apply

apply executes a block of code on an object and returns the object itself. Inside the block, the object is referenced by this. This function is handy for initializing objects.

data class Person(var name: String, var age: Int, var about: String) {
    constructor() : this("", 0, "")
}

fun main() {
    val jake = Person()                                     // 1
    val stringDescription = jake.apply {                    // 2
        name = "Jake"                                       // 3
        age = 30
        about = "Android developer"
    }.toString()                                            // 4
    println(stringDescription)
}

1. Creates a Person() instance with default property values.
2. Applies the code block (next 3 lines) to the instance.
3. Inside apply, it's equivalent to jake.name = "Jake".
4. The return value is the instance itself, so you can chain other operations.