集合 API

Kotlin 标准库对集合提供了丰富的扩展函数：filter、map、fold、groupBy 等链式调用，比 Java Stream 更简洁，且区分可变与不可变集合。

学习目标

• 掌握 List、Set、Map 的创建与基本操作
• 熟练使用 filter、map、fold、groupBy 等函数式 API
• 理解只读集合与可变集合的区别

创建与基本操作

fun main() {
    // 只读 List
    val slugs = listOf("kt-intro", "kt-syntax", "kt-control")
    println(slugs[0])
    println(slugs.size)

    // 可变 List
    val mutable = mutableListOf<String>()
    mutable.add("kt-functions")
    mutable.removeAt(0)

    // Set 去重
    val tags = setOf("kotlin", "jvm", "kotlin")
    println(tags)   // [kotlin, jvm]

    // Map
    val lessonCount = mapOf(
        "kotlin-essentials" to 12,
        "java-fundamentals" to 14
    )
    println(lessonCount["kotlin-essentials"])
    println(lessonCount.getOrDefault("missing", 0))
}

listOf/setOf/mapOf 返回只读视图；mutableListOf 等可修改。

函数式变换

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

fun main() {
    val lessons = listOf(
        Lesson("kt-intro", "Kotlin 是什么", 1),
        Lesson("kt-syntax", "基础语法", 2),
        Lesson("kt-control", "控制流", 3),
        Lesson("kt-functions", "函数与 Lambda", 4)
    )

    val titles = lessons
        .filter { it.order <= 3 }
        .map { it.title }
    println(titles)

    val byPrefix = lessons.groupBy { it.slug.substringBefore("-") }
    println(byPrefix.keys)

    val totalOrders = lessons.fold(0) { acc, l -> acc + l.order }
    println("序号总和: $totalOrders")

    val sorted = lessons.sortedBy { it.order }
    println(sorted.map { "${it.order}. ${it.title}" })
}

链式调用可读性高；groupBy 按 key 分组，fold 聚合。

遍历与解构

fun main() {
    val counts = mapOf("kt-intro" to 1, "kt-syntax" to 2)

    for ((slug, order) in counts) {
        println("$slug -> $order")
    }

    val slugs = listOf("a", "b", "c")
    slugs.forEachIndexed { index, slug ->
        println("${index + 1}. $slug")
    }

    // 序列：惰性求值，适合大集合
    val result = (1..1_000_000)
        .asSequence()
        .filter { it % 2 == 0 }
        .take(5)
        .toList()
    println(result)
}