Constructor trong Kotlin - Primary và Secondary

class Person(val name: String, var age: Int)
 
// Với từ khóa constructor (optional)
class User constructor(val email: String)
 
fun main() {
    val person = Person("Alice", 25)
    println(person.name)  // Alice
}

class Person(val name: String) {
    val nameLength: Int
    
    init {
        println("Đang tạo person: $name")
        nameLength = name.length
    }
    
    init {
        println("Có thể có nhiều init blocks")
    }
}

class Config(
    val host: String = "localhost",
    val port: Int = 8080,
    val secure: Boolean = false
)
 
fun main() {
    val default = Config()
    val custom = Config(port = 3000)
    val full = Config("api.example.com", 443, true)
}

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
    }
}

class Person(val name: String, val age: Int) {
    var email: String = ""
    
    constructor(name: String, age: Int, email: String) : this(name, age) {
        this.email = email
    }
}

class Singleton private constructor() {
    companion object {
        private val instance = Singleton()
        fun getInstance() = instance
    }
}
 
fun main() {
    // val s = Singleton()  // Error! Private constructor
    val s = Singleton.getInstance()
}

class Age(value: Int) {
    val value: Int
    
    init {
        require(value >= 0) { "Age must be non-negative" }
        require(value <= 150) { "Age seems unrealistic" }
        this.value = value
    }
}
 
fun main() {
    val age = Age(25)      // OK
    // val bad = Age(-5)   // IllegalArgumentException
}

open class Animal(val name: String)
 
class Dog(name: String, val breed: String) : Animal(name)
 
class Cat : Animal {
    val color: String
    
    constructor(name: String, color: String) : super(name) {
        this.color = color
    }
}

// Kotlin - ngắn gọn
class Person(val name: String, var age: Int)
 
// Java - dài dòng
/*
public class Person {
    private final String name;
    private int age;
    
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String getName() { return name; }
    public int getAge() { return age; }
    public void setAge(int age) { this.age = age; }
}
*/