/**
 * 第 5 章 · Null 安全 — 综合示例
 *
 * 包含：
 *   - 可空类型与非空类型
 *   - ?. 安全调用
 *   - ?: Elvis 操作符
 *   - !! 非空断言（演示其危险性）
 *   - as? 安全转换
 *   - ?.let { } 链式
 *   - 集合的可空处理
 *   - 平台类型陷阱
 */

data class Address(val city: String?, val zipcode: String?)
data class User(val name: String, val nickname: String?, val address: Address?)

fun main() {
    println("=== 1. 可空 vs 非空 ===")
    val name: String = "Alice"
    val nick: String? = null
    println("name = $name (非空)")
    println("nick = $nick (可空)")
    // val s: String = nick   // ❌ 编译报错：Type mismatch

    println("\n=== 2. ?. 安全调用 ===")
    println("nick?.length = ${nick?.length}")             // null
    println("name?.length = ${name?.length}")             // 5

    println("\n=== 3. ?: Elvis 默认值 ===")
    val displayNick = nick ?: "(无昵称)"
    println("displayNick = $displayNick")
    val len = nick?.length ?: 0
    println("len = $len")

    println("\n=== 4. !! 危险演示 ===")
    val safeName: String? = "Bob"
    println("safeName!!.length = ${safeName!!.length}")   // 5

    try {
        val nullName: String? = null
        println("nullName!!.length = ${nullName!!.length}")
    } catch (e: NullPointerException) {
        println("⚠️ 捕获到 NPE: ${e.message ?: "no message"}")
        println("   → !! 是自杀式操作符，能不用就别用！")
    }

    println("\n=== 5. as? 安全转换 ===")
    val obj: Any = "hello"
    val asInt: Int? = obj as? Int           // null
    val asString: String? = obj as? String  // "hello"
    println("obj as? Int = $asInt")
    println("obj as? String = $asString")
    val n: Int = (obj as? Int) ?: -1
    println("用默认值: $n")

    println("\n=== 6. Smart Cast ===")
    fun describe(x: Any?) = when {
        x == null -> "null"
        x is String -> "字符串，长度 ${x.length}"   // 自动当 String 用
        x is Int -> "整数，平方 ${x * x}"
        else -> "其他类型 ${x::class.simpleName}"
    }
    listOf(null, "hello", 5, 3.14).forEach {
        println("${it} → ${describe(it)}")
    }

    println("\n=== 7. ?.let { } 链式 ===")
    val users = listOf(
        User("Alice", "alice123", Address("Beijing", "100000")),
        User("Bob", null, Address("Shanghai", null)),
        User("Charlie", "charlie", null),
    )

    users.forEach { user ->
        // 老 Java 风格
        if (user.address != null && user.address.city != null) {
            // ↑ 这里 smart cast 不一定成功（开放属性问题），实际可能要本地变量
            // println("城市: ${user.address.city}")
        }
        // Kotlin 风格
        user.address?.city?.let { city ->
            println("${user.name} 住在 $city")
        } ?: println("${user.name} 没填城市")
    }

    println("\n=== 8. 集合的 null 处理 ===")
    val mixed: List<String?> = listOf("a", null, "b", null, "c")
    println("原始: $mixed")
    println("过滤掉 null: ${mixed.filterNotNull()}")
    println("用空字符串代替 null: ${mixed.map { it ?: "" }}")
    println("非 null 元素长度之和: ${mixed.sumOf { it?.length ?: 0 }}")

    println("\n=== 9. requireNotNull / checkNotNull ===")
    fun process(input: String?) {
        val name = requireNotNull(input) { "input 不能为空" }
        println("处理: $name (长度 ${name.length})")
    }
    process("hello")
    try {
        process(null)
    } catch (e: IllegalArgumentException) {
        println("⚠️ 捕获: ${e.message}")
    }

    println("\n=== 10. 实战：解析嵌套结构 ===")
    val u = User("David", null, Address(null, null))
    val info = buildString {
        append("姓名：${u.name}\n")
        append("昵称：${u.nickname ?: "(未设置)"}\n")
        append("城市：${u.address?.city ?: "(未填写)"}\n")
        append("邮编：${u.address?.zipcode ?: "(未填写)"}")
    }
    println(info)
}