Deep Kotlin tutorial covering composition, delegation, by, repository interface with examples, diagram, mistakes, and interview answers.

Composition and Delegation

What is Composition and Delegation?

Composition and Delegation is an important Kotlin topic from 1.5 Object-Oriented vs Functional Kotlin. In this lesson we focus only on the concepts that belong together: composition, delegation, by, repository interface, testability. This keeps the lesson deep enough for interviews but not mixed with unrelated ideas.

Why students should learn it

• Understand Composition and Delegation in simple language
• Use composition in real Kotlin or Android code
• Explain Composition and Delegation in interviews with examples and trade-offs

How it works

Composition and Delegation: composition, delegation, by, repository interface, testability works by shaping responsibilities. Kotlin lets you combine OOP modeling with functional transformations: use objects for behavior and boundaries, then use pure functions for predictable data changes.

1. Identify what problem Composition and Delegation: composition, delegation, by, repository interface, testability solves.
2. Write the smallest Kotlin snippet that demonstrates it.
3. Add one Android or JVM use case where it matters.
4. Compare it with a similar concept so the difference is clear.
5. Finish by naming one common mistake and the safer alternative.

Interview tip: keep the answer simple first, then add example, trade-off, and mistake.

Types / Classification

• composition: core concept in this lesson
• delegation: core concept in this lesson
• by: core concept in this lesson
• repository interface: core concept in this lesson
• testability: core concept in this lesson
• Class: blueprint that owns state and behavior
• Interface: a contract with no state of its own
• Inheritance: reusing behavior through an is-a relationship

Important differences

Diagram

flowchart TD
A["1.5 Object-Oriented vs Functional Kotlin"] --> B["Composition and Delegation"]
B --> C["Why it exists"]
C --> D["How it works in Kotlin"]
D --> E["Android or JVM example"]
E --> F["Mistakes and interview answer"]

Examples

Sealed class for UI state

sealed interface UiState {
object Loading : UiState
data class Content(val items: List<String>) : UiState
data class Error(val message: String) : UiState
}

Delegation instead of inheritance

interface Repository { fun load(): List<String> }
class CachingRepository(private val source: Repository) : Repository by source

When to use it

Use it when: Use this to model real domain concepts clearly - a data class for a User or Order, a sealed class for a finite set of UI states, an interface for a contract that several classes share.

Avoid it when: Avoid deep inheritance hierarchies for code reuse alone; prefer composition when the relationship is not a genuine is-a relationship.

Common mistakes

• Reaching for inheritance when composition would be more flexible
• Building deep inheritance hierarchies that are hard to change safely
• Putting mutable shared state directly on an open base class
• Using a plain class where a sealed class would make invalid states unrepresentable
• Skipping interfaces and coding directly against a concrete implementation, which hurts testability

Interview questions

Read each answer in four parts: simple answer, code example, trade-off, and common mistake.

sealed interface UiState {
object Loading : UiState
data class Content(val items: List<String>) : UiState
data class Error(val message: String) : UiState
}

interface Repository { fun load(): List<String> }
class CachingRepository(private val source: Repository) : Repository by source

sealed interface UiState {
object Loading : UiState
data class Content(val items: List<String>) : UiState
data class Error(val message: String) : UiState
}

sealed interface UiState {
object Loading : UiState
data class Content(val items: List<String>) : UiState
data class Error(val message: String) : UiState
}

interface Repository { fun load(): List<String> }
class CachingRepository(private val source: Repository) : Repository by source

Practice task

Write one Kotlin example for Composition and Delegation. Then explain it using: what is it, how it works, where it is used, mistake to avoid, and one interview answer.