apply

// Basic form of apply
val result = object.apply {
    // this = the object
    property = value
    method()
}
// result is the object itself

data class Person(var name: String = "", var age: Int = 0, var email: String = "")

fun main() {
    // Initialize an object with apply (this can be omitted).
    val person = Person().apply {
        name = "Alice"
        age = 30
        email = "alice@example.com"
    }
    println(person)
    // Person(name=Alice, age=30, email=alice@example.com)

    // apply is also commonly used to build a StringBuilder.
    val sb = StringBuilder().apply {
        append("Hello")
        append(", ")
        append("Kotlin!")
    }
    println(sb.toString())  // Hello, Kotlin!

    // apply chaining — you can chain multiple configurations together.
    val list = mutableListOf<String>().apply {
        add("Apple")
        add("Banana")
        add("Cherry")
    }
    println(list)  // [Apple, Banana, Cherry]

    // Because apply returns the receiver, you can call another method on it directly.
    val size = mutableListOf<Int>().apply {
        addAll(listOf(1, 2, 3, 4, 5))
    }.size
    println(size)  // 5
}

apply is commonly used as an initialization block for an object. Inside the block, the receiver is referenced via this, so you can assign to properties directly by name.

Because the return value is the receiver itself, it is convenient to assign the newly created object directly to a variable or use it as part of a method chain.

For inserting side effects, see also. For null checks, see let. When you need to return a computed result, see run / with.