Kotlin Classes – Structure, Constructors & Types - Learn ANDROID-KOTLIN | Revochamp

Name: Kotlin Classes – Structure, Constructors & Types
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Author: Revochamp

Modern Class Declaration

In Kotlin, classes are declared using the class keyword. The most efficient feature is the Primary Constructor, defined directly in the class header. This allows you to declare and initialize properties in a single line, drastically reducing the boilerplate code found in Java.

KOTLINRead-only

// Standard class with primary constructor
class Project(val id: String, var title: String, val isAiGenerated: Boolean = false) {
    init {
        // Initialization logic runs here
        println("Project $title initialized.")
    }
}

// Usage: No 'new' keyword needed (just like Dart)
val myProject = Project("rev_01", "Mobile UI Builder")

// Standard class with primary constructor
class Project(val id: String, var title: String, val isAiGenerated: Boolean = false) {
    init {
        // Initialization logic runs here
        println("Project $title initialized.")
    }
}

// Usage: No 'new' keyword needed (just like Dart)
val myProject = Project("rev_01", "Mobile UI Builder")

Data Classes: The Model Standard

When you need a class strictly to hold data—such as a JSON response for a Revochamp widget—you use a data class. The Kotlin compiler automatically generates equals(), hashCode(), toString(), and the invaluable copy() method. This is a massive upgrade over manual Dart model implementation.

KOTLINRead-only

data class Widget(val id: String, val type: String)

val button = Widget("b1", "Button")
// Create a copy with a modified property
val icon = button.copy(type = "Icon")

data class Widget(val id: String, val type: String)

val button = Widget("b1", "Button")
// Create a copy with a modified property
val icon = button.copy(type = "Icon")

Sealed Classes: State Management

Sealed classes represent restricted class hierarchies. For an Engineering Manager, they are the 'Gold Standard' for representing UI States (Loading, Success, Error). When used with a when expression, the compiler ensures you have handled every possible state, preventing runtime crashes.

KOTLINRead-only

sealed class GenerationState {
    object Idle : GenerationState()
    object Loading : GenerationState()
    data class Success(val code: String) : GenerationState()
    data class Failure(val error: String) : GenerationState()
}

// Pattern matching ensures all cases are covered
fun handleState(state: GenerationState) = when(state) {
    is GenerationState.Idle -> println("Waiting...")
    is GenerationState.Loading -> showSpinner()
    is GenerationState.Success -> renderCode(state.code)
    is GenerationState.Failure -> showError(state.error)
}

sealed class GenerationState {
    object Idle : GenerationState()
    object Loading : GenerationState()
    data class Success(val code: String) : GenerationState()
    data class Failure(val error: String) : GenerationState()
}

// Pattern matching ensures all cases are covered
fun handleState(state: GenerationState) = when(state) {
    is GenerationState.Idle -> println("Waiting...")
    is GenerationState.Loading -> showSpinner()
    is GenerationState.Success -> renderCode(state.code)
    is GenerationState.Failure -> showError(state.error)
}

Objects and Companion Objects

Kotlin doesn't use the static keyword. For Singletons, you use the object keyword. For members that should belong to a class (like constants or factory methods), you use a companion object. This keeps the namespace clean and organized.

KOTLINRead-only

class RevoEngine {
    companion object {
        const val VERSION = "2.0.0"
        fun create() = RevoEngine()
    }
}

// Usage: RevoEngine.VERSION

class RevoEngine {
    companion object {
        const val VERSION = "2.0.0"
        fun create() = RevoEngine()
    }
}

// Usage: RevoEngine.VERSION

Class Type Comparison

Class Type	Primary Purpose	Key Feature
Standard Class	General logic/UI	Encapsulation & Inheritance
Data Class	Data Models	Autogenerated copy/toString
Sealed Class	State/Type safety	Exhaustive pattern matching
Object	Singletons	Thread-safe lazy initialization
Abstract Class	Templates	Cannot be instantiated

Test Your Knowledge

of 3

Which keyword is used to create a class specifically for holding data with autogenerated utility methods?

model

data

sealed

value

of 3

How do you allow a Kotlin class to be inherited by another class?

public

open

extendable

abstract

of 3

Which structure is used in Kotlin to replace static methods and constants within a class?

StaticBlock

Companion Object

GlobalObject

InnerClass

Frequently Asked Questions

What is the difference between a Data Class and a regular Class?

Data classes automatically provide methods for structural equality (==) and copying. Regular classes require you to override these methods manually. Data classes also require at least one property in the primary constructor.

Why are classes 'final' by default in Kotlin?

To promote 'Composition over Inheritance.' If you want a class to be subclassable, you must explicitly mark it with the 'open' keyword. This prevents accidental overriding of logic in complex native modules.

Can a Sealed Class be instantiated?

No. A Sealed Class is abstract by nature. You can only instantiate its subclasses. This makes it perfect for representing a closed set of types.

// Standard class with primary constructor class Project(val id: String, var title: String, val isAiGenerated: Boolean = false) { init { // Initialization logic runs here println("Project $title initialized.") } } // Usage: No 'new' keyword needed (just like Dart) val myProject = Project("rev_01", "Mobile UI Builder")

sealed class GenerationState { object Idle : GenerationState() object Loading : GenerationState() data class Success(val code: String) : GenerationState() data class Failure(val error: String) : GenerationState() } // Pattern matching ensures all cases are covered fun handleState(state: GenerationState) = when(state) { is GenerationState.Idle -> println("Waiting...") is GenerationState.Loading -> showSpinner() is GenerationState.Success -> renderCode(state.code) is GenerationState.Failure -> showError(state.error) }

Class Type

Primary Purpose

Key Feature

Standard Class

General logic/UI

Encapsulation & Inheritance

Data Class

Data Models

Autogenerated copy/toString

Sealed Class

State/Type safety

Exhaustive pattern matching

Object

Singletons

Thread-safe lazy initialization

Abstract Class

Templates

Cannot be instantiated

Test Your Knowledge

of 3

Which keyword is used to create a class specifically for holding data with autogenerated utility methods?

model

data

sealed

value

of 3

How do you allow a Kotlin class to be inherited by another class?

public

open

extendable

abstract

of 3

Which structure is used in Kotlin to replace static methods and constants within a class?

StaticBlock

Companion Object

GlobalObject

InnerClass

Frequently Asked Questions

What is the difference between a Data Class and a regular Class?

Why are classes 'final' by default in Kotlin?

Can a Sealed Class be instantiated?

No. A Sealed Class is abstract by nature. You can only instantiate its subclasses. This makes it perfect for representing a closed set of types.

Kotlin Classes – Structure, Constructors & Types

Kotlin Classes – Structure, Constructors & Types

Modern Class Declaration

Data Classes: The Model Standard

Sealed Classes: State Management

Objects and Companion Objects

Class Type Comparison

Test Your Knowledge

Frequently Asked Questions

Need help?

Kotlin Classes – Structure, Constructors & Types

Kotlin Classes – Structure, Constructors & Types

Modern Class Declaration

Data Classes: The Model Standard

Sealed Classes: State Management

Objects and Companion Objects

Class Type Comparison

Test Your Knowledge

Frequently Asked Questions

Need help?