Interface trong Kotlin

interface Drawable {
    fun draw()
}
 
class Circle : Drawable {
    override fun draw() {
        println("Drawing circle")
    }
}
 
fun main() {
    val circle = Circle()
    circle.draw()
}

interface Printable {
    fun print()
    
    fun preview() {
        println("Previewing...")
        print()
    }
}
 
class Document : Printable {
    override fun print() {
        println("Printing document")
    }
    // preview() đã có sẵn implementation
}

interface Named {
    val name: String  // abstract property
    
    val greeting: String
        get() = "Hello, $name"  // với default getter
}
 
class Person(override val name: String) : Named
 
fun main() {
    val person = Person("Alice")
    println(person.greeting)  // Hello, Alice
}

interface Flyable {
    fun fly()
}
 
interface Swimmable {
    fun swim()
}
 
class Duck : Flyable, Swimmable {
    override fun fly() = println("Flying")
    override fun swim() = println("Swimming")
}

interface A {
    fun greet() = println("Hello from A")
}
 
interface B {
    fun greet() = println("Hello from B")
}
 
class C : A, B {
    override fun greet() {
        super<A>.greet()
        super<B>.greet()
    }
}

// Interface - không có state
interface Movable {
    fun move()
    val speed: Int  // abstract
}
 
// Abstract class - có thể có state
abstract class Vehicle(val brand: String) {
    abstract fun start()
    
    fun honk() {
        println("Beep!")
    }
}

fun interface Clickable {
    fun onClick()
}
 
fun main() {
    // Lambda expression
    val button = Clickable { println("Clicked!") }
    button.onClick()
}

interface Repository<T> {
    fun findById(id: Int): T?
    fun findAll(): List<T>
    fun save(item: T)
    fun delete(id: Int)
}
 
class UserRepository : Repository<User> {
    private val users = mutableListOf<User>()
    
    override fun findById(id: Int) = users.find { it.id == id }
    override fun findAll() = users.toList()
    override fun save(item: User) { users.add(item) }
    override fun delete(id: Int) { users.removeIf { it.id == id } }
}

interface Printer {
    fun print(message: String)
}
 
class ConsolePrinter : Printer {
    override fun print(message: String) = println(message)
}
 
class Document(printer: Printer) : Printer by printer {
    // print() được delegate cho printer
}
 
fun main() {
    val doc = Document(ConsolePrinter())
    doc.print("Hello!")  // Hello!
}