Contracts
- Kotlin course
- Expert
- Lesson 1/47
- 5 min
Learn compile-time guarantees with Kotlin contracts.
Contracts
Contracts are commitments that tell the compiler which conditions hold after a function is called. They are written with Kotlin's kotlin.contracts API; it is an experimental API.
The most common use is to declare that a value is non-null once the function returns. Example: contract { returns() implies (x != null) }. Then the call site needs no extra !! or if (x != null) block. Contracts only inform the compiler; they emit no runtime code.
returns() implies
A contract block opens with contract { ... }. The most common clause is returns() implies (condition).
Example: contract { returns() implies (x != null) }. This says: when this function returns normally, x is not null. After a requireNonNull(name) call the compiler treats name as String. A wrong contract causes runtime errors, so write them with care.
Practice Write the requireNonNull function
Define requireNonNull(x: String?): String. Inside it, place a contract { returns() implies (x != null) } clause. In main, after calling requireNonNull(name), write println(name.length).
Starter code
import kotlin.contracts.*
fun requireNonNull(x: String?): String {
// add the contract
return x!!
}
fun main() {
val name: String? = "Ayse"
requireNonNull(name)
// print the length
}Sample solution
fun requireNonNull(x: String?): String {
contract { returns() implies (x != null) }
return x!!
}
fun main() {
val name: String? = "Ayse"
requireNonNull(name)
println(name.length)
}Expected output
4
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does a contract do?
- Throws an error at runtime
- Awaits an async call
- Generates a new class
- Gives the compiler a behaviour commitment Correct answer
Explanation: A contract informs the compiler; it emits no runtime code.
2 Complete the contract block
fun requireNonNull(x: String?): String {
____ { returns() implies (x != null) }
return x!!
}fun requireNonNull(x: String?): String {
contract { returns() implies (x != null) }
return x!!
}Explanation: contract { } starts a contract.
3 Match contract parts
| Term | Meaning |
|---|---|
contract |
Contract block |
returns() |
Return clause |
implies |
States result |
x != null |
Non-null guarantee |