Delegation vs inheritance decision in Kotlin - Hands-On Comparison

Delegation vs Inheritance Decision in Kotlin

📖 Scenario: Imagine you are building a simple app to manage different types of workers in a company. Some workers have special skills, and you want to reuse code efficiently. You will learn when to use inheritance and when to use delegation in Kotlin.

🎯 Goal: You will create classes using inheritance and delegation to share behavior. Then you will decide which approach fits better by comparing outputs.

📋 What You'll Learn

Create a base class with a common function

Create a subclass using inheritance

Create a class using delegation to reuse behavior

Print outputs from both inheritance and delegation examples

💡 Why This Matters

🌍 Real World

In real apps, choosing between inheritance and delegation helps keep code flexible and easier to maintain.

💼 Career

Understanding these concepts is important for Kotlin developers working on clean, reusable code in Android or backend projects.

1

Create a base class with a common function

Create a Kotlin class called Worker with a function work() that returns the string "Working hard".

Kotlin

# Create the Worker class with work() function
// Your code here

Hint

Define a class named Worker and inside it a function work() that returns the exact string "Working hard".

2

Create a subclass using inheritance

Create a class called Manager that inherits from Worker. Override the work() function to return "Managing team".

Kotlin

open class Worker {
    open fun work(): String {
        return "Working hard"
    }
}

// Create Manager class inheriting Worker and override work()
// Your code here

Hint

Use class Manager : Worker() to inherit. Use override fun work() to change the behavior.

3

Create a class using delegation to reuse behavior

Create a class called Assistant that takes a Worker object as a constructor parameter and delegates the work() function to it. Use Kotlin's by keyword for delegation.

Kotlin

open class Worker {
    open fun work(): String {
        return "Working hard"
    }
}

class Manager : Worker() {
    override fun work(): String {
        return "Managing team"
    }
}

// Create Assistant class delegating work() to Worker
// Your code here

Hint

Define class Assistant(private val worker: Worker) : Worker by worker to delegate using the by keyword.

4

Print outputs from inheritance and delegation examples

Create instances of Manager and Assistant (passing a Worker to it). Print the results of calling work() on both instances.

Kotlin

open class Worker {
    open fun work(): String {
        return "Working hard"
    }
}

class Manager : Worker() {
    override fun work(): String {
        return "Managing team"
    }
}

class Assistant(private val worker: Worker) : Worker by worker

// Create Manager and Assistant instances and print their work() results
// Your code here

Hint

Create val manager = Manager() and val assistant = Assistant(Worker()). Then print manager.work() and assistant.work().

Practice

1. Which statement best describes when to use inheritance in Kotlin?

easy

A. When you want to hide implementation details completely.

B. When you want to reuse code without creating a parent-child relationship.

C. When a class is a specialized type of another class ("is-a" relationship).

D. When you want to avoid any relationship between classes.

Solution

1. Step 1: Understand inheritance purpose

   Inheritance models an "is-a" relationship where a subclass extends a superclass.

2. Step 2: Match the correct use case

   Using inheritance is appropriate when a class is a specialized version of another class.

3. Final Answer:

   When a class is a specialized type of another class ("is-a" relationship). -> Option C

4. Quick Check:

   Inheritance = "is-a" relationship [OK]

Hint: Use inheritance for clear "is-a" relationships [OK]

Common Mistakes:

• Confusing delegation with inheritance
• Using inheritance for code reuse only
• Thinking inheritance hides implementation

2. Which Kotlin syntax correctly shows delegation of interface Printer by class Document?

easy

A. class Document : Printer(printer)

B. class Document(val printer: Printer) : Printer by printer

C. class Document : Printer implements printer

D. class Document(printer: Printer) extends Printer

Solution

1. Step 1: Recall Kotlin delegation syntax

   Kotlin uses : Interface by instance to delegate interface implementation.

2. Step 2: Identify correct syntax

   class Document(val printer: Printer) : Printer by printer correctly shows delegation with class Document(val printer: Printer) : Printer by printer.

3. Final Answer:

   class Document(val printer: Printer) : Printer by printer -> Option B

4. Quick Check:

   Delegation syntax = : Interface by instance [OK]

Hint: Delegation uses ': Interface by instance' syntax [OK]

Common Mistakes:

• Using extends keyword (not Kotlin)
• Writing implements instead of by
• Missing val/var for delegated instance

3. What is the output of this Kotlin code?

interface Logger {
    fun log(message: String)
}

class ConsoleLogger : Logger {
    override fun log(message: String) {
        println("Console: $message")
    }
}

class FileLogger : Logger {
    override fun log(message: String) {
        println("File: $message")
    }
}

class Service(logger: Logger) : Logger by logger

fun main() {
    val service = Service(ConsoleLogger())
    service.log("Start")
}

medium

A. Console: Start

B. File: Start

C. Start

D. Compilation error

Solution

1. Step 1: Understand delegation in Service class

   Service delegates Logger interface to the passed logger instance, here ConsoleLogger.

2. Step 2: Trace the log call

   Calling service.log("Start") calls ConsoleLogger.log, printing "Console: Start".

3. Final Answer:

   Console: Start -> Option A

4. Quick Check:

   Delegation calls ConsoleLogger.log = Console: Start [OK]

Hint: Delegation calls passed instance methods [OK]

Common Mistakes:

• Confusing which logger is used
• Expecting FileLogger output
• Thinking delegation copies methods

4. Identify the error in this Kotlin code using delegation:

interface Worker {
    fun work()
}

class Employee : Worker {
    override fun work() {
        println("Employee working")
    }
}

class Manager(worker: Worker) : Worker by worker {
    fun manage() {
        println("Managing team")
    }
}

fun main() {
    val manager = Manager()
    manager.work()
    manager.manage()
}

medium

A. Manager class missing argument for Worker in constructor call.

B. Employee class does not implement work() method.

C. Delegation syntax is incorrect; should use extends instead of by.

D. manage() method cannot be called on Manager instance.

Solution

1. Step 1: Check Manager constructor usage

   Manager requires a Worker parameter in constructor but none is provided in main().

2. Step 2: Identify compilation error

   Calling Manager() without argument causes a compilation error due to missing parameter.

3. Final Answer:

   Manager class missing argument for Worker in constructor call. -> Option A

4. Quick Check:

   Constructor argument missing = compilation error [OK]

Hint: Check constructor parameters when using delegation [OK]

Common Mistakes:

• Ignoring constructor parameters
• Confusing delegation syntax with inheritance
• Assuming manage() is inaccessible

5. You want to design a Kotlin class SmartDevice that can either behave like a Phone or a Camera depending on user choice at runtime. Which approach is best and why?

hard

A. Use static methods in SmartDevice to mimic Phone and Camera features.

B. Use inheritance with SmartDevice extending Phone or Camera based on a flag.

C. Use multiple inheritance to inherit both Phone and Camera directly.

D. Use delegation to delegate Phone or Camera behavior to SmartDevice dynamically.

Solution

1. Step 1: Analyze design needs

   SmartDevice needs to switch behavior dynamically between Phone and Camera at runtime.

2. Step 2: Choose delegation for flexibility

   Delegation allows SmartDevice to hold and switch between Phone or Camera instances easily without inheritance constraints.

3. Step 3: Exclude other options

   Inheritance is static and cannot switch base class at runtime; Kotlin does not support multiple inheritance; static methods do not provide instance behavior.

4. Final Answer:

   Use delegation to delegate Phone or Camera behavior to SmartDevice dynamically. -> Option D

5. Quick Check:

   Dynamic behavior switching = delegation [OK]

Hint: Use delegation for flexible runtime behavior changes [OK]

Common Mistakes:

• Trying to use inheritance for runtime behavior switch
• Assuming Kotlin supports multiple inheritance
• Using static methods instead of instance delegation