Lazy Initialization is a common pattern for delaying creation of an object, calculation of a value, or an expensive process until it’s accessed for the first time. It is helpful when the initialization process consumes significant resources and the data isn’t always required when the object is used. Kotlin provides a good solution for that with lazy function. In this tutorial, we’re gonna look at a Kotlin Lazy Initialization example.
>>> Refer to: JavaSampleApproach.com
I. Technology
– Java 1.8
– Kotlin 1.1.2
II. Overview
Assume that we have a Person(name,books) class that lets us access a list of the books own by a person. Books list is stored in a database that we need time to access.
We want to load books on first access to the books property and do it only once.
It can be done with a delegated property:
|
1 2 3 4 |
data class Person(val name: String) { val books by lazy { loadBooks(this) } } |
– lazy function (thread-safe by default) returns an object that has getValuemethod called.
– parameter of lazy is a lambda to initialize the value.
III. Practice
1. Helper Class
BookManager class handles processing data:
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 |
package com.kotlination.lazyinit class BookManager { companion object { fun loadBooks(person: Person): List<String> { println("Load books for ${person.name}") return listOf("Master Kotlin at Kotlination", "Be Happy to become Kotlineer") } } } |
2. Class with delegated property
|
1 2 3 4 5 6 |
package com.kotlination.lazyinit data class Person(val name: String) { val books by lazy { BookManager.loadBooks(this) } } |
3. Run and check Result
Run the code below:
|
1 2 3 4 5 6 7 8 9 10 11 |
package com.kotlination.lazyinit fun main(args: Array<String>) { val person = Person("Andrien") println(person.books) println("--- call again ---") println(person.books) } |
Check Result:
|
1 2 3 4 |
Load books for Andrien [Master Kotlin at Kotlination, Be Happy to become Kotlineer] --- call again --- [Master Kotlin at Kotlination, Be Happy to become Kotlineer] |