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.

Answer 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.

Answer 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.

Answer 5

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.

Example:

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]}" }
println(bundleString)

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

Answer 6

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

val result = buildString{
    append("a")
    append("b")
}

println(result)

// you will see "ab" in console.

Answer 7

There are various way to concatenate strings in kotlin Example -

a = "Hello" , b= "World"

1. Using + operator a+b

2. Using plus() operator

   a.plus(b)

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")

Answer 8

yourString += "newString"

This way you can concatenate a string

Answer 9

I suggest if you have a limited and predefined set of values then the most efficient and readable approach is to use the String Template (It uses String Builder to perform concatination).

val a = "Hello"
val b = "World"
val c = "$a  ${b.toUpperCase()} !"

println(c) //prints: Hello  WORLD !

On the other hand if you have a collection of values then use joinToString method.

val res = (1..100).joinToString(",")
println(res) //prints: 1,2,3,...,100

I believe that some suggested solutions on this post are are not efficient. Like using plus or + or creating a collection for a limited set of entris and then applying joinToString on them.

Answer 10

If you have an object and want to concatenate two values of an object like

data class Person(
val firstName: String,
val lastName: String
)

Then simply following won't work

val c = "$person.firstName $person.lastName"

Correct way in this case will be

"${person.firstName} ${person.lastName}"

If you want any string in between concatenated values, just write there without any helper symbol. For example if I want "Name is " and then a hyphen in between first and last name then

 "Name is ${person.firstName}-${person.lastName}"

Answer 11

The most simplest way so far which will add separator and exclude the empty/null strings from concatenation:

val finalString = listOf(a, b, c)
    .filterNot { it.isNullOrBlank() }
    .joinToString(separator = " ")

Answer 12

In Kotlin we can join string array using joinToString()

val tags=arrayOf("hi","bye")

val finalString=tags.joinToString (separator = ","){ "#$it" }

Result is :

#hi,#bye

---

if list coming from server

var tags = mutableListOf<Tags>() // list from server

val finalString=tags.joinToString (separator = "-"){ "#${it.tagname}" }

Result is :

#hi-#bye