Kotlin Object-Oriented Programming: Classes, Inheritance, and Interfaces

In the previous articles, we covered the basics of Kotlin syntax and key differences between Kotlin and Java. In this article, we'll explore Kotlin's object-oriented programming features, focusing on classes, inheritance, and interfaces. These concepts will help you create clean and maintainable code. Let's get started!

Classes

In Kotlin, classes are defined using the 'class' keyword. A class can have properties (variables) and member functions (methods). Here's a simple example:

class Person(val name: String, val age: Int) {
    fun introduce() {
        println("Hello, my name is $name and I am $age years old.")
    }
}

// Usage
val person = Person("John Doe", 30)
person.introduce() // Output: Hello, my name is John Doe and I am 30 years old.

Inheritance

By default, Kotlin classes are 'final', which means they cannot be inherited. To enable inheritance, use the 'open' keyword. The 'super' keyword is used to call the parent class constructor or methods.

open class Animal(val name: String) {
    open fun speak() {
        println("$name makes a sound.")
    }
}

class Dog(name: String) : Animal(name) {
    override fun speak() {
        println("$name barks.")
    }
}

// Usage
val dog = Dog("Buddy")
dog.speak() // Output: Buddy barks.

Interfaces

Interfaces in Kotlin are similar to Java interfaces. They can contain abstract methods and properties, as well as default implementations. Classes can implement multiple interfaces using the 'interface' keyword.

interface Flyable {
    fun fly()
}

interface Swimmable {
    fun swim()
}

class Duck : Flyable, Swimmable {
    override fun fly() {
        println("The duck flies.")
    }

    override fun swim() {
        println("The duck swims.")
    }
}

// Usage
val duck = Duck()
duck.fly() // Output: The duck flies.
duck.swim() // Output: The duck swims.

Data Classes

Kotlin provides data classes, which automatically generate useful methods like equals(), hashCode(), and toString(). To create a data class, use the 'data' keyword:

data class Point(val x: Int, val y: Int)

// Usage
val point1 = Point(3, 4)
val point2 = Point(3, 4)
println(point1 == point2) // Output: true
println(point1) // Output: Point(x=3, y=4)

Conclusion

In this article, we have explored Kotlin's object-oriented programming features, such as classes, inheritance, and interfaces. These concepts are essential for creating clean, maintainable, and reusable code in Kotlin. As you continue learning Kotlin, you'll discover more advanced features that will help you build powerful applications.

In the next article, we'll dive into Kotlin's functional programming features, including lambdas, collections, and extension functions. Stay tuned for more insights into Kotlin's capabilities!

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