambda implements kotlin.jvm.functions.Function0<kotlin.Unit>

final class Stackoverflow_53384931Kt$printKotlin$1 extends kotlin/jvm/internal/Lambda  implements kotlin/jvm/functions/Function0  {





  // access flags 0x1041

  public synthetic bridge invoke()Ljava/lang/Object;

    ALOAD 0

    INVOKEVIRTUAL Stackoverflow_53384931Kt$printKotlin$1.invoke ()V

    GETSTATIC kotlin/Unit.INSTANCE : Lkotlin/Unit;

    ARETURN

    MAXSTACK = 1

    MAXLOCALS = 1



  // access flags 0x11

  public final invoke()V

   L0

    LINENUMBER 4 L0

    LDC "Hello Kotlin"

    ASTORE 1

   L1

    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;

    ALOAD 1

    INVOKEVIRTUAL java/io/PrintStream.print (Ljava/lang/Object;)V

   L2

   L3

    LINENUMBER 5 L3

    RETURN

   L4

    LOCALVARIABLE this LStackoverflow_53384931Kt$printKotlin$1; L0 L4 0

    MAXSTACK = 2

    MAXLOCALS = 2



  // access flags 0x0

  <init>()V

    ALOAD 0

    ICONST_0

    INVOKESPECIAL kotlin/jvm/internal/Lambda.<init> (I)V

    RETURN

    MAXSTACK = 2

    MAXLOCALS = 1



  // access flags 0x19

  public final static LStackoverflow_53384931Kt$printKotlin$1; INSTANCE



  // access flags 0x8

  static <clinit>()V

    NEW Stackoverflow_53384931Kt$printKotlin$1

    DUP

    INVOKESPECIAL Stackoverflow_53384931Kt$printKotlin$1.<init> ()V

    PUTSTATIC Stackoverflow_53384931Kt$printKotlin$1.INSTANCE : LStackoverflow_53384931Kt$printKotlin$1;

    RETURN

    MAXSTACK = 2

    MAXLOCALS = 0



  @Lkotlin/Metadata; ...

  OUTERCLASS Stackoverflow_53384931Kt null

  // access flags 0x18

  final static INNERCLASS Stackoverflow_53384931Kt$printKotlin$1 null null

  // compiled from: stackoverflow-53384931.kt

  // debug info: SMAP

  ...



}