Kotlin - How to correctly concatenate a String

Question

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?

Answer 1

String Templates/Interpolation

In Kotlin, you can concatenate using String interpolation/templates:

val a = "Hello" val b = "World" val c = "$a $b" 

The output will be: Hello World

• The compiler uses StringBuilder for String templates which is the most efficient approach in terms of memory because +/plus() creates new String objects.

Or you can concatenate using the StringBuilder explicitly.

val a = "Hello" val b = "World"  val sb = StringBuilder() sb.append(a).append(b) val c = sb.toString()  print(c) 

The output will be: HelloWorld

New String Object

Or you can concatenate using the + / plus() operator:

val a = "Hello" val b = "World" val c = a + b   // same as calling operator function a.plus(b)  print(c) 

The output will be: HelloWorld

• This will create a new String object.

Answer 2

kotlin.String has a plus method:

a.plus(b) 

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