Why is this an issue?

Kotlin features language support for the delegator pattern using by clauses. Because this is a built-in language feature, it should be used as an idiom instead of resorting to custom idioms.

What is the potential impact?

Readability and Understanding

This change makes it easier to understand the code because this is how delegation is intended to be used in Kotlin. When developers share common standards and idioms, they need to spend less effort understanding each other’s code.

Code Redundancy

Using a built-in language feature or a standard API is always better than a custom implementation, because the reimplementation of something that already exists is unnecessary.

How to fix it

Remove all override functions from your class that delegate to the function with the same signature in the delegee. In the class header, add a by clause to delegate the entire interface to the delegee.

Code examples

Noncompliant code example

interface Network { fun connect() }

class PoorNetwork : Network {
    override fun connect() { println("cannot connect") }
}

class GoodNetwork : Network {
    override fun connect() { println("connected") }
}

interface Graphics { fun render() }

class Nvidia : Graphics {
    override fun render() { println("Neat 3D world") }
}

class AsusCardFrom2010 : Graphics {
    override fun render() { println("~8 fps") }
}

abstract class OS : Network, Graphics

class Linux : OS() {
    private val network = GoodNetwork()
    override fun connect() = network.connect() // Noncompliant, explicit function delegation
    private val graphics= Nvidia()
    override fun render() = graphics.render() // Noncompliant, explicit function delegation
    // ...
}

class Windows : OS() {
    private val network = PoorNetwork()
    override fun connect() = network.connect() // Noncompliant, explicit function delegation
    private val graphics = AsusCardFrom2010()
    override fun render() = graphics.render() // Noncompliant, explicit function delegation
    // ...
}

Compliant solution

class Linux:
    OS(),
    Network by GoodNetwork(), // Compliant, interface delegation using `by`
    Graphics by Nvidia() // Compliant, interface delegation using `by`
{
    // ...
}
class Windows:
    OS(),
    Network by PoorNetwork(), // Compliant, interface delegation using `by`
    Graphics by AsusCardFrom2010() // Compliant, interface delegation using `by`
{
    // ...
}

Resources

Documentation

• Kotlin Docs, Delegation