KOTLIN Tutorial

The `init` Block in Kotlin

The init block in Kotlin is used to initialize properties and execute code when an object is created. It's part of the class's primary constructor and allows you to execute code as soon as the object is instantiated.

Let's dive into how and when you can use the init block in Kotlin!

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What is the `init` Block?

The init block is used for initialization code. It runs immediately when an object of the class is created, and it's the place to perform any setup you need that isn't part of the constructor's parameter initialization.

// Example of using init block
class Student(val name: String, var grade: Int) {
    init {
        // Code inside init block is executed when the object is created
        println("Student Created: Name = $name, Grade = $grade")
    }
}

// Creating an object of Student
val student1 = Student("John", 90)
  

In this example, when the Student object is created, the init block is executed, printing the details of the student.

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Multiple `init` Blocks

In Kotlin, you can have multiple init blocks in a class. These blocks will be executed in the order they are defined.

// Multiple init blocks
class Book(val title: String, val author: String) {
    init {
        println("Initializing Book: $title by $author")
    }
    
    init {
        println("Book Initialized: $title")
    }
}

// Creating an object of Book
val book1 = Book("Kotlin Programming", "Jane Smith")
  

Here, we have two init blocks in the Book class. The output will print the initialization message twice, in the order the blocks appear.

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Using `init` to Validate or Modify Inputs

The init block is a good place to validate or modify the properties of an object when it's created.

// Using init block for validation
class Account(val accountNumber: String, var balance: Double) {
    init {
        if (balance < 0) {
            balance = 0.0
            println("Negative balance is not allowed. Setting balance to 0.")
        }
    }
}

// Creating an object of Account
val account1 = Account("12345", -500.0)
println("Account Balance: ${account1.balance}")
  

In this case, the init block checks whether the balance is negative. If it is, it sets the balance to 0 and prints a message.

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Accessing Constructor Parameters in `init` Block

Since the init block runs when the object is created, you can access constructor parameters directly within the block to perform additional initialization tasks.

// Accessing constructor parameters in init block
class Product(val name: String, var price: Double) {
    init {
        println("Product Initialized: $name, Price: $price")
    }
    
    fun applyDiscount(discount: Double) {
        price -= price * (discount / 100)
    }
}

// Creating a Product object
val product1 = Product("Smartphone", 799.99)
product1.applyDiscount(10)
println("Price after discount: ${product1.price}")
  

The init block prints the initial product details when the object is created. After that, you can apply a discount using the applyDiscount method, which modifies the price.

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🧪 Try It Yourself

Try these challenges to get hands-on experience with the init block:

• Create a class called Employee with properties name and salary. Use the init block to set a minimum salary (e.g., $1000) if the provided salary is too low.
• Define a class Rectangle with properties length and width. In the init block, calculate the area and perimeter of the rectangle.
• Write a Car class with properties make, model, and year. In the init block, ensure that the year is a valid, non-negative number, and if it's not, set it to the current year.