Class delegation with by keyword in Kotlin - Step-by-Step Execution

Concept Flow - Class delegation with by keyword

Define Interface

↓

Create Delegate Class

↓

Create Delegating Class with 'by'

↓

Call Method on Delegating Class

↓

Method Call Forwarded to Delegate

↓

Delegate Executes Method

The delegating class forwards calls to the delegate class using 'by', so it reuses delegate's behavior.

Execution Sample

Kotlin

interface Printer {
    fun print()
}

class ConsolePrinter : Printer {
    override fun print() = println("Printing to console")
}

class DelegatingPrinter(printer: Printer) : Printer by printer

fun main() {
    val printer = DelegatingPrinter(ConsolePrinter())
    printer.print()
}

This code shows a class delegating the print() method to another class using 'by'.

Execution Table

Step	Action	Object	Method Called	Delegate Used	Output
1	Create ConsolePrinter instance	ConsolePrinter	-	-	-
2	Create DelegatingPrinter with ConsolePrinter	DelegatingPrinter	-	ConsolePrinter	-
3	Call print() on DelegatingPrinter	DelegatingPrinter	print()	ConsolePrinter	-
4	DelegatingPrinter forwards print() to ConsolePrinter	ConsolePrinter	print()	-	Printing to console
5	print() method completes	ConsolePrinter	-	-	-
6	Program ends	-	-	-	-

💡 All method calls delegated; program ends after print() output.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
printer	null	null	DelegatingPrinter(ConsolePrinter())	DelegatingPrinter(ConsolePrinter())	DelegatingPrinter(ConsolePrinter())

Key Moments - 2 Insights

Why does calling print() on DelegatingPrinter actually run ConsolePrinter's print()?

Because DelegatingPrinter uses 'by printer' to delegate all Printer interface calls to the ConsolePrinter instance, as shown in execution_table step 4.

Does DelegatingPrinter need to implement print() method explicitly?

No, the 'by' keyword automatically forwards calls to the delegate, so no explicit implementation is needed (see execution_table steps 3 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the print() method actually executed?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Check the 'Output' column in execution_table rows to see when 'Printing to console' appears.

According to variable_tracker, what does 'printer' hold after step 2?

A DelegatingPrinter(ConsolePrinter())

B null

C ConsolePrinter instance

D print() method

💡 Hint

Look at the 'After Step 2' column for 'printer' in variable_tracker.

If DelegatingPrinter did not use 'by', what would happen when calling print()?

A It would call ConsolePrinter's print() anyway

B It would print twice

C It would cause a compile error unless print() is implemented

D It would do nothing silently

💡 Hint

Recall that 'by' keyword delegates method calls; without it, methods must be implemented explicitly.

Concept Snapshot

interface Printer { fun print() }
class ConsolePrinter : Printer { override fun print() = println("...") }
class DelegatingPrinter(printer: Printer) : Printer by printer

'by' keyword forwards interface calls to delegate instance automatically.

Full Transcript

Class delegation in Kotlin lets one class reuse another's behavior by forwarding interface method calls using the 'by' keyword. Here, DelegatingPrinter implements Printer by delegating to ConsolePrinter. When print() is called on DelegatingPrinter, it forwards the call to ConsolePrinter's print(), which prints to the console. This avoids writing boilerplate code. The execution table shows object creation, method calls, delegation, and output. Variable tracker shows how the 'printer' variable holds the delegate instance. Key moments clarify why delegation works and that explicit method implementation is not needed. The quiz tests understanding of delegation steps and effects. This is a simple way to share behavior between classes in Kotlin.

Practice

1. What does the by keyword do in Kotlin class delegation?

easy

A. It overrides all methods of a class.

B. It creates a new instance of a class.

C. It marks a class as abstract.

D. It lets a class reuse another class's implementation automatically.

Solution

1. Step 1: Understand the role of by keyword

   The by keyword in Kotlin is used for class delegation, which means one class can reuse the implementation of another class without rewriting the code.

2. Step 2: Identify what by does

   It automatically forwards calls to the delegated class, so the delegating class behaves like the delegated one.

3. Final Answer:

   It lets a class reuse another class's implementation automatically. -> Option D

4. Quick Check:

   Delegation = reuse implementation [OK]

Hint: Remember: by means reuse another class's code [OK]

Common Mistakes:

• Thinking by creates a new instance
• Confusing by with inheritance
• Assuming by overrides methods automatically

2. Which of the following is the correct syntax to delegate interface Printer to class ConsolePrinter in Kotlin?

easy

A. class MyPrinter : Printer by ConsolePrinter()

B. class MyPrinter implements Printer by ConsolePrinter()

C. class MyPrinter extends Printer by ConsolePrinter()

D. class MyPrinter delegate Printer to ConsolePrinter()

Solution

1. Step 1: Recall Kotlin delegation syntax

   In Kotlin, delegation uses the syntax: class ClassName : InterfaceName by instance.

2. Step 2: Match the correct syntax

   class MyPrinter : Printer by ConsolePrinter() matches this pattern exactly: class MyPrinter : Printer by ConsolePrinter().

3. Final Answer:

   class MyPrinter : Printer by ConsolePrinter() -> Option A

4. Quick Check:

   Syntax = class X : Interface by instance [OK]

Hint: Use colon and 'by' for delegation syntax [OK]

Common Mistakes:

• Using 'implements' or 'extends' keywords (Java style)
• Writing 'delegate' keyword which doesn't exist
• Missing colon before interface name

3. What is the output of this Kotlin code?

interface Logger {
    fun log(msg: String)
}

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

class AppLogger(logger: Logger) : Logger by logger

fun main() {
    val logger = AppLogger(ConsoleLogger())
    logger.log("Hello")
}

medium

A. No output

B. Console: Hello

C. Error: log() not implemented

D. Hello

Solution

1. Step 1: Understand delegation in AppLogger

   The class AppLogger delegates Logger interface to the passed logger instance, which is ConsoleLogger.

2. Step 2: Trace the log call

   Calling logger.log("Hello") calls ConsoleLogger.log, which prints "Console: Hello".

3. Final Answer:

   Console: Hello -> Option B

4. Quick Check:

   Delegation calls delegated method output [OK]

Hint: Delegated methods run from the delegated instance [OK]

Common Mistakes:

• Expecting just 'Hello' without prefix
• Thinking delegation requires manual override
• Assuming no output because of delegation

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

interface Speaker {
    fun speak()
}

class Human : Speaker {
    override fun speak() {
        println("Hello")
    }
}

class Robot(speaker: Speaker) : Speaker {
    speak()
}

medium

A. Missing delegation with 'by' keyword in Robot class.

B. Robot class should not implement Speaker interface.

C. Human class must not override speak().

D. speak() method call is correct as is.

Solution

1. Step 1: Check Robot class declaration

   Robot implements Speaker but does not delegate or implement speak() method itself.

2. Step 2: Identify missing delegation

   Robot should delegate Speaker to the passed speaker instance using by speaker to avoid error.

3. Final Answer:

   Missing delegation with 'by' keyword in Robot class. -> Option A

4. Quick Check:

   Delegation needs 'by' keyword [OK]

Hint: Delegate with 'by' or implement all methods [OK]

Common Mistakes:

• Calling method directly without delegation
• Forgetting to use 'by' keyword
• Assuming interface methods auto-implement

5. You want to create a class SmartSpeaker that delegates Speaker interface to a Human instance but adds a new method playMusic(). Which is the best way to do this?

hard

A. class SmartSpeaker : Human() {
fun playMusic() = println("Playing music")
}

B. class SmartSpeaker : Speaker {
val human = Human()
override fun speak() = human.speak()
fun playMusic() = println("Playing music")
}

C. class SmartSpeaker(human: Human) : Speaker by human {
fun playMusic() {
println("Playing music")
}
}

D. class SmartSpeaker(human: Human) : Speaker {
fun playMusic() = println("Playing music")
}

Solution

1. Step 1: Understand delegation with added methods

   Using by human delegates all Speaker methods to Human automatically.

2. Step 2: Add new method in delegating class

   We can add playMusic() directly in SmartSpeaker without affecting delegation.

3. Final Answer:

   class SmartSpeaker(human: Human) : Speaker by human { fun playMusic() { println("Playing music") } } -> Option C

4. Quick Check:

   Delegate with 'by' and add methods [OK]

Hint: Delegate with 'by' and add new methods in class body [OK]

Common Mistakes:

• Not delegating interface methods properly
• Extending class instead of delegating interface
• Forgetting to implement interface methods