Why Class delegation with by keyword in Kotlin? - Purpose & Use Cases

The Big Idea

What if you could reuse code without rewriting or copying it over and over?

The Scenario

Imagine you have a class that needs to reuse behavior from another class. Without delegation, you must write lots of code to forward each function call manually.

The Problem

This manual forwarding is slow to write, easy to forget or make mistakes, and hard to maintain when the delegated class changes.

The Solution

Class delegation with the by keyword lets Kotlin automatically forward calls to another class, saving time and avoiding errors.

Before vs After

✗ Before

class Printer {
    fun print() = println("Printing...")
}

class Manager {
    private val printer = Printer()
    fun print() = printer.print()
}

✓ After

class Printer {
    fun print() = println("Printing...")
}

class Manager(printer: Printer) : Printer by printer

What It Enables

You can easily reuse and extend behavior without writing repetitive forwarding code.

Real Life Example

When building a user interface, you can delegate event handling to another class without rewriting all event methods.

Key Takeaways

Manual forwarding is tedious and error-prone.

by keyword automates delegation.

Code becomes cleaner and easier to maintain.