Kotlin: Kotlin入門と設計思想

// Java: 20+ lines just to define a simple POJO
public class Order {
    private String id;
    private BigDecimal total;
    private String status;

    public Order(String id, BigDecimal total, String status) {
        this.id = id;
        this.total = total;
        this.status = status;
    }

    public String getId() { return id; }
    public BigDecimal getTotal() { return total; }
    public String getStatus() { return status; }

    @Override
    public boolean equals(Object o) { /* 10+ lines */ }
    @Override
    public int hashCode() { /* 5+ lines */ }
    @Override
    public String toString() { /* 3+ lines */ }
}

// Kotlin: 1 line does it all
data class Order(val id: String, val total: BigDecimal, val status: String)

// commonMain - shared across all platforms
expect fun logMessage(message: String)

// JVM platform
actual fun logMessage(message: String) = println("[JVM] $message")

// JS platform
actual fun logMessage(message: String) = console.log("[JS] $message")

// Non-null type: CANNOT be null
val orderId: String = "ORD-001"

// Nullable type: MUST use ? suffix
val customerName: String? = null

// Safe call operator
val length = customerName?.length  // null if customerName is null

// Elvis operator: provide default
val name = customerName ?: "Unknown"

// Non-null assertion: throw NPE if null (use sparingly)
val forced = customerName!!

// ============================================
// OrderProcessor - First Kotlin Program
// Feature: Define order model and print summary
// ============================================

// Data class: auto-generates equals/hashCode/toString/copy
data class Order(val id: String, val total: Double, val status: String)

data class Customer(val name: String, val email: String?)

fun main() {
    // Create orders
    val order1 = Order("ORD-001", 299.99, "SHIPPED")
    val order2 = Order("ORD-002", 1_500.00, "PROCESSING")

    // Nullable customer
    val customer: Customer? = Customer("Alice", "alice@example.com")

    // Safe call + Elvis operator
    val email = customer?.email ?: "no-email"

    // String template
    println("Order: ${order1.id}, Total: \$${order1.total} USD, Status: ${order1.status}")
    println("Customer email: $email")

    // when expression (like switch but more powerful)
    val priority = when {
        order2.total > 1_000 -> "HIGH"
        order2.total > 500 -> "MEDIUM"
        else -> "LOW"
    }
    println("Order ${order2.id} priority: $priority")

    // Smart cast
    val result: Any = order1
    if (result is Order) {
        // No explicit cast needed!
        println("Smart cast: ${result.id} - ${result.total} USD")
    }
}

Order: ORD-001, Total: $299.99 USD, Status: SHIPPED
Customer email: alice@example.com
Order ORD-002 priority: HIGH
Smart cast: ORD-001 - 299.99 USD

fun main() {
    println("Hello, Kotlin!")
}

Hello, Kotlin!

val name = "Alice"
var age = 30
fun greet(n: String): String = "Hello, $n!"
println(greet(name))

Hello, Alice!

data class User(val name: String, val age: Int)
val alice = User("Alice", 30)
println(alice)

User(name=Alice, age=30)