Extensions in Kotlin are very powerful mechanism. It allows for add any method to any of existing classes. Each instance has (as in Java) equals, toString and hashCode methods, but there is much more in Kotlin.

Example classes

Let's define some simple classes describing person: normal class and data class.

class PersonJaxb {
    var firstName: String? = null
    var lastName: String? = null
    var age: Int? = null
}

data class Person(val firstName: String, val lastName: String, val age: Int)

Normal class extensions

All instances have methods described below.

apply method

I often work with jaxb classes similar to PersonJaxb, which has not all arg constructor and all fields must be set via setters. Kotlin helps to deal with it via apply method. Target instance is provided as delagate to closure so we could define all fields values in it and returns this. The signature is T.apply(f: T.() -> Unit): T.

@Test
fun applyTest() {
    //when
    val person = PersonJaxb().apply {
        firstName = "John"
        lastName = "Smith"
        age = 20
    }

    //then
    assertEquals(20, person.age)
    assertEquals("John", person.firstName)
    assertEquals("Smith", person.lastName)
}

let method

Another extension is let method which is similar to map operation for collections. It has signature T.let(f: (T) -> R): R. this is passed as parameter to given closure/function.

@Test
fun letTest() {
    //when
    val fullName = Person("John", "Smith", 20).let {
        "${it.firstName} ${it.lastName}"
    }

    //then
    assertEquals("John Smith", fullName)
}

run method

run method looks like merge of apply and let methods: access to this is via delegate as in apply, but it also returns value as in let method. It has signature T.run(f: T.() -> R): R.

@Test
fun runTest() {
    //when
    val fullName = Person("John", "Smith", 20).run {
        "$firstName $lastName"
    }

    //then
    assertEquals("John Smith", fullName)
}

to method

Each instance has also defined to infix operator, which is used to create Pair. Pairs is helpful to create map entries. It has signature A.to(that: B): Pair<A, B>.

@Test
fun toTest() {
    //when
    val pair = Person("John", "Smith", 20) to 5

    //then
    assertEquals(Person("John", "Smith", 20), pair.first)
    assertEquals(5, pair.second)
}

Data class methods

Data class instances have also some other helpful methods (which are not extensions, but are generated for us).

componentX methods

Data class Person has three fields and it has component method generated for each of them: component1 for firstName, component2 for lastName and component3 for age.

@Test
fun componentsTest() {
    //when
    val p = Person("John", "Smith", 20)

    //then
    assertEquals("John", p.component1())
    assertEquals("Smith", p.component2())
    assertEquals(20, p.component3())
}

Why is it helpful? componentX methods are used in extracting (similar to Scala case classes extracting mechanism), e. g.:

@Test
fun extractingTest() {
    //when
    val (first, last, age) = Person("John", "Smith", 20)

    //then
    assertEquals(20, age)
    assertEquals("John", first)
    assertEquals("Smith", last)
}

copy method

copy method allows to create new instance based on current instance.

@Test
fun copyTest() {
    //when
    val person = Person("John", "Smith", 20).copy(lastName = "Kowalski", firstName = "Jan")

    //then
    assertEquals(Person("Jan", "Kowalski", 20), person)
}

Summary

Kotlin's extensions for each instances are very simple and help to solve many problems. The code written with these extensions is much more readable and concise than written in Java.

Sources are available here.