Secondary constructors in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of secondary constructor call

What is the output of this Kotlin code using a secondary constructor?

Kotlin

class Person(val name: String) {
    var age: Int = 0

    constructor(name: String, age: Int) : this(name) {
        this.age = age
    }

    fun info() = "$name is $age years old"
}

fun main() {
    val p = Person("Alice", 30)
    println(p.info())
}

A Alice is 30 years old

B Alice is 0 years old

C Runtime error: Uninitialized property

D Compilation error due to secondary constructor

💡 Hint

Remember that the secondary constructor calls the primary constructor first.

The secondary constructor calls the primary constructor with 'this(name)', initializing 'name'. Then it sets 'age' to 30. So info() returns 'Alice is 30 years old'.

❓ Predict Output

intermediate

Value of property after secondary constructor

What is the value of 'age' after creating an instance with the secondary constructor?

Kotlin

class Animal(val species: String) {
    var age: Int = 5

    constructor(species: String, age: Int) : this(species) {
        this.age = age
    }
}

fun main() {
    val a = Animal("Dog", 7)
    println(a.age)
}

A 7

B 5

C 0

D Compilation error

💡 Hint

Check how the secondary constructor sets the 'age' property.

The primary constructor sets age to 5, but the secondary constructor overrides it to 7.

🔧 Debug

advanced

Identify the error in secondary constructor usage

What error does this Kotlin code produce?

Kotlin

class Car(val brand: String) {
    var year: Int

    constructor(brand: String, year: Int) {
        this.brand = brand
        this.year = year
    }
}

fun main() {
    val c = Car("Toyota", 2020)
    println(c.brand + " " + c.year)
}

A Compilation error: Val cannot be reassigned

B Runtime error: Uninitialized property 'year'

C Compilation error: Secondary constructor must delegate to primary

D No error, prints 'Toyota 2020'

💡 Hint

Check how the secondary constructor calls the primary constructor.

In Kotlin, a secondary constructor must delegate to the primary constructor using 'this(...)'. This code does not, causing a compilation error.

📝 Syntax

advanced

Correct syntax for secondary constructor delegation

Which option shows the correct syntax for a secondary constructor delegating to the primary constructor?

Kotlin

class Book(val title: String) {
    var pages: Int = 0

    // Secondary constructor here
}

A constructor(title: String, pages: Int) { this.pages = pages }

B constructor(title: String, pages: Int) : this(title) { this.pages = pages }

C constructor(title: String, pages: Int) -> this(title) { this.pages = pages }

D constructor(title: String, pages: Int) : super(title) { this.pages = pages }

💡 Hint

Look for the correct delegation syntax using ': this(...)'.

Option B correctly delegates to the primary constructor using ': this(title)'. Option B misses delegation, C uses invalid arrow syntax, D incorrectly uses 'super'.

🚀 Application

expert

Number of instances created with secondary constructor

Given this Kotlin code, how many times is the primary constructor called when creating an instance with the secondary constructor?

Kotlin

class User(val username: String) {
    init {
        println("Primary constructor called for $username")
    }

    constructor(username: String, email: String) : this(username) {
        println("Secondary constructor called for $username with email $email")
    }
}

fun main() {
    val u = User("john_doe", "[e-mail]")
}

A Primary constructor is called after secondary constructor

B Primary constructor is called twice

C Primary constructor is not called

D Primary constructor is called once

💡 Hint

Remember how secondary constructors delegate to primary constructors in Kotlin.

The secondary constructor delegates to the primary constructor exactly once before executing its own body.

Practice

1. What is the main purpose of a secondary constructor in Kotlin?

easy

A. To create static methods

B. To replace the primary constructor completely

C. To provide additional ways to create an object with different parameters

D. To define functions inside a class

Solution

1. Step 1: Understand constructors in Kotlin

   Kotlin classes can have a primary constructor and one or more secondary constructors to create objects.

2. Step 2: Role of secondary constructors

   Secondary constructors allow creating objects with different sets of parameters, providing flexibility.

3. Final Answer:

   To provide additional ways to create an object with different parameters -> Option C

4. Quick Check:

   Secondary constructors = multiple ways to create objects [OK]

Hint: Secondary constructors add extra ways to create objects [OK]

Common Mistakes:

• Thinking secondary constructors replace the primary constructor
• Confusing constructors with functions
• Assuming secondary constructors create static methods

2. Which of the following is the correct syntax for a secondary constructor in Kotlin?

easy

A. secondary constructor(name: String) { this.name = name }

B. fun constructor(name: String) { this.name = name }

C. init constructor(name: String) { this.name = name }

D. constructor(name: String) { this.name = name }

Solution

1. Step 1: Recall Kotlin secondary constructor syntax

   Secondary constructors use the keyword constructor followed by parameters and a body.

2. Step 2: Check each option

   constructor(name: String) { this.name = name } uses correct syntax: constructor(name: String) { ... }. Others use invalid keywords.

3. Final Answer:

   constructor(name: String) { this.name = name } -> Option D

4. Quick Check:

   Keyword 'constructor' starts secondary constructors [OK]

Hint: Look for 'constructor' keyword without fun or init [OK]

Common Mistakes:

• Adding 'fun' before constructor
• Using 'secondary' keyword
• Confusing with init blocks

3. What will be the output of the following Kotlin code?

class Person(val name: String) {
    var age: Int = 0
    constructor(name: String, age: Int) : this(name) {
        this.age = age
    }
}

fun main() {
    val p = Person("Alice", 30)
    println("Name: ${p.name}, Age: ${p.age}")
}

medium

A. Name: Alice, Age: 0

B. Name: Alice, Age: 30

C. Name: , Age: 30

D. Compilation error

Solution

1. Step 1: Understand constructor calls

   The secondary constructor calls the primary constructor with this(name), initializing name.

2. Step 2: Assign age in secondary constructor

   The secondary constructor sets this.age = age, so age becomes 30.

3. Final Answer:

   Name: Alice, Age: 30 -> Option B

4. Quick Check:

   Secondary constructor sets age correctly [OK]

Hint: Secondary constructor calls primary, then sets extra properties [OK]

Common Mistakes:

• Assuming age stays 0
• Thinking name is empty
• Expecting compilation error

4. Identify the error in this Kotlin class with a secondary constructor:

class Car(val model: String) {
    constructor(model: String, year: Int) {
        println("Year: $year")
    }
}

medium

A. Secondary constructor must call the primary constructor

B. Cannot assign to val property 'model'

C. Missing return type in secondary constructor

D. println cannot be used inside constructor

Solution

1. Step 1: Check secondary constructor rules

   In Kotlin, a secondary constructor must delegate to the primary constructor using : this(...).

2. Step 2: Analyze given code

   The secondary constructor does not call this(model), so it violates the rule.

3. Final Answer:

   Secondary constructor must call the primary constructor -> Option A

4. Quick Check:

   Secondary constructors require delegation [OK]

Hint: Always delegate secondary constructors to primary with ': this(...)' [OK]

Common Mistakes:

• Forgetting to delegate to primary constructor
• Trying to assign val property directly
• Thinking println is disallowed

5. You want to create a Kotlin class Book with a primary constructor that takes title and a secondary constructor that also takes author. The secondary constructor should reuse the primary constructor and set author. Which code correctly implements this?

hard

A. class Book(val title: String) {
var author: String = "Unknown"
constructor(title: String, author: String) : this(title) {
this.author = author
}
}

B. class Book(val title: String, var author: String) {
constructor(title: String) {
this.title = title
this.author = "Unknown"
}
}

C. class Book(val title: String) {
constructor(title: String, author: String) {
this.title = title
this.author = author
}
}

D. class Book {
constructor(title: String) {
this.title = title
}
constructor(title: String, author: String) {
this.title = title
this.author = author
}
}

Solution

1. Step 1: Check primary constructor and property initialization

   class Book(val title: String) { var author: String = "Unknown" constructor(title: String, author: String) : this(title) { this.author = author } } defines primary constructor with title and a mutable author property with default value.

2. Step 2: Verify secondary constructor delegates properly

   The secondary constructor calls : this(title) to reuse primary constructor and sets author inside its body.

3. Final Answer:

   class Book(val title: String) { var author: String = "Unknown" constructor(title: String, author: String) : this(title) { this.author = author } } -> Option A

4. Quick Check:

   Secondary constructor delegates and sets extra property [OK]

Hint: Secondary constructors must delegate and set extra properties inside body [OK]

Common Mistakes:

• Not delegating secondary constructor to primary
• Trying to assign val properties
• Missing property initialization