How Kotlin can be used as an alternative to the builder pattern that we so often use in Java

1. Kotlin DSL beats the builder patte
Arild Nilsen, December 2018

2. Person.builder()
.name("John")
.age(25)
.addresses(
Address.builder()
.street("Main Street")
.number(12)
.city("London")
.build(),
Address.builder()
.street("Dev Avenue")
.number(42)
.city("Paris")
.build()
)
.build();

3. DSL building blocks
• Lambdas without parentheses
• Lambdas with receivers
• Extension functions

4. Lambdas without parentheses
fun person(block: (Person) -> Unit): Person {
val p = Person()
block(p)
return p
}
val person = person {
it.name = "John"
it.age = 25
}
val person = person({ p ->
p.name = "John"
p.age = 25
})

5. Lambdas with receivers
fun person(block: Person.() -> Unit): Person {
val p = Person()
p.block()
return p
}
val person = person {
name = "John"
age = 25
}
fun person(block: Person.() -> Unit): Person
= Person().apply(block)

6. Extension functions
fun Person.address(block: Address.() -> Unit) {
address = Address().apply(block)
}
val person = person {
name = "John"
age = 25
address {
street = "Main Street"
number = 42
city = "London"
}
}

7. val person = person {
"John" age 25
addresses {
address {
street = "Main Street"
number = 12
city = "London"
}
address {
street = "Dev Avenue"
number = 42
city = "Paris"
}
}
}

8. Who uses the DSL?
• Libraries: Coroutines, Ktor, Exposed
• Gradle Kotlin DSL