lazy Delegate in Kotlin

Introduction

lazy Delegate 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.

Lazy initializes value only on first access. 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

• lazy initializes value only on first access.
• Subsequent access returns cached value.
• Useful for expensive one-time initialization.

Syntax

val data by lazy { compute() }

lazy Delegate in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val message by lazy {
        println("Initializing...")
        "Hello Kotlin"
    }
    println("First access: $message")
    println("Second access: $message")
}

Sample Output

First access: Initializing...
Hello Kotlin
Second access: Hello Kotlin

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("Initializing...") statement writes a line to the console — this produces part of the sample output below.

3. The println("First access: $message") statement writes a line to the console — this produces part of the sample output below.

4. The println("Second access: $message") statement writes a line to the console — this produces part of the sample output below.

5. Lazy initializes value only on first access.

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

• lazy initializes value only on first access.
• Subsequent access returns cached value.
• Useful for expensive one-time initialization.
• 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.