Why does Kotlin string interpolation concatenate strings in Java?

Question

So basically, I was wondering how to interpret a string interpretation in a Java context (decompiled bytecode). At first I thought it would use StringBuffer (or StringBuilder) or String.format (), but it seems like it is doing concatenation. Is this really optimal for this and what about the row pool?

Here is my Kotlin code:

fun main(args: Array<String>) {

    val age : Int = 24
    var mySuperString : String = "Dato is ${age} years old!"

    println(mySuperString)
}

And here is the Java code that I decompiled in * .class format:

import java.io.PrintStream;
import kotlin.Metadata;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;

@Metadata(mv={1, 1, 6}, bv={1, 0, 1}, k=2, d1={"\000\024\n\000\n\002\020\002\n\000\n\002\020\021\n\002\020\016\n\002\b\002\032\031\020\000\032\0020\0012\f\020\002\032\b\022\004\022\0020\0040\003 \006\002\020\005 \006\006"}, d2={"main", "", "args", "", "", "([Ljava/lang/String;)V", "KotlinProject"})
public final class MainKt
{
  public static final void main(@NotNull String[] args)
  {
    Intrinsics.checkParameterIsNotNull(args, "args");int age = 24;
    String mySuperString = "Dato is " + age + " years old!";

    System.out.println(mySuperString);
  }
}

Even when the line is huge and there is a lot of value, it still uses plus signs!

Is this the fastest way or why do they do it?

Answer 1

The answer is simple: it is not

, , StringBuilder -append + .

, -, , StringBuilder.

public main(java.lang.String[] arg0) { //([Ljava/lang/String;)V
     <localVar:index=3 , name=mySuperString , desc=Ljava/lang/String;, sig=null, start=L1, end=L2>
     <localVar:index=2 , name=age , desc=I, sig=null, start=L3, end=L2>
     <localVar:index=0 , name=this , desc=Lorg/guenhter/springboot/kt/RandomReceivingController;, sig=null, start=L4, end=L2>
     <localVar:index=1 , name=args , desc=[Ljava/lang/String;, sig=null, start=L4, end=L2>

     L4 {
         aload1
         ldc "args" (java.lang.String)
         invokestatic kotlin/jvm/internal/Intrinsics checkParameterIsNotNull((Ljava/lang/Object;Ljava/lang/String;)V);
     }
     L5 {
         bipush 24
         istore2
     }
     L3 {
         new java/lang/StringBuilder
         dup
         invokespecial java/lang/StringBuilder <init>(()V);
         ldc "Dato is " (java.lang.String)
         invokevirtual java/lang/StringBuilder append((Ljava/lang/String;)Ljava/lang/StringBuilder;);
         iload2
         invokevirtual java/lang/StringBuilder append((I)Ljava/lang/StringBuilder;);
         ldc " years old!" (java.lang.String)
         invokevirtual java/lang/StringBuilder append((Ljava/lang/String;)Ljava/lang/StringBuilder;);
         invokevirtual java/lang/StringBuilder toString(()Ljava/lang/String;);
         astore3
     }
     L1 {
         getstatic java/lang/System.out:java.io.PrintStream
         aload3
         invokevirtual java/io/PrintStream println((Ljava/lang/Object;)V);
     }
     L6 {
         return
     }
     L2 {
     }
 }