Data Classes and Sealed Classes

# CHAPTER 17

Data Classes and Sealed Classes #

1. Chapter Introduction #

2. Learning Objectives #

• Declare and use a data class.

• Understand the automatically generated methods (toString, equals, copy).

• Destructure objects using component functions.

• Define a sealed class.

• Use sealed classes with the when expression for state modeling.

3. Data Classes #

// That's it! 1 line of code.
data class User(val name: String, val age: Int)

By adding the data keyword, the compiler automatically generates:

1. 1. toString() (Prints nicely formatted data instead of memory addresses).

1. 2. equals() / == (Compares the actual data, not the memory reference).

1. 3. hashCode()

1. 4. copy() (To easily duplicate and alter immutable objects).

1. 5. componentN() functions (For destructuring).

fun main() {
    val u1 = User("Alice", 25)
    val u2 = User("Alice", 25)

    // toString() is generated!
    println(u1) // Output: User(name=Alice, age=25)

    // equals() checks structural data!
    println(u1 == u2) // Output: true
}

4. The copy() Method #

fun main() {
    val user1 = User("Bob", 30)
    
    // Creates a copy of Bob, but changes the age to 31
    val user2 = user1.copy(age = 31)
    
    println(user2) // Output: User(name=Bob, age=31)
}

5. Destructuring Declarations #

fun main() {
    val user = User("Charlie", 40)
    
    // Destructuring!
    val (extractedName, extractedAge) = user
    
    println("Extracted: $extractedName is $extractedAge")
}

6. Sealed Classes (Advanced State Modeling) #

Sealed Classes are Enums on steroids. A sealed class restricts inheritance: all subclasses MUST be defined in the same file. This allows the compiler to know exactly how many states exist.

// Define the restricted hierarchy
sealed class NetworkResult {
    object Loading : NetworkResult() // Object (Singleton) because Loading has no data
    data class Success(val data: String) : NetworkResult() // Holds data
    data class Error(val exception: String) : NetworkResult() // Holds an error
}

fun processResult(result: NetworkResult) {
    // The compiler forces us to handle ALL sealed class subclasses in a 'when' expression!
    when (result) {
        is NetworkResult.Loading -> println("Show loading spinner...")
        is NetworkResult.Success -> println("Display data: ${result.data}")
        is NetworkResult.Error -> println("Show alert: ${result.exception}")
    }
}

fun main() {
    val response = NetworkResult.Success("Weather is 72 Degrees")
    processResult(response)
}

7. Common Mistakes #

• Empty Data Classes: A data class MUST have at least one parameter in its primary constructor. data class Empty() is an error.

• Using var in Data Classes: While allowed, it is terrible practice. Data classes should represent a snapshot of state. Always use val. If state changes, use .copy().

8. Best Practices #

• Exhaustive when: Always use when as an expression (assigning it to a variable or using return when) when dealing with Sealed Classes. This forces the compiler to error out if you add a new subclass to the Sealed Class but forget to handle it in the when block!

9. Exercises #

1. 1. Create a data class Product(val id: Int, val name: String, val price: Double).

1. 2. Instantiate Product(1, "Laptop", 999.99).

1. 3. Print the object.

1. 4. Use .copy() to create a new product with the same ID and Name, but a price of 899.99.

10. MCQs with Answers #

11. Interview Questions #

• Q: Explain what the Kotlin compiler generates behind the scenes when you use the data keyword.

• Q: Explain the difference between an Enum and a Sealed Class. Give an example of when you would use a Sealed Class in Android Development.

12. Summary #

13. Next Chapter Recommendation #