Extension properties
- Kotlin course
- Intermediate
- Lesson 37/116
- 5 min
Learn extension properties with val Receiver.ext.
Extension property
An extension property adds a property to an existing type from outside. It is written as val Receiver.prop: Type and uses a getter.
Example: val String.isLong: Boolean get() = length > 10. This adds isLong to String. Calling "hello world".isLong returns true. Extension properties can only have a getter, because an extension cannot store a backing field. They suit computed values.
The isLong property
The getter of an extension property accesses the receiver's methods and properties through this. Because there is no backing field, only computation runs.
Example: val String.isLong: Boolean get() = length > 10. Here length means this.length. Calling "hello world".isLong returns true; "short" gives false. An extension property is a short way to add a computed property without changing the class source.
Practice The String.isLong extension property
Define an extension property val String.isLong: Boolean whose getter returns length > 10. In main print "hello world".isLong and "short".isLong on separate lines with println.
Starter code
// Define the String.isLong extension property
// getter returns length > 10
// print both values in mainSample solution
val String.isLong: Boolean
get() = length > 10
fun main() {
println("hello world".isLong)
println("short".isLong)
}Expected output
true false
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the extension property parts
| Term | Meaning |
|---|---|
val String.isLong |
Receiver and property |
get() = length > 10 |
Getter body |
"abc".isLong |
Call |
Boolean |
Return type |
2 Why can an <strong>extension property</strong> only be val (not var)?
- Because val is faster
- Because it cannot store a backing field Correct answer
- Because var syntax is invalid
- Because it is only declared inside a class
Explanation: An extension property cannot store a backing field; that is why it is val only.
3 Complete the extension property definition
val String.isLong: Boolean
get() = ____ > 10val String.isLong: Boolean
get() = length > 10Explanation: length returns the String length.