Kế thừa (Inheritance) trong Kotlin

open class Animal(val name: String) {
    open fun makeSound() {
        println("$name makes a sound")
    }
}
 
class Dog(name: String) : Animal(name) {
    override fun makeSound() {
        println("$name barks")
    }
}
 
fun main() {
    val dog = Dog("Buddy")
    dog.makeSound()  // Buddy barks
}

open class Vehicle(val brand: String) {
    open fun start() {
        println("$brand is starting")
    }
}
 
class Car(brand: String) : Vehicle(brand) {
    override fun start() {
        super.start()
        println("Car engine running")
    }
}

open class Shape {
    open val sides: Int = 0
}
 
class Triangle : Shape() {
    override val sides: Int = 3
}
 
class Square : Shape() {
    override val sides: Int = 4
}

abstract class Animal(val name: String) {
    abstract fun makeSound()
    
    fun eat() {
        println("$name is eating")
    }
}
 
class Cat(name: String) : Animal(name) {
    override fun makeSound() {
        println("$name meows")
    }
}

sealed class Result {
    data class Success(val data: String) : Result()
    data class Error(val message: String) : Result()
    object Loading : Result()
}
 
fun handleResult(result: Result) {
    when (result) {
        is Result.Success -> println("Data: ${result.data}")
        is Result.Error -> println("Error: ${result.message}")
        Result.Loading -> println("Loading...")
    }
}

open class Person(val name: String, var age: Int)
 
class Student(
    name: String,
    age: Int,
    val school: String
) : Person(name, age)
 
// Với secondary constructor
class Employee : Person {
    val company: String
    
    constructor(name: String, age: Int, company: String) : super(name, age) {
        this.company = company
    }
}

open class Shape {
    open fun area(): Double = 0.0
}
 
class Circle(val radius: Double) : Shape() {
    override fun area() = 3.14159 * radius * radius
}
 
class Rectangle(val width: Double, val height: Double) : Shape() {
    override fun area() = width * height
}
 
fun main() {
    val shapes: List<Shape> = listOf(Circle(5.0), Rectangle(4.0, 3.0))
    shapes.forEach { println(it.area()) }
}

open class Parent {
    open fun method() { println("Parent") }
}
 
open class Child : Parent() {
    final override fun method() { println("Child") }
}
 
class GrandChild : Child() {
    // override fun method() {}  // Error! method is final
}

// Mọi class đều kế thừa Any
class MyClass  // Ngầm định: class MyClass : Any()
 
// Any có 3 methods:
// equals(), hashCode(), toString()