类与 OOP

Kotlin 的 OOP 更简洁：data class 自动生成 equals/hashCode/toString，sealed class 表达受限继承，object 单例，companion object 替代 static。

学习目标

• 掌握 class、属性、构造函数与 data class
• 理解 sealed class、object、companion object
• 了解泛型与接口默认实现

类与属性

class Track(
    val slug: String,
    var title: String,
    val lessons: MutableList<Lesson> = mutableListOf()
) {
    fun addLesson(lesson: Lesson) {
        lessons.add(lesson)
    }

    val lessonCount: Int
        get() = lessons.size
}

data class Lesson(val slug: String, val title: String, val order: Int)

fun main() {
    val track = Track("kotlin-essentials", "Kotlin 语言入门")
    track.addLesson(Lesson("kt-intro", "Kotlin 是什么", 1))
    track.addLesson(Lesson("kt-syntax", "基础语法", 2))

    println("${track.title}: ${track.lessonCount} 课")
    println(track.lessons)
}

主构造函数参数可直接声明为属性；data class 适合 DTO、实体。

sealed class 与 object

sealed class Result<out T> {
    data class Success<T>(val data: T) : Result<T>()
    data class Error(val message: String) : Result<Nothing>()
    object Loading : Result<Nothing>()
}

object TrackRegistry {
    private val tracks = mutableMapOf<String, Track>()

    fun register(track: Track) {
        tracks[track.slug] = track
    }

    fun get(slug: String): Track? = tracks[slug]
}

fun handleResult(result: Result<List<Lesson>>) = when (result) {
    is Result.Success -> println("加载 ${result.data.size} 课")
    is Result.Error   -> println("错误: ${result.message}")
    Result.Loading    -> println("加载中...")
}

sealed class 子类受限，配合 when 可 exhaustiveness check；object 是线程安全单例。

接口与 companion

interface Repository {
    fun findBySlug(slug: String): Track?

    fun findAll(): List<Track> = emptyList()   // 默认实现
}

class InMemoryRepository : Repository {
    companion object {
        fun createDefault(): InMemoryRepository = InMemoryRepository()
    }

    private val store = mutableMapOf<String, Track>()

    override fun findBySlug(slug: String): Track? = store[slug]
}

fun main() {
    val repo = InMemoryRepository.createDefault()
    println(repo.findBySlug("kotlin-essentials"))
}