Summary - Extension functions

Combine extension functions, properties and nullable extensions to write a string helper extension.

Summary - Extension functions

In this group you learned extension function, extension property, nullable extension and companion extension. They all add methods or properties to existing types from outside.

In this summary you will combine all of them into a String helper extension set. fun String.shout() gives uppercase, val String.isLong checks length, fun String?.shoutOrEmpty() gives null-safe behavior. When all parts come together, you get a typesafe and short helper set.

Exercise String.shout and isLong extensions

Define an extension function fun String.shout(): String whose body returns uppercase() + "!". Define an extension property val String.isLong: Boolean whose getter returns length > 10. In main print "hello world".shout() and "hello world".isLong on separate lines with println.

Show example solution

fun String.shout(): String {
    return uppercase() + "!"
}

val String.isLong: Boolean
    get() = length > 10

fun main() {
    println("hello world".shout())
    println("hello world".isLong)
}

Exercise The nullable shoutOrEmpty extension

Define an extension function fun String?.shoutOrEmpty(): String whose body returns this?.uppercase() ?: "". In main print the result of "hi".shoutOrEmpty() and, after declaring val s: String? = null, the result of s.shoutOrEmpty() on separate lines with println.

Show example solution

fun String?.shoutOrEmpty(): String {
    return this?.uppercase() ?: ""
}

fun main() {
    println("hi".shoutOrEmpty())
    val s: String? = null
    println(s.shoutOrEmpty())
}

Test yourself

Recall why an <strong>extension property</strong> can only be val. What is the reason?
- Because it cannot store a backing field
- Because it is faster
- Because syntax forbids it
- Because it is only declared inside a companion object
Show answer

Because it cannot store a backing field — An extension property cannot store a backing field; that is why it is val.

Complete the nullable extension definition

fun String____.shoutOrEmpty(): String {
    return this?.uppercase() ?: ""
}

Show answer

fun String?.shoutOrEmpty(): String {
    return this?.uppercase() ?: ""
}

Next lesson: Quiz model

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies