Kotlin OOP – Classes, Objects, and Inheritance - Learn ANDROID-KOTLIN | Revochamp

Name: Kotlin OOP – Classes, Objects, and Inheritance
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Author: Revochamp

Classes and Constructors

In Kotlin, classes are declared using the class keyword. The most striking difference from Java is the Primary Constructor, which is part of the class header itself. This makes property declaration incredibly concise. As an Architect, you'll appreciate how this reduces 'boilerplate' noise in your native Android models.

KOTLINRead-only

// A class with a primary constructor and default values
class Project(val id: String, var name: String, val isPublic: Boolean = false) {
    init {
        println("Initializing project: $name")
    }
}

// Usage
val myProject = Project("rev_01", "AI Builder")

// A class with a primary constructor and default values
class Project(val id: String, var name: String, val isPublic: Boolean = false) {
    init {
        println("Initializing project: $name")
    }
}

// Usage
val myProject = Project("rev_01", "AI Builder")

Data Classes: The Perfect Models

When a class is created primarily to hold data (like a JSON response from your Python backend), you mark it as a data class. The compiler automatically generates equals(), hashCode(), toString(), and the powerful copy() function. This is far more advanced than standard Dart classes and saves hours of manual coding.

KOTLINRead-only

data class User(val uid: String, val email: String)

val user1 = User("123", "k@test.com")
// Create a new instance with one property changed
val user2 = user1.copy(email = "new@test.com")

data class User(val uid: String, val email: String)

val user1 = User("123", "k@test.com")
// Create a new instance with one property changed
val user2 = user1.copy(email = "new@test.com")

Inheritance and 'open' Classes

By default, all classes in Kotlin are final (cannot be inherited). To allow a class to be a parent, you must explicitly mark it with the open keyword. This 'Design for Inheritance' philosophy prevents accidental architectural fragility in your native Android modules.

KOTLINRead-only

open class BaseWidget(val type: String) {
    open fun render() = println("Rendering $type")
}

class ButtonWidget : BaseWidget("Button") {
    override fun render() {
        super.render()
        println("Applying shadow...")
    }
}

open class BaseWidget(val type: String) {
    open fun render() = println("Rendering $type")
}

class ButtonWidget : BaseWidget("Button") {
    override fun render() {
        super.render()
        println("Applying shadow...")
    }
}

Sealed Classes: State Management Pro

Sealed classes are used for representing restricted class hierarchies. They are essentially 'Enums on steroids.' For a Flutter Architect, they are the ideal way to represent UI States (Loading, Success, Error) when passing data back through a Method Channel.

KOTLINRead-only

sealed class UIState {
    object Loading : UIState()
    data class Success(val data: List<String>) : UIState()
    data class Error(val message: String) : UIState()
}

// Comprehensive handling in 'when'
fun handleState(state: UIState) = when(state) {
    is UIState.Loading -> showSpinner()
    is UIState.Success -> showList(state.data)
    is UIState.Error -> showError(state.message)
}

sealed class UIState {
    object Loading : UIState()
    data class Success(val data: List<String>) : UIState()
    data class Error(val message: String) : UIState()
}

// Comprehensive handling in 'when'
fun handleState(state: UIState) = when(state) {
    is UIState.Loading -> showSpinner()
    is UIState.Success -> showList(state.data)
    is UIState.Error -> showError(state.message)
}

Objects and Companion Objects

Kotlin doesn't have static members. Instead, it uses object for Singletons and companion object for members that belong to a class. This makes the Singleton pattern a first-class language feature.

Feature	Kotlin OOP	Dart (Flutter) OOP
Default Visibility	Public	Public
Immutability	Classes are final by default	Classes are open by default
Static Members	Companion Objects	static keyword
Data Models	Data Classes (Autogen)	Manual / Code Gen
Enums	Enums / Sealed Classes	Enums / Sealed Classes
Constructors	Primary & Secondary	Generative & Named

Test Your Knowledge

of 3

Which keyword is required to allow a Kotlin class to be inherited by another class?

public

open

abstract

static

of 3

Which specific class type provides autogenerated copy() and toString() methods?

Sealed Class

Object Class

Data Class

Inner Class

of 3

What is the Kotlin equivalent of a static block or static method?

Companion Object

Static Object

Init Block

Module Object

Frequently Asked Questions

What is the difference between a Class and an Object?

A 'class' is a blueprint for creating instances. An 'object' is a keyword used to define a Singleton—a class that has exactly one instance created automatically by the system.

Can a class have multiple constructors?

Yes. While the primary constructor is most common, you can define secondary constructors using the 'constructor' keyword. However, they must always call the primary constructor.

Why use Sealed Classes over Enums?

Enums are for constant values. Sealed classes are for types. Each branch of a sealed class can have its own unique properties and state (like an Error message or a Data list), which Enums cannot do effectively.

// A class with a primary constructor and default values class Project(val id: String, var name: String, val isPublic: Boolean = false) { init { println("Initializing project: $name") } } // Usage val myProject = Project("rev_01", "AI Builder")

open class BaseWidget(val type: String) { open fun render() = println("Rendering $type") } class ButtonWidget : BaseWidget("Button") { override fun render() { super.render() println("Applying shadow...") } }

sealed class UIState { object Loading : UIState() data class Success(val data: List<String>) : UIState() data class Error(val message: String) : UIState() } // Comprehensive handling in 'when' fun handleState(state: UIState) = when(state) { is UIState.Loading -> showSpinner() is UIState.Success -> showList(state.data) is UIState.Error -> showError(state.message) }

Feature

Kotlin OOP

Dart (Flutter) OOP

Default Visibility

Public

Immutability

Classes are final by default

Classes are open by default

Static Members

Companion Objects

static keyword

Data Models

Data Classes (Autogen)

Manual / Code Gen

Enums

Enums / Sealed Classes

Constructors

Primary & Secondary

Generative & Named

Test Your Knowledge

of 3

Which keyword is required to allow a Kotlin class to be inherited by another class?

public

open

abstract

static

of 3

Which specific class type provides autogenerated copy() and toString() methods?

Sealed Class

Object Class

Data Class

Inner Class

of 3

What is the Kotlin equivalent of a static block or static method?

Companion Object

Static Object

Init Block

Module Object

Frequently Asked Questions

What is the difference between a Class and an Object?

A 'class' is a blueprint for creating instances. An 'object' is a keyword used to define a Singleton—a class that has exactly one instance created automatically by the system.

Can a class have multiple constructors?

Yes. While the primary constructor is most common, you can define secondary constructors using the 'constructor' keyword. However, they must always call the primary constructor.

Why use Sealed Classes over Enums?

Kotlin OOP – Classes, Objects, and Inheritance

Kotlin OOP – Classes, Objects, and Inheritance

Classes and Constructors

Data Classes: The Perfect Models

Inheritance and 'open' Classes

Sealed Classes: State Management Pro

Objects and Companion Objects

Test Your Knowledge

Frequently Asked Questions

Need help?

Kotlin OOP – Classes, Objects, and Inheritance

Kotlin OOP – Classes, Objects, and Inheritance

Classes and Constructors

Data Classes: The Perfect Models

Inheritance and 'open' Classes

Sealed Classes: State Management Pro

Objects and Companion Objects

Test Your Knowledge

Frequently Asked Questions

Need help?