Data classes in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Correct syntax for a Kotlin data class

Which option correctly defines a Kotlin data class named Person with properties name (String) and age (Int)?

A class data Person(val name: String, val age: Int)

B data class Person(val name: String, val age: Int)

C data Person class(val name: String, val age: Int)

D data class Person(name: String, age: Int)

💡 Hint

Remember the correct order is data class ClassName(...) with property types declared.

In Kotlin, a data class is declared with the keyword data before class. Properties must have types and be declared inside the primary constructor.

❓ ui_behavior

intermediate

Output of toString() on a Kotlin data class instance

Given this data class:

data class User(val username: String, val id: Int)

What is the output of:

println(User("alice", 42))

Android Kotlin

data class User(val username: String, val id: Int)
fun main() {
  println(User("alice", 42))
}

A User(alice, 42)

B User@42

C User(username=alice, id=42)

D alice 42

💡 Hint

Data classes automatically generate a toString() method showing property names and values.

Kotlin data classes generate a toString() method that prints the class name and all properties with their values in the format ClassName(prop1=value1, prop2=value2).

❓ lifecycle

advanced

Copying and modifying a Kotlin data class instance

Given this data class and instance:

data class Book(val title: String, val author: String, val year: Int)
val original = Book("1984", "Orwell", 1949)

Which option creates a copy of original but changes the year to 1950?

Android Kotlin

data class Book(val title: String, val author: String, val year: Int)
fun main() {
  val original = Book("1984", "Orwell", 1949)
  val copy = /* fill here */
  println(copy)
}

A original.copy(year = 1950)

B Book(original.title, original.author, 1950)

C original.clone(year = 1950)

D original.copy(1950)

💡 Hint

Data classes have a built-in copy() method to create modified copies.

The copy() method allows creating a new instance with some properties changed. You specify the property name and new value as a named argument.

🔧 Debug

advanced

Error caused by missing property types in data class

What error will this Kotlin code produce?

data class Car(val make, val model: String)

Android Kotlin

data class Car(val make, val model: String)

A Error: Property 'make' must have a type

B No error, compiles fine

C Error: Missing parentheses in class declaration

D Error: Duplicate property names

💡 Hint

Every property in a data class constructor must declare its type.

Kotlin requires all constructor properties to have explicit types. Missing type for 'make' causes a compilation error.

🧠 Conceptual

expert

Equality check between Kotlin data class instances

Consider these two instances:

data class Point(val x: Int, val y: Int)
val p1 = Point(1, 2)
val p2 = Point(1, 2)

What is the result of p1 == p2?

Android Kotlin

data class Point(val x: Int, val y: Int)
fun main() {
  val p1 = Point(1, 2)
  val p2 = Point(1, 2)
  println(p1 == p2)
}

A Reference equality check, result depends on JVM

B false

C Compilation error

D true

💡 Hint

Data classes automatically generate equals() to compare property values.

Kotlin data classes override equals() to compare all properties for equality, so two instances with same property values are equal.