[string] Kotlin - How to correctly concatenate a String

A very basic question, what is the right way to concatenate a String in Kotlin?

In Java you would use the concat() method, e.g.

String a = "Hello ";
String b = a.concat("World"); // b = Hello World

The concat() function isn't available for Kotlin though. Should I use the + sign?

This question is related to string kotlin

The answer is

yourString += "newString"

This way you can concatenate a string

There are various way to concatenate strings in kotlin Example -

a = "Hello" , b= "World"
  1. Using + operator a+b

  2. Using plus() operator


Note - + is internally converted to .plus() method only

In above 2 methods, a new string object is created as strings are immutable. if we want to modify the existing string, we can use StringBuilder

StringBuilder str = StringBuilder("Hello").append("World")

kotlin.String has a plus method:


See https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-string/plus.html for details.

Yes, you can concatenate using a + sign. Kotlin has string templates, so it's better to use them like:

var fn = "Hello"
var ln = "World"

"$fn $ln" for concatenation.

You can even use String.plus() method.

Try this, I think this is a natively way to concatenate strings in Kotlin:

val result = buildString{


// you will see "ab" in console.

I agree with the accepted answer above but it is only good for known string values. For dynamic string values here is my suggestion.

// A list may come from an API JSON like
   "names": [
      "Person 1",
      "Person 2",
      "Person 3",
      "Person N"
var listOfNames = mutableListOf<String>() 

val stringOfNames = listOfNames.joinToString(", ") 
// ", " <- a separator for the strings, could be any string that you want

// Posible result
// Person 1, Person 2, Person 3, ..., Person N

This is useful for concatenating list of strings with separator.

Similar to @Rhusfer answer I wrote this. In case you have a group of EditTexts and want to concatenate their values, you can write:

listOf(edit_1, edit_2, edit_3, edit_4).joinToString(separator = "") { it.text.toString() }

If you want to concatenate Map, use this:

map.entries.joinToString(separator = ", ")

To concatenate Bundle, use

bundle.keySet().joinToString(", ") { key -> "$key=${bundle[key]}" }

It sorts keys in alphabetical order.


val map: MutableMap<String, Any> = mutableMapOf("price" to 20.5)
map += "arrange" to 0
map += "title" to "Night cream"
println(map.entries.joinToString(separator = ", "))

// price=20.5, arrange=0, title=Night cream

val bundle = bundleOf("price" to 20.5)
bundle.putAll(bundleOf("arrange" to 0))
bundle.putAll(bundleOf("title" to "Night cream"))
val bundleString =
    bundle.keySet().joinToString(", ") { key -> "$key=${bundle[key]}" }

// arrange=0, price=20.5, title=Night cream