Companion Object trong Kotlin

class MyClass {
    companion object {
        const val CONSTANT = "Hello"
        
        fun create(): MyClass {
            return MyClass()
        }
    }
}
 
fun main() {
    println(MyClass.CONSTANT)  // Hello
    val instance = MyClass.create()
}

class User private constructor(val name: String) {
    companion object Factory {
        fun fromName(name: String) = User(name)
        fun fromEmail(email: String) = User(email.substringBefore("@"))
    }
}
 
fun main() {
    val user1 = User.fromName("Alice")
    val user2 = User.Factory.fromEmail("bob@email.com")
}

interface Factory<T> {
    fun create(): T
}
 
class MyClass private constructor() {
    companion object : Factory<MyClass> {
        override fun create() = MyClass()
    }
}
 
fun main() {
    val factory: Factory<MyClass> = MyClass
    val instance = factory.create()
}

class Person(val name: String) {
    companion object
}
 
fun Person.Companion.fromJson(json: String): Person {
    // Parse JSON
    return Person(json)
}
 
fun main() {
    val person = Person.fromJson("{name: 'Alice'}")
}

class Logger private constructor() {
    fun log(message: String) = println("[LOG] $message")
    
    companion object {
        private val instance = Logger()
        fun getInstance() = instance
    }
}
 
fun main() {
    Logger.getInstance().log("Hello")
}

class Config {
    companion object {
        const val MAX_SIZE = 100      // Compile-time constant
        val DEFAULT_NAME = "Default"   // Runtime value
        
        // const chỉ cho primitive và String
        // const val list = listOf(1, 2)  // Error!
    }
}

class Utils {
    companion object {
        @JvmStatic
        fun helper() = println("Helper")
    }
}
 
// Từ Java có thể gọi: Utils.helper()
// Thay vì: Utils.Companion.helper()

// Companion object - thuộc về class
class MyClass {
    companion object {
        fun foo() = println("Companion")
    }
}
 
// Object declaration - standalone singleton
object MySingleton {
    fun bar() = println("Singleton")
}
 
fun main() {
    MyClass.foo()      // Companion
    MySingleton.bar()  // Singleton
}

data class User(val id: Int, val name: String) {
    companion object {
        const val TABLE_NAME = "users"
        
        fun fromMap(map: Map<String, Any>): User {
            return User(
                id = map["id"] as Int,
                name = map["name"] as String
            )
        }
        
        fun empty() = User(0, "")
    }
}
 
fun main() {
    val user = User.fromMap(mapOf("id" to 1, "name" to "Alice"))
    println(user)
}