Property Delegates in Kotlin

Introduction

Property Delegates 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.

Property delegation moves getter/setter logic to delegate object. 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

• Property delegation moves getter/setter logic to delegate object.
• Syntax: var name by Delegate().
• Reduces boilerplate for common property patterns.

Syntax

var value by Delegate()

Property Delegates in Kotlin Example Program in Kotlin

import kotlin.properties.Delegates

fun main(args: Array<String>) {
    var score by Delegates.observable(0) { _, old, new ->
        println("Score changed from $old to $new")
    }
    score = 10
    score = 15
}

Sample Output

Score changed from 0 to 10
Score changed from 10 to 15

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. The println("Score changed from $old to $new") statement writes a line to the console — this produces part of the sample output below.

3. score = 10 assigns or updates a value used later in the program.

4. score = 15 assigns or updates a value used later in the program.

5. Property delegation moves getter/setter logic to delegate object.

6. 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

• Property delegation moves getter/setter logic to delegate object.
• Syntax: var name by Delegate().
• Reduces boilerplate for common property patterns.
• 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.