Delegated Properties Overview in Kotlin

Introduction

Delegated Properties Overview is a fundamental concept every Kotlin developer should understand. Property delegates move repetitive getter/setter logic into reusable classes — lazy initialization and observable properties are common examples.

Kotlin standard library provides lazy, observable and vetoable delegates. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Kotlin standard library provides lazy, observable and vetoable delegates.
• Custom delegates can be created for reusable property behavior.
• Delegation keeps classes focused on business logic.

Syntax

var field by customDelegate()

Delegated Properties Overview in Kotlin Example Program in Kotlin

import kotlin.properties.ReadOnlyProperty
import kotlin.reflect.KProperty

class Constant(val value: String) : ReadOnlyProperty<Any?, String> {
    override fun getValue(thisRef: Any?, property: KProperty<*>): String = value
}

fun main(args: Array<String>) {
    val appName by Constant("Kotlin App")
    println(appName)
}

Sample Output

Kotlin App

When to use

Use delegates when multiple properties share the same access pattern — lazy init, logging, or validation.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. override fun getValue(thisRef: Any?, property: KProperty<*>): String = value assigns or updates a value used later in the program.

3. The println(appName) statement writes a line to the console — this produces part of the sample output below.

4. Kotlin standard library provides lazy, observable and vetoable delegates.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

Common Mistakes

Key Points

• Kotlin standard library provides lazy, observable and vetoable delegates.
• Custom delegates can be created for reusable property behavior.
• Delegation keeps classes focused on business logic.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.