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.

Practice

1. What is the main purpose of a data class in Kotlin?

easy

A. To hold data and automatically provide useful functions like toString() and equals()

B. To create user interface components

C. To manage app navigation between screens

D. To handle background tasks and threading

Solution

1. Step 1: Understand the role of data classes

   Data classes are designed to store data and automatically generate functions like toString(), equals(), and hashCode().

2. Step 2: Compare with other options

   Options B, C, and D relate to UI, navigation, and threading, which are not the purpose of data classes.

3. Final Answer:

   To hold data and automatically provide useful functions like toString() and equals() -> Option A

4. Quick Check:

   Data class purpose = hold data with auto functions [OK]

Hint: Data classes store data and auto-generate helper functions [OK]

Common Mistakes:

• Confusing data classes with UI components
• Thinking data classes manage app navigation
• Assuming data classes handle threading

2. Which of the following is the correct syntax to declare a data class named Person with properties name (String) and age (Int)?

easy

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

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

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

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

Solution

1. Step 1: Recall correct data class syntax

   The correct syntax starts with data class followed by the class name and constructor parameters.

2. Step 2: Check each option

   data class Person(val name: String, val age: Int) matches the correct syntax. Options A, B, and D have wrong order or missing keywords.

3. Final Answer:

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

4. Quick Check:

   Data class syntax = data class ClassName(params) [OK]

Hint: Start with 'data class' keyword to declare data classes [OK]

Common Mistakes:

• Omitting the 'data' keyword
• Swapping order of 'class' and 'data'
• Using incorrect parameter syntax

3. Given the data class:

data class Book(val title: String, val author: String)

What will be the output of:

val book1 = Book("Kotlin Basics", "Anna")
val book2 = Book("Kotlin Basics", "Anna")
println(book1 == book2)

medium

A. false

B. Runtime exception

C. Compilation error

D. true

Solution

1. Step 1: Understand equality in data classes

   Data classes automatically generate equals() to compare property values, so two objects with same data are equal.

2. Step 2: Evaluate the code

   Both book1 and book2 have identical title and author, so book1 == book2 returns true.

3. Final Answer:

   true -> Option D

4. Quick Check:

   Data class equality compares properties = true [OK]

Hint: Data class equality checks property values, not references [OK]

Common Mistakes:

• Thinking '==' compares references, not values
• Expecting false because objects are different instances
• Confusing with regular class equality

4. What is wrong with this data class declaration?

data class User(val name: String, var age: Int) {
  fun copyWithNewAge(newAge: Int): User {
    return copy(age = newAge)
  }
}

medium

A. The copy function is automatically generated and can be used here

B. The copy function cannot be used inside the class

C. Data classes cannot have functions inside them

D. The age property cannot be var in a data class

Solution

1. Step 1: Understand the copy function in data classes

   Kotlin automatically generates a copy function for data classes to create modified copies.

2. Step 2: Check usage inside the class

   Using copy inside the class is valid and common to create new instances with changed properties.

3. Final Answer:

   The copy function is automatically generated and can be used here -> Option A

4. Quick Check:

   copy() is auto-generated and usable inside data class [OK]

Hint: copy() is built-in and usable inside data classes [OK]

Common Mistakes:

• Thinking copy() is unavailable inside the class
• Believing data classes cannot have functions
• Assuming var properties are disallowed

5. You want to create a data class Employee with properties id (Int), name (String), and department (String). You also want to exclude department from the automatically generated equals() and hashCode() methods because it can change often. How can you achieve this?

hard

A. Make department a var property and mark it with @Transient

B. Use data class Employee(val id: Int, val name: String) and add var department: String inside the class body

C. You cannot exclude properties from equals() and hashCode() in data classes

D. Declare department inside the class body, not in the primary constructor

Solution

1. Step 1: Understand how data classes generate equals and hashCode

   Only properties declared in the primary constructor are used in equals() and hashCode().

2. Step 2: Exclude a property by declaring it outside the constructor

   Declaring department inside the class body excludes it from these methods.

3. Final Answer:

   Use data class Employee(val id: Int, val name: String) and add var department: String inside the class body -> Option B

4. Quick Check:

   Only constructor properties affect equals/hashCode [OK]

Hint: Exclude property by declaring it outside constructor [OK]

Common Mistakes:

• Assuming @Transient excludes from equals/hashCode
• Trying to exclude constructor properties directly
• Believing all properties must be in constructor