Kotlinにおける型変換の方法と注意点を徹底解説

val obj: Any = "Hello, Kotlin"
val str: String? = obj as? String  // 成功すれば"Hello, Kotlin"、失敗すればnull

fun safeCastExample(input: Any) {
    val number: Int? = input as? Int
    if (number != null) {
        println("Number is $number")
    } else {
        println("Input is not an Int")
    }
}

fun main() {
    safeCastExample(42)       // 出力: Number is 42
    safeCastExample("Kotlin") // 出力: Input is not an Int
}

val obj: Any = "Hello, Kotlin"
val str: String = obj as String  // 成功すれば"Hello, Kotlin"、失敗すれば例外発生

val obj: Any = 123
val str: String = obj as String  // ClassCastExceptionが発生

fun processString(input: Any) {
    try {
        val str: String = input as String
        println("Input string: $str")
    } catch (e: ClassCastException) {
        println("Error: Input is not a String")
    }
}

fun main() {
    processString("Hello, World!")  // 出力: Input string: Hello, World!
    processString(123)              // 出力: Error: Input is not a String
}

if (obj is String) {
    println("objはString型です")
}

fun printLength(obj: Any) {
    if (obj is String) {
        // スマートキャストによりobjがStringとして扱われる
        println("Stringの長さ: ${obj.length}")
    }
}

fun main() {
    printLength("Kotlin")  // 出力: Stringの長さ: 6
    printLength(123)       // 出力なし
}

class Example(var obj: Any) {
    fun check() {
        if (obj is String) {
            // objはスマートキャストされない
            // println(obj.length) はコンパイルエラー
        }
    }
}

fun checkType(obj: Any) {
    when (obj) {
        is String -> println("Stringの長さ: ${obj.length}")
        is Int -> println("Intの値: $obj")
        else -> println("不明な型")
    }
}

fun main() {
    checkType("Hello")  // 出力: Stringの長さ: 5
    checkType(42)       // 出力: Intの値: 42
}

val obj: Any = 42
val str: String = obj as String  // ClassCastExceptionが発生

val obj: Any = 42
val str: String? = obj as? String  // nullが代入される

val obj: Any = 123
val str: String? = obj as? String
println(str.length)  // NullPointerExceptionが発生

val obj: Any = 123
val str: String? = obj as? String
if (str != null) {
    println(str.length)
} else {
    println("strはnullです")
}

val obj: Any = "Kotlin"
if (obj is String) {
    println("String型です: ${obj.length}")
} else {
    println("String型ではありません")
}

val obj: Any = 123
println("objの型: ${obj::class.simpleName}")

val obj: Any = 123
val str: String = obj as? String ?: "デフォルト値"
println(str)  // 出力: デフォルト値

val intList: List<Int> = listOf(1, 2, 3, 4)
val stringList: List<String> = intList.map { it.toString() }

println(stringList)  // 出力: [1, 2, 3, 4]

val mixedList: List<Any> = listOf(1, "Kotlin", 3, "Programming", 5)
val stringList: List<String> = mixedList.filterIsInstance<String>()

println(stringList)  // 出力: [Kotlin, Programming]

val originalMap = mapOf(1 to "One", 2 to "Two")
val newMap = originalMap.mapKeys { it.key.toString() }

println(newMap)  // 出力: {1=One, 2=Two}

val originalMap = mapOf("One" to 1, "Two" to 2)
val newMap = originalMap.mapValues { it.value * 10 }

println(newMap)  // 出力: {One=10, Two=20}

val numbers = arrayOf(1, 2, 3, 4)
val stringArray = numbers.map { it.toString() }.toTypedArray()

println(stringArray.joinToString())  // 出力: 1, 2, 3, 4

val anyList: List<Any> = listOf(1, "Two", 3, "Four")
val intList: List<Int?> = anyList.map { it as? Int }

println(intList)  // 出力: [1, null, 3, null]

class Box<T>(val value: T)

fun <T> printValue(value: T) {
    println(value)
}

fun main() {
    val intBox = Box(42)
    val stringBox = Box("Kotlin")

    printValue(intBox.value)      // 出力: 42
    printValue(stringBox.value)   // 出力: Kotlin
}

fun <T> checkType(value: T) {
    if (value is List<String>) {  // コンパイルエラー: 不正な型チェック
        println("This is a List of Strings")
    }
}

fun checkList(value: Any) {
    if (value is List<*>) {
        if (value.all { it is String }) {
            println("This is a List of Strings")
        }
    }
}

val list: List<Any> = listOf("Hello", "World")
val stringList = list as List<String>  // 警告: 不正なキャストの可能性

println(stringList[0].length)  // 実行時にClassCastExceptionが発生する可能性

val stringList = list as? List<String>
if (stringList != null) {
    println(stringList[0].length)
} else {
    println("キャストに失敗しました")
}

fun <T : Number> addNumbers(a: T, b: T): Double {
    return a.toDouble() + b.toDouble()
}

fun main() {
    println(addNumbers(4, 5))           // 出力: 9.0
    println(addNumbers(4.5, 3.2))       // 出力: 7.7
    // addNumbers("4", "5")             // コンパイルエラー
}

interface Producer<out T> {
    fun produce(): T
}

interface Consumer<in T> {
    fun consume(item: T)
}

import org.json.JSONObject

fun parseJsonResponse(response: String) {
    val json = JSONObject(response)
    val name = json.optString("name")
    val age = json.optInt("age")
    val skills = json.optJSONArray("skills")

    if (skills != null) {
        for (i in 0 until skills.length()) {
            val skill = skills.get(i) as? String
            if (skill != null) {
                println("Skill: $skill")
            }
        }
    }

    println("Name: $name, Age: $age")
}

fun main() {
    val jsonResponse = """
        {
            "name": "John Doe",
            "age": 30,
            "skills": ["Kotlin", "Java", "JavaScript"]
        }
    """.trimIndent()

    parseJsonResponse(jsonResponse)
}

Skill: Kotlin  
Skill: Java  
Skill: JavaScript  
Name: John Doe, Age: 30

sealed class ListItem {
    data class Header(val title: String) : ListItem()
    data class Content(val description: String) : ListItem()
}

class MyAdapter(private val items: List<ListItem>) : RecyclerView.Adapter<RecyclerView.ViewHolder>() {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
        return if (viewType == 0) {
            HeaderViewHolder(LayoutInflater.from(parent.context).inflate(R.layout.header_item, parent, false))
        } else {
            ContentViewHolder(LayoutInflater.from(parent.context).inflate(R.layout.content_item, parent, false))
        }
    }

    override fun getItemViewType(position: Int): Int {
        return if (items[position] is ListItem.Header) 0 else 1
    }

    override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
        when (val item = items[position]) {
            is ListItem.Header -> (holder as? HeaderViewHolder)?.bind(item)
            is ListItem.Content -> (holder as? ContentViewHolder)?.bind(item)
        }
    }

    override fun getItemCount() = items.size
}

fun <T> convertAndPrint(value: Any) {
    when (value) {
        is T -> println("Converted value: $value")
        else -> println("Value is not of the expected type")
    }
}

fun main() {
    convertAndPrint<String>("Hello, World!")  // 出力: Converted value: Hello, World!
    convertAndPrint<Int>("Not an Int")        // 出力: Value is not of the expected type
}

fun handleDbResult(result: Any) {
    val rows = result as? List<Map<String, Any>> ?: return

    for (row in rows) {
        val id = row["id"] as? Int ?: continue
        val name = row["name"] as? String ?: continue
        println("ID: $id, Name: $name")
    }
}

fun main() {
    val dbResult: Any = listOf(
        mapOf("id" to 1, "name" to "Alice"),
        mapOf("id" to 2, "name" to "Bob")
    )

    handleDbResult(dbResult)
}

ID: 1, Name: Alice  
ID: 2, Name: Bob