Classes and Objects in Kotlin

Let's dive into a foundational concept in Object-Oriented Programming (OOP): Classes and Objects. If you have already explored OOP concepts in other programming languages or previous units, this might serve as a good reminder. If not, no worries; we'll start from the basics.

Classes and objects are the building blocks of OOP. A class acts as a blueprint for creating objects, which are instances of the class. Understanding these basics is essential before moving on to more advanced OOP topics like inheritance, polymorphism, and encapsulation.

An object is an instance of a class. It represents a specific example of the class and holds the characteristics that define the class.

Objects have three main characteristics:

• State: The data or attributes of the object. In the Person class, the name and age represent the object's state.
• Behavior: The methods and functions that the object can perform.
• Identity: A unique identifier that distinguishes the object from others, even if they have the same state.

To understand object identity in Kotlin, consider this example:

fun main() {
    val person1 = Person("Alice", 25)
    val person2 = Person("Alice", 25)
    val person3 = person1

    println(person1 == person2)  // true (compares values)
    println(person1 === person2)  // false (compares references)
    println(person1 === person3)  // true (same object reference)
}

Kotlin integrates the constructor directly into the class declaration as a primary constructor. You can initialize the object's properties using this constructor:

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

For more complex initialization, use the init block:

class Person(val name: String, val age: Int) {
    init {
        require(age > 0) { "Age must be positive" }
    }
}