The problem is quite simple:
What is the difference between these 2 versions (both have initialized var name: String and val someInt: Int)?
name += " $someInt"
name = "$name $someInt"
I also read that the String interpolation $ uses a StringBuilder, so the second code should be preferred to the first one (which uses concatenation)?
The best way to find the difference is to look at the generated bytecode (I will use Kotlin 1.4.10).
Bytecode generated for name += " $someInt":
ALOAD 0
NEW java/lang/StringBuilder
DUP
INVOKESPECIAL java/lang/StringBuilder.<init> ()V
SWAP
INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
BIPUSH 32
INVOKEVIRTUAL java/lang/StringBuilder.append (C)Ljava/lang/StringBuilder;
ILOAD 1
INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/StringBuilder;
INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
ASTORE 0
Bytecode generated for name = "$name $someInt":
NEW java/lang/StringBuilder
DUP
INVOKESPECIAL java/lang/StringBuilder.<init> ()V
ALOAD 0
INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
BIPUSH 32
INVOKEVIRTUAL java/lang/StringBuilder.append (C)Ljava/lang/StringBuilder;
ILOAD 1
INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/StringBuilder;
INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
ASTORE 0
Results are pretty much the same, but in the first case there is an extra SWAP operation, cause argument for first append operation was loaded onto stack too early (before StringBuilder was created, and now they need to be swapped).
TL;DR
Turns out that:
StringBuilder is created in both casesname, whitespace & someInt) in both cases
The semantics are equivalent, and the second one is probably slightly more efficient, but not exactly for the reason you give: it's that there's only one concatenation instead of two. (The first one first generates " " + someInt, and then generates name + (" " + someInt).)
