Secondary constructors in Kotlin - Mini Project: Build & Apply

Using Secondary Constructors in Kotlin

📖 Scenario: You are creating a simple Kotlin class to represent a Book in a library system. Sometimes you know all the details about the book, and sometimes you only know the title and author.

🎯 Goal: Build a Kotlin class Book with a primary constructor and a secondary constructor to handle different ways of creating book objects.

📋 What You'll Learn

Create a class called Book with a primary constructor that takes title, author, and year.

Add a secondary constructor that takes only title and author, and sets year to 0.

Create two Book objects using both constructors.

Print the details of both books.

💡 Why This Matters

🌍 Real World

Secondary constructors let you create objects in different ways depending on the information you have, like when some details are missing or optional.

💼 Career

Understanding constructors is essential for Kotlin developers to write flexible and readable code, especially when working with data models in apps.

1

Create the Book class with a primary constructor

Create a Kotlin class called Book with a primary constructor that has three parameters: title of type String, author of type String, and year of type Int. Store these parameters as properties in the class.

Kotlin

class Book(val title: String, val author: String, val year: Int) {
    // Your code here
}

Hint

Use class Book(val title: String, val author: String, val year: Int) to create the class with properties.

2

Add a secondary constructor to the Book class

Inside the Book class, add a secondary constructor that takes only title and author as parameters. This constructor should call the primary constructor and set year to 0.

Kotlin

class Book(val title: String, val author: String, val year: Int) {
    constructor(title: String, author: String) : this(title, author, 0) {
        // Your code here
    }
}

Hint

Use constructor(title: String, author: String) : this(title, author, 0) inside the class.

3

Create two Book objects using both constructors

Create a variable called book1 using the primary constructor with "1984", "George Orwell", and 1949. Then create a variable called book2 using the secondary constructor with "Animal Farm" and "George Orwell".

Kotlin

class Book(val title: String, val author: String, val year: Int) {
    constructor(title: String, author: String) : this(title, author, 0) {
    }
}

// Create book1 and book2 below
// Your code here

Hint

Use val book1 = Book("1984", "George Orwell", 1949) and val book2 = Book("Animal Farm", "George Orwell").

4

Print the details of both books

Print the details of book1 and book2 in the format: Title: [title], Author: [author], Year: [year]. Use two separate println statements.

Kotlin

class Book(val title: String, val author: String, val year: Int) {
    constructor(title: String, author: String) : this(title, author, 0) {
    }
}

val book1 = Book("1984", "George Orwell", 1949)
val book2 = Book("Animal Farm", "George Orwell")

// Print book1 and book2 details below
// Your code here

Hint

Use println("Title: ${book1.title}, Author: ${book1.author}, Year: ${book1.year}") and similarly for book2.

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