Kotlin 集合函数map 和 first 的使用场景分析

data class User(val id: Int, val name: String)
val users = listOf(
    User(1, "Alice"),
    User(2, "Bob"),
    User(3, "Charlie")
)
val names = users.map { it.name }
println(names) // 输出: [Alice, Bob, Charlie]

val numbers = listOf(1, 2, 3, 4, 5)
val stringNumbers = numbers.map { "Number: $it" }
println(stringNumbers) // 输出: [Number: 1, Number: 2, Number: 3, Number: 4, Number: 5]

val numbers = listOf(10, 20, 30, 40, 50)
val firstEven = numbers.first { it % 2 == 0 }
println(firstEven) // 输出: 10

val emptyList = listOf<Int>()
val firstOrNull = emptyList.firstOrNull()
println(firstOrNull) // 输出: null

val numbers = listOf(1, 2, 3, 4, 5)
val evenNumbers = numbers.filter { it % 2 == 0 }
println(evenNumbers) // 输出: [2, 4]

val numbers = listOf(1, 2, 3, 4, 5)
val sum = numbers.reduce { acc, num -> acc + num }
println(sum) // 输出: 15

val numbers = listOf(1, 2, 3, 4, 5)
val sumWithInitial = numbers.fold(10) { acc, num -> acc + num }
println(sumWithInitial) // 输出: 25

data class User(val name: String, val age: Int)
val users = listOf(
    User("Alice", 30),
    User("Bob", 25),
    User("Charlie", 28)
)
val sortedUsers = users.sortedBy { it.age }
println(sortedUsers) // 输出: [User(name=Bob, age=25), User(name=Charlie, age=28), User(name=Alice, age=30)]

val list = listOf("A", "B", "C")
val result = list.joinToString(", ")  // 用 ", " 连接元素
println(result)  
// 输出: A, B, C

val numbers = listOf(1, 2, 3, 4)
println(numbers.joinToString(" - "))  
// 输出: 1 - 2 - 3 - 4

val words = listOf("Kotlin", "Java", "Python")
println(words.joinToString(", ", prefix = "[", postfix = "]"))
// 输出: [Kotlin, Java, Python]

val letters = listOf("A", "B", "C", "D", "E")
println(letters.joinToString(", ", limit = 3, truncated = "..."))
// 输出: A, B, C, ...

data class Person(val name: String, val age: Int)
val people = listOf(Person("Alice", 25), Person("Bob", 30))
println(people.joinToString { "${it.name} (${it.age})" })
// 输出: Alice (25), Bob (30)