Kotlin 数据类 #

data class User(val id: Int, val name: String, val email: String)

val user = User(1, "Kotlin", "kotlin@example.com")
println(user)  // User(id=1, name=Kotlin, email=kotlin@example.com)

// 正确
data class User(val id: Int, val name: String)

// 错误
// data class Invalid(id: Int)  // 参数没有 val/var
// abstract data class AbstractUser(val id: Int)  // 不能是 abstract

data class User(val id: Int, val name: String)

val user1 = User(1, "Kotlin")
val user2 = User(1, "Kotlin")

println(user1 == user2)  // true（内容相等）
println(user1.hashCode() == user2.hashCode())  // true

data class User(val id: Int, val name: String)

val user = User(1, "Kotlin")
println(user)  // User(id=1, name=Kotlin)

data class User(val id: Int, val name: String, val email: String)

val user = User(1, "Kotlin", "kotlin@example.com")

// 复制并修改部分属性
val updated = user.copy(name = "Updated")
// User(id=1, name=Updated, email=kotlin@example.com)

// 完全复制
val same = user.copy()

data class User(val id: Int, val name: String)

val user = User(1, "Kotlin")

// 通过 componentN 访问
val id = user.component1()    // 1
val name = user.component2()  // "Kotlin"

data class User(val id: Int, val name: String)

val user = User(1, "Kotlin")
val (id, name) = user

println("ID: $id, Name: $name")

data class User(val id: Int, val name: String)

val users = listOf(
    User(1, "Alice"),
    User(2, "Bob"),
    User(3, "Charlie")
)

for ((id, name) in users) {
    println("$id: $name")
}

val map = mapOf("A" to 1, "B" to 2, "C" to 3)

for ((key, value) in map) {
    println("$key -> $value")
}

data class User(val id: Int, val name: String, val email: String)

val user = User(1, "Kotlin", "kotlin@example.com")

val (id, _, _) = user  // 使用 _ 忽略
println("ID: $id")

data class User(val id: Int, val name: String)

val users = listOf(
    User(1, "Alice"),
    User(2, "Bob"),
    User(1, "Alice")  // 重复
)

// 去重
val unique = users.distinct()
// [User(1, Alice), User(2, Bob)]

data class User(val id: Int, val name: String)

val users = setOf(
    User(1, "Alice"),
    User(2, "Bob"),
    User(1, "Alice")  // 自动去重
)

println(users.size)  // 2

data class Key(val id: Int, val type: String)

val map = mapOf(
    Key(1, "A") to "Value1",
    Key(2, "B") to "Value2"
)

map[Key(1, "A")]  // "Value1"

open class BaseEntity(val createdAt: Long)

data class User(
    val id: Int,
    val name: String
) : BaseEntity(System.currentTimeMillis())

interface Identifiable {
    val id: Int
}

data class User(
    override val id: Int,
    val name: String
) : Identifiable

sealed class Result {
    data class Success(val data: String) : Result()
    data class Error(val message: String) : Result()
    object Loading : Result()
}

fun handle(result: Result) = when (result) {
    is Result.Success -> println("Data: ${result.data}")
    is Result.Error -> println("Error: ${result.message}")
    Result.Loading -> println("Loading...")
}

data class ApiResponse<T>(
    val success: Boolean,
    val data: T?,
    val error: String?,
    val timestamp: Long = System.currentTimeMillis()
)

val response = ApiResponse(
    success = true,
    data = User(1, "Kotlin"),
    error = null
)

data class DatabaseConfig(
    val host: String = "localhost",
    val port: Int = 5432,
    val name: String = "mydb",
    val user: String = "admin",
    val password: String = ""
)

val config = DatabaseConfig(
    host = "production-db",
    password = "secret"
)

data class Point(val x: Int, val y: Int) {
    fun move(dx: Int, dy: Int): Point {
        return copy(x = x + dx, y = y + dy)
    }
    
    fun distanceTo(other: Point): Double {
        val dx = x - other.x
        val dy = y - other.y
        return Math.sqrt((dx * dx + dy * dy).toDouble())
    }
}

val p1 = Point(0, 0)
val p2 = p1.move(3, 4)
println(p1.distanceTo(p2))  // 5.0

data class DateRange(
    val start: LocalDate,
    val end: LocalDate
) {
    fun contains(date: LocalDate): Boolean {
        return date in start..end
    }
    
    val days: Long
        get() = ChronoUnit.DAYS.between(start, end) + 1
}

// 数据类
data class User(val id: Int, val name: String)

// 等价的普通类
class NormalUser(val id: Int, val name: String) {
    override fun equals(other: Any?): Boolean {
        if (this === other) return true
        if (other !is NormalUser) return false
        return id == other.id && name == other.name
    }
    
    override fun hashCode(): Int {
        var result = id
        result = 31 * result + name.hashCode()
        return result
    }
    
    override fun toString(): String {
        return "NormalUser(id=$id, name=$name)"
    }
    
    fun copy(id: Int = this.id, name: String = this.name): NormalUser {
        return NormalUser(id, name)
    }
}

// 推荐
data class User(val id: Int, val name: String)

// 不推荐
data class MutableUser(var id: Int, var name: String)

data class User(val id: Int, val name: String)

val user = User(1, "Kotlin")

// 推荐：使用 copy
val updated = user.copy(name = "Updated")

// 不推荐：使用 var
// user.name = "Updated"

// 推荐：包含所有需要比较的属性
data class User(val id: Int, val name: String, val email: String)

// 不推荐：包含不需要比较的属性
data class UserWithTimestamp(
    val id: Int,
    val name: String,
    val timestamp: Long  // 每次创建都不同，影响相等性比较
)