by map

Learn dynamic properties with by map.

by map delegation

You can delegate a property to a Map. Example: class User(val map: Map<String, Any?>) { val name: String by map; val age: Int by map }. Here name and age are read from map; the key is the property name.

This is handy for dynamic data: it makes key-value data like JSON accessible as a class. val properties delegate to a Map, var properties delegate to a MutableMap. For var the assignment map[name] = value works.

by map example

A User class is created with mapOf("name" to "Alex", "age" to 30). u.name reads the "name" key from the map and returns it as String. u.age returns 30.

Type safety matters: the age: Int declaration assumes the map value is an Int. A wrong type throws at runtime. Wrapping dynamic data with static types is the power of this pattern. With MutableMap, var properties can be both read and written.

Exercise Define the User class with by map

Define class User(val map: Map<String, Any?>). Add val name: String by map and val age: Int by map properties inside. In main create User(mapOf("name" to "Alex", "age" to 30)) and call println(u.name) and println(u.age).

Show example solution

class User(val map: Map<String, Any?>) {
    val name: String by map
    val age: Int by map
}

fun main() {
    val u = User(mapOf("name" to "Alex", "age" to 30))
    println(u.name)
    println(u.age)
}

Test yourself

Match the by map concepts

by map

Map delegation
map[key]

Read value
var name by map

Mutable
val name by map

Read-only
What type does val name by map require?
- List
- Array
- Set
- Map
Show answer

Map — by map requires a Map; the key is the property name.

Complete the by map declaration

class User(val map: Map<String, Any?>) {
    val name: String ____ map
}

Show answer

class User(val map: Map<String, Any?>) {
    val name: String by map
}

Next lesson: vetoable

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies