KotlinでNull可能なMap要素を安全に取得する方法を徹底解説

val nonNullable: String = "Hello"  // 非Null型
val nullable: String? = null       // Null可能型

val map: Map<String, String?> = mapOf("key1" to "value1", "key2" to null)

// 安全呼び出し
val result1 = map["key1"]?.length    // result1は4になる
val result2 = map["key2"]?.length    // result2はnullになる

// Elvis演算子でデフォルト値を指定
val result3 = map["key2"]?.length ?: 0  // result3は0になる

val map = mapOf("key1" to "value1", "key2" to "value2")

val nullableMap: Map<String, String?> = mapOf(
    "name" to "John",
    "nickname" to null
)

val mapWithNullKey: Map<String?, String> = mapOf(
    null to "No Key",
    "key1" to "Value1"
)

val map: Map<String, String?> = mapOf("key1" to "value1", "key2" to null)

// Null安全に取得
val value = map["key2"] ?: "default value"  // "default value"が返る

val map = mapOf("name" to "Alice", "age" to "25", "city" to null)

val name = map.get("name")  // "Alice"
val city = map.get("city")  // null
val country = map.get("country")  // null（キーが存在しない）

map.getOrElse(key) { defaultValue }

val map = mapOf("name" to "Alice", "age" to "25")

val name = map.getOrElse("name") { "Unknown" }       // "Alice"
val country = map.getOrElse("country") { "Unknown" } // "Unknown"（デフォルト値）

val scores = mapOf("Math" to 85, "English" to null)

val mathScore = scores.getOrElse("Math") { 0 }       // 85
val englishScore = scores.getOrElse("English") { 0 } // 0（Nullの場合のデフォルト値）

map.getOrDefault(key, defaultValue)

val userMap = mapOf("name" to "Alice", "age" to "25")

val name = userMap.getOrDefault("name", "Unknown")       // "Alice"
val country = userMap.getOrDefault("country", "Unknown") // "Unknown"（デフォルト値）

val scores = mapOf("Math" to 85, "English" to null)

val mathScore = scores.getOrDefault("Math", 0)        // 85
val englishScore = scores.getOrDefault("English", 0)  // 0（`null`の代わりにデフォルト値）
val scienceScore = scores.getOrDefault("Science", 0)  // 0（キーが存在しないためデフォルト値）

val map = mapOf("key1" to 100)

val defaultWithGetOrDefault = map.getOrDefault("key2", 0) // 0（固定値のデフォルト）

val defaultWithGetOrElse = map.getOrElse("key2") { 
    calculateDefaultValue()  // 動的にデフォルト値を計算
}

val result = nullableValue ?: defaultValue

val userMap = mapOf("name" to "Alice", "city" to null)

val name = userMap["name"] ?: "Unknown"       // "Alice"
val city = userMap["city"] ?: "No City"       // "No City"（値がnullの場合のデフォルト）
val country = userMap["country"] ?: "Unknown" // "Unknown"（キーが存在しない場合のデフォルト）

fun getUsername(): String? {
    return null  // ユーザー名が取得できない場合
}

val username = getUsername() ?: "Guest"  // "Guest"が返る

val preferredName = userMap["nickname"] ?: userMap["username"] ?: "DefaultUser"

val userId = userMap["id"] ?: throw IllegalArgumentException("User ID is required")

val result = requireNotNull(value) { "エラーメッセージ" }

fun processUserName(name: String?) {
    val validName = requireNotNull(name) { "Name cannot be null" }
    println("Hello, $validName")
}

processUserName("Alice")  // 出力: Hello, Alice
processUserName(null)     // IllegalArgumentException: Name cannot be null

val result = checkNotNull(value) { "エラーメッセージ" }

fun fetchData(data: String?) {
    val validData = checkNotNull(data) { "Data must not be null" }
    println("Data: $validData")
}

fetchData("Some data")  // 出力: Data: Some data
fetchData(null)         // IllegalStateException: Data must not be null

val userMap = mapOf("id" to "123", "name" to "Alice")

fun getUserId(map: Map<String, String?>): String {
    return requireNotNull(map["id"]) { "User ID is missing" }
}

fun getUserName(map: Map<String, String?>): String {
    return checkNotNull(map["name"]) { "User name cannot be null" }
}

println(getUserId(userMap))   // 出力: 123
println(getUserName(userMap)) // 出力: Alice

val userMap = mapOf("name" to "Alice", "city" to "Tokyo", "country" to null)

for ((key, value) in userMap) {
    println("$key: $value")
}

name: Alice  
city: Tokyo  
country: null  

for ((key, value) in userMap) {
    println("$key: ${value?.uppercase()}")
}

for ((key, value) in userMap) {
    val safeValue = value ?: "Unknown"
    println("$key: $safeValue")
}

name: Alice  
city: Tokyo  
country: Unknown  

val filteredMap = userMap.filter { it.value != null }

for ((key, value) in filteredMap) {
    println("$key: $value")
}

name: Alice  
city: Tokyo  

for (key in userMap.keys) {
    println("Key: $key")
}

for (value in userMap.values) {
    println("Value: ${value ?: "Unknown"}")
}

val upperCaseMap = userMap.mapValues { it.value?.uppercase() ?: "Unknown" }

for ((key, value) in upperCaseMap) {
    println("$key: $value")
}

name: ALICE  
city: TOKYO  
country: Unknown  

val userProfile = mapOf(
    "name" to "Alice",
    "email" to null,
    "city" to "Tokyo"
)

fun getUserInfo(profile: Map<String, String?>) {
    val name = profile["name"] ?: "Unknown Name"
    val email = profile["email"] ?: "Email not provided"
    val city = profile["city"] ?: "Unknown City"

    println("Name: $name")
    println("Email: $email")
    println("City: $city")
}

getUserInfo(userProfile)

Name: Alice  
Email: Email not provided  
City: Tokyo  

val inventory = mapOf(
    "Apple" to 50,
    "Banana" to null,
    "Orange" to 20
)

fun checkStock(item: String, stockMap: Map<String, Int?>) {
    val stock = stockMap[item] ?: 0
    println("$item stock: $stock")
}

checkStock("Apple", inventory)   // 出力: Apple stock: 50
checkStock("Banana", inventory)  // 出力: Banana stock: 0
checkStock("Grapes", inventory)  // 出力: Grapes stock: 0

val apiResponse = mapOf(
    "status" to "success",
    "message" to null,
    "data" to "User data loaded"
)

fun processApiResponse(response: Map<String, String?>) {
    val status = response["status"] ?: "unknown"
    val message = response["message"] ?: "No message available"
    val data = response["data"] ?: "No data"

    println("Status: $status")
    println("Message: $message")
    println("Data: $data")
}

processApiResponse(apiResponse)

Status: success  
Message: No message available  
Data: User data loaded  

val rawScores = mapOf(
    "Math" to 85,
    "English" to null,
    "Science" to 90
)

val safeScores = rawScores.mapValues { it.value ?: 0 }
println(safeScores)

// 合計スコアを計算
val totalScore = safeScores.values.sum()
println("Total Score: $totalScore")

{Math=85, English=0, Science=90}  
Total Score: 175