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.