Constructors and Initialization

# CHAPTER 14

Constructors and Initialization #

1. Chapter Introduction #

2. Learning Objectives #

• Define a Primary Constructor.

• Automatically declare properties inside the constructor.

• Run setup code using the init block.

• Define Secondary Constructors (Constructor Overloading).

3. The Primary Constructor #

Verbose Way (Similar to Java):

class Person constructor(firstName: String) {
    var name: String = firstName // Assign the parameter to the property
}

The Kotlin Way (Concise): Kotlin allows you to declare properties directly in the constructor by adding val or var before the parameter name. This eliminates boilerplate!

// The Primary Constructor is right here in the parentheses!
class Person(val name: String, var age: Int)

fun main() {
    // We MUST pass "Alice" and 28 when creating the object
    val user = Person("Alice", 28)
    
    println("${user.name} is ${user.age}")
}

4. The init Block #

class BankAccount(val owner: String, startingBalance: Double) {
    var balance: Double = startingBalance

    // This runs automatically when the object is created!
    init {
        println("Initializing account for $owner...")
        if (balance < 0) {
            println("Warning: Account opened with negative balance!")
            balance = 0.0
        }
    }
}

fun main() {
    val myAccount = BankAccount("Bob", -50.0)
    println("Final Balance: $${myAccount.balance}")
}

5. Secondary Constructors (Overloading) #

*Rule: Every secondary constructor MUST call the primary constructor using this().*

class User(val email: String) { // Primary Constructor
    var phone: String = "Not Provided"

    // Secondary Constructor
    constructor(email: String, phone: String) : this(email) {
        this.phone = phone
        println("Secondary constructor used!")
    }
}

fun main() {
    val u1 = User("admin@test.com") // Calls Primary
    val u2 = User("user@test.com", "555-1234") // Calls Secondary
}

6. Kotlin's Better Way: Default Arguments #

// This ONE line replaces multiple secondary constructors!
class User(val email: String, var phone: String = "Not Provided")

fun main() {
    val u1 = User("admin@test.com") 
    val u2 = User("user@test.com", "555-1234") 
}

7. Common Mistakes #

• Forgetting val or var in the Primary Constructor: If you write class Person(name: String), name is just a temporary parameter passed to the init block. It is NOT saved as a property of the class! You must write class Person(val name: String) for it to become a permanent property.

8. Best Practices #

• Use Default Arguments: Avoid writing multiple constructor blocks. Use default values in your primary constructor. It is significantly more idiomatic and concise.

9. Exercises #

1. 1. Create a class Product with a primary constructor taking val name: String and var price: Double.

1. 2. Add an init block that prints "Product created: [name]".

1. 3. Instantiate a Product in main() and print its price.

10. MCQs with Answers #

11. Interview Questions #

• Q: Explain how Kotlin handles constructor overloading differently than Java. (Answer: While Kotlin supports Secondary Constructors, it strongly encourages using Default Arguments in the Primary Constructor, reducing boilerplate code massively).

• Q: What is the purpose of the init block?

12. Summary #

13. Next Chapter Recommendation #