Class Delegation in Kotlin

Introduction

Class Delegation 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.

Class delegation forwards interface calls to another 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

• Class delegation forwards interface calls to another object.
• Syntax: class A(b: B) : Interface by b.
• Reduces boilerplate when implementing interfaces.

Syntax

class Printer(base: Base) : Printable by base

Class Delegation in Kotlin Example Program in Kotlin

interface Printer {
    fun printLine(text: String)
}
class ConsolePrinter : Printer {
    override fun printLine(text: String) = println(text)
}
class LoggingPrinter(base: Printer) : Printer by base

fun main(args: Array<String>) {
    LoggingPrinter(ConsolePrinter()).printLine("Delegated print")
}

Sample Output

Delegated print

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

3. Class delegation forwards interface calls to another object.

4. 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

• Understand the core idea: class delegation forwards interface calls to another object.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• Class delegation forwards interface calls to another object.
• Syntax: class A(b: B) : Interface by b.
• Reduces boilerplate when implementing interfaces.
• 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.