Kotlin - How to correctly concatenate a String

Solution 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.

Solution 2:

kotlin.String has a plus method:

a.plus(b)

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

Solution 3:

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.

Solution 4:

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.