Class và Object trong Kotlin

class Person {
    var name: String = ""
    var age: Int = 0
    
    fun introduce() {
        println("Tôi là $name, $age tuổi")
    }
}
 
fun main() {
    val person = Person()
    person.name = "Alice"
    person.age = 25
    person.introduce()
}

class Person(val name: String, var age: Int) {
    fun introduce() {
        println("Tôi là $name, $age tuổi")
    }
}
 
fun main() {
    val person = Person("Bob", 30)
    println(person.name)  // Bob
    person.age = 31       // Có thể thay đổi vì var
}

class Person(val name: String) {
    val nameLength: Int
    
    init {
        nameLength = name.length
        println("Person created: $name")
    }
}
 
fun main() {
    val p = Person("Alice")  // In: Person created: Alice
    println(p.nameLength)     // 5
}

class Person {
    var name: String
    var age: Int
    
    constructor(name: String) {
        this.name = name
        this.age = 0
    }
    
    constructor(name: String, age: Int) {
        this.name = name
        this.age = age
    }
}
 
fun main() {
    val p1 = Person("Alice")
    val p2 = Person("Bob", 30)
}

class Rectangle(val width: Double, val height: Double) {
    // Computed property
    val area: Double
        get() = width * height
    
    // Property với backing field
    var color: String = "white"
        set(value) {
            field = value.lowercase()
        }
        get() = field.uppercase()
}
 
fun main() {
    val rect = Rectangle(5.0, 3.0)
    println(rect.area)  // 15.0
    
    rect.color = "RED"
    println(rect.color)  // RED
}

class BankAccount(private val accountNumber: String) {
    private var balance = 0.0
    
    fun deposit(amount: Double) {
        if (amount > 0) balance += amount
    }
    
    fun getBalance() = balance
}
 
// public - mặc định, visible everywhere
// private - chỉ trong class
// protected - class và subclasses
// internal - trong cùng module

class Calculator {
    fun add(a: Int, b: Int) = a + b
    fun subtract(a: Int, b: Int) = a - b
    fun multiply(a: Int, b: Int) = a * b
    fun divide(a: Int, b: Int) = if (b != 0) a / b else null
}
 
fun main() {
    val calc = Calculator()
    println(calc.add(5, 3))  // 8
}

class User private constructor(val name: String) {
    companion object {
        fun create(name: String): User {
            println("Creating user: $name")
            return User(name)
        }
        
        const val MAX_AGE = 150
    }
}
 
fun main() {
    val user = User.create("Alice")
    println(User.MAX_AGE)  // 150
}

class MyClass {
    lateinit var name: String
    
    fun initialize() {
        name = "Initialized"
    }
    
    fun checkInit() {
        if (::name.isInitialized) {
            println(name)
        }
    }
}