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?
In Kotlin, you can concatenate using String interpolation/templates:
val a = "Hello" val b = "World" val c = "$a $b" The output will be: Hello World
StringBuilder for String templates which is the most efficient approach in terms of memory because +/plus() creates new String objects.Or you can concatenate using the StringBuilder explicitly.
val a = "Hello" val b = "World" val sb = StringBuilder() sb.append(a).append(b) val c = sb.toString() print(c) The output will be: HelloWorld
Or you can concatenate using the + / plus() operator:
val a = "Hello" val b = "World" val c = a + b // same as calling operator function a.plus(b) print(c) The output will be: HelloWorld
a.plus(b) or a + b and the same bytecode is generated
"Hello" + "Word" is just as performant as using StringBuilder to append to a string, someHelloStringBuilder.append("World")?
kotlin.String has a plus method:
a.plus(b) See https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-string/plus.html for details.
+ operator is normal, not calling the translated operator function plus ... this is not idiomatic
a = a.plus(b) for instance
plus() is less practical than a StringBuilder's append method ie.Try this, I think this is a natively way to concatenate strings in Kotlin:
val result = buildString{ append("a") append("b") } println(result) // you will see "ab" in console. 
buildString { instead of stringBuild {
I agree with the accepted answer above but it is only good for known string values. For dynamic string values here is my suggestion.
// A list may come from an API JSON like { "names": [ "Person 1", "Person 2", "Person 3", ... "Person N" ] } var listOfNames = mutableListOf<String>() val stringOfNames = listOfNames.joinToString(", ") // ", " <- a separator for the strings, could be any string that you want // Posible result // Person 1, Person 2, Person 3, ..., Person N This is useful for concatenating list of strings with separator.
Yes, you can concatenate using a + sign. Kotlin has string templates, so it's better to use them like:
var fn = "Hello" var ln = "World" "$fn $ln" for concatenation.
You can even use String.plus() method.
+ operator is normal, not calling the translated operator function plus ... this is not idiomaticSimilar to @Rhusfer answer I wrote this. In case you have a group of EditTexts and want to concatenate their values, you can write:
listOf(edit_1, edit_2, edit_3, edit_4).joinToString(separator = "") { it.text.toString() } If you want to concatenate Map, use this:
map.entries.joinToString(separator = ", ") To concatenate Bundle, use
bundle.keySet().joinToString(", ") { key -> "$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=${bundle[key]}" } println(bundleString) // arrange=0, price=20.5, title=Night cream The most simplest way so far which will add separator and exclude the empty/null strings from concatenation:
val finalString = listOf(a, b, c) .filterNot { it.isNullOrBlank() } .joinToString(separator = " ") There are various way to concatenate strings in kotlin Example -
a = "Hello" , b= "World" Using + operator a+b
Using plus() operator
a.plus(b)
Note - + is internally converted to .plus() method only
In above 2 methods, a new string object is created as strings are immutable. if we want to modify the existing string, we can use StringBuilder
StringBuilder str = StringBuilder("Hello").append("World") If you have an object and want to concatenate two values of an object like
data class Person( val firstName: String, val lastName: String ) Then simply following won't work
val c = "$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}" The best way of string concatenation is String Interpolation in Kotlin.
val a = "Hello" val b = "World" val mString = "$a $b" val sString = "${a, b}, I am concatenating different string with." //Output of mString : Hello World //Output of sString : Hello World, I am concatenating different string with. The idiomatically correct way to do string interpolation in Kotlin is as follows
val helloString = "Hello" val worldString = "World" val concatString = "$helloString $worldString" Output
Hello World yourString += "newString"
This way you can concatenate a string
In Kotlin we can join string array using joinToString()
val tags=arrayOf("hi","bye") val finalString=tags.joinToString (separator = ","){ "#$it" } Result is :
#hi,#bye
if list coming from server
var tags = mutableListOf<Tags>() // list from server val finalString=tags.joinToString (separator = "-"){ "#${it.tagname}" } Result is :
#hi-#bye
I suggest if you have a limited and predefined set of values then the most efficient and readable approach is to use the String Template (It uses String Builder to perform concatination).
val a = "Hello" val b = "World" val c = "$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.
Using Addition Assignment (+=) Operator:
| Expression | Equivalent | Translates |
|---|---|---|
| a +=b | a = a + b | a.plusAssign(b) |
Basic usage:
var message = "My message" message += ", add message 2" Will output: My message, add message 2
Generic functions, with optional spaces between words:
fun concat(vararg words: String, space: Boolean = false): String { var message = "" words.forEach { word -> message += word + if (space) " " else "" } return message } fun concat(vararg words: String, separator: String = ""): String { return string.joinToString(separator) } Calling generic function:
concat("one", "two", "three", "four", "five") concat("one", "two", "three", "four", "five", space = true) Output:
onetwothreefourfive one two three four five 