String helpers
- Kotlin course
- Intermediate
- Lesson 20/116
- 6 min
Add and test string processing functions.
String extension function
An extension function adds a method to an existing type from outside. The declaration puts the receiver type (ReceiverType.) before the function name. The type itself does not change.
Example: fun String.titleCase(): String { return split(" ").joinToString(" ") { it.replaceFirstChar(Char::uppercase) } }. This function adds titleCase to String. Calling "hello world".titleCase() returns Hello World. Extension functions are resolved statically; they do not modify the type itself.
The titleCase function
This lesson adds the titleCase extension to String. It uppercases the first letter of each word and lowercases the rest.
Example: fun String.titleCase(): String { return split(" ").joinToString(" ") { word -> word.lowercase().replaceFirstChar { it.uppercase() } } }. Use split(" ") to break into words, joinToString(" ") to rejoin. replaceFirstChar uppercases the first letter. Call: "hello world".titleCase(). This produces a title-style output.
Practice Add the String.titleCase extension
Alongside the existing Helpers object, add a String.titleCase() extension function. Its body returns split(" ").joinToString(" ") { word -> word.lowercase().replaceFirstChar { it.uppercase() } }. In main print first Helpers.greet("Alex"), then "hello world".titleCase() with println.
Starter code
object Helpers {
fun greet(name: String): String {
return "Hello, $name!"
}
}
// String extension: titleCase
// uppercase the first letter of each word
// call greet and titleCase in mainSample solution
object Helpers {
fun greet(name: String): String {
return "Hello, $name!"
}
}
fun String.titleCase(): String {
return split(" ").joinToString(" ") { word ->
word.lowercase().replaceFirstChar { it.uppercase() }
}
}
fun main() {
println(Helpers.greet("Alex"))
println("hello world".titleCase())
}Expected output
Hello, Alex! Hello World
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the extension parts
| Term | Meaning |
|---|---|
fun String.titleCase() |
Receiver type and method |
split(" ") |
Split on space |
joinToString(" ") |
Join |
"abc".titleCase() |
Call |
2 How is an extension function declared to add <code>titleCase</code> to <code>String</code>?
- fun String.titleCase(): String Correct answer
- class String { fun titleCase() }
- fun titleCase(): String
- extension String.titleCase()
Explanation: In an extension the receiver type goes before the function name.
3 Complete the extension function signature
fun ____.titleCase(): String {
return split(" ").joinToString(" ") { word ->
word.lowercase().replaceFirstChar { it.uppercase() }
}
}fun String.titleCase(): String {
return split(" ").joinToString(" ") { word ->
word.lowercase().replaceFirstChar { it.uppercase() }
}
}Explanation: String is the receiver type for the titleCase extension.