Delegation vs inheritance decision in Kotlin - Visual Side-by-Side Comparison

Concept Flow - Delegation vs inheritance decision

Start

↓

Need to reuse behavior?

↓

Is "is-a" relationship clear?

↓

Use Inheritance

↓

Use Delegation

↓

No reuse needed

↓

End

Decide reuse by checking if a clear "is-a" relationship exists; if yes, use inheritance, else use delegation.

Execution Sample

Kotlin

interface Printer {
    fun print()
}

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

class Report(private val printer: Printer) {
    fun generate() {
        // Delegation
        printer.print()
    }
}

This code shows delegation: Report class uses a Printer interface implementation to print, instead of inheriting.

Execution Table

Step	Action	Evaluation	Result
1	Check if reuse of behavior is needed	Yes	Proceed to next step
2	Check if "is-a" relationship is clear	No	Choose delegation
3	Create interface Printer	Interface defined	Printer interface ready
4	Implement ConsolePrinter class	Class implements Printer	ConsolePrinter ready
5	Create Report class with Printer property	Report delegates printing	Delegation setup complete
6	Call Report.generate()	Calls printer.print()	Prints "Printing to console"
7	End	All steps done	Execution complete

💡 Decision made to use delegation because "is-a" relationship was not clear

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	After Step 6	Final
printer	null	Interface defined	Instance of ConsolePrinter	Assigned to Report	Used in generate()	Print output done
report	null	null	null	Created with printer	generate() called	Delegation works

Key Moments - 2 Insights

Why do we choose delegation when the "is-a" relationship is unclear?

Because inheritance implies a strong "is-a" relationship. If that is not clear, delegation allows reuse without forcing that relationship, as shown in execution_table step 2.

Can delegation still reuse behavior like inheritance?

Yes, delegation reuses behavior by calling methods on another object, as seen in execution_table step 6 where Report calls printer.print().

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the decision made to use delegation?

A Step 5

B Step 4

C Step 2

D Step 6

💡 Hint

Check the 'Evaluation' column where the choice between inheritance and delegation is decided.

According to variable_tracker, what is the state of 'printer' after Step 4?

A Instance of ConsolePrinter

B null

C Interface defined

D Assigned to Report

💡 Hint

Look at the 'printer' row and the column 'After Step 4' in variable_tracker.

If the "is-a" relationship was clear, which step would change in the execution_table?

A Step 6 would not call printer.print()

B Step 2 would choose inheritance instead of delegation

C Step 3 would be skipped

D Step 5 would create a different class

💡 Hint

Focus on the decision step about relationship clarity in execution_table step 2.

Concept Snapshot

Delegation vs Inheritance Decision:
- Use inheritance if there is a clear "is-a" relationship.
- Use delegation to reuse behavior without "is-a".
- Delegation involves calling methods on another object.
- Inheritance involves extending a class.
- Delegation offers more flexibility and less tight coupling.

Full Transcript

This visual execution shows how to decide between delegation and inheritance in Kotlin. First, check if you need to reuse behavior. If yes, check if the "is-a" relationship is clear. If it is, use inheritance. If not, use delegation. The example code defines a Printer interface and a ConsolePrinter class. The Report class delegates printing to a Printer instance instead of inheriting. The execution table traces each step, showing the decision and how delegation works. The variable tracker shows how the printer variable changes from interface definition to instance and usage. Key moments clarify why delegation is chosen without a clear "is-a" relationship and how delegation still reuses behavior. The quiz tests understanding of decision points and variable states. The snapshot summarizes the key rules for choosing delegation or inheritance.