📚 Kotlin 常用集合操作符速查表

// map 示例 val numbers = listOf(1, 2, 3) val doubled = numbers.map { it * 2 } println(doubled) // [2, 4, 6] // mapNotNull 示例 val strings = listOf("1", "a", "3", null, "5") val ints = strings.mapNotNull { it?.toIntOrNull() } println(ints) // [1, 3, 5] // mapIndexed 示例 val indexed = numbers.mapIndexed { index, n -> "索引$index = $n" } println(indexed) // [索引0 = 1, 索引1 = 2, 索引2 = 3] // flatMap 示例 val nested = listOf(listOf(1, 2), listOf(3, 4), listOf(5, 6)) val flattened = nested.flatMap { it } println(flattened) // [1, 2, 3, 4, 5, 6] // flatMap 用于展开嵌套结构 val words = listOf("hello", "world") val chars = words.flatMap { it.toList() } println(chars) // [h, e, l, l, o, w, o, r, l, d]

// filter 示例 val numbers = listOf(1, 2, 3, 4, 5) val evens = numbers.filter { it % 2 == 0 } println(evens) // [2, 4] val odds = numbers.filterNot { it % 2 == 0 } println(odds) // [1, 3, 5] // find 示例 val firstGreaterThan3 = numbers.find { it > 3 } println(firstGreaterThan3) // 4 val firstGreaterThan10 = numbers.find { it > 10 } println(firstGreaterThan10) // null // findLast 示例 val lastGreaterThan3 = numbers.findLast { it > 3 } println(lastGreaterThan3) // 5

// groupBy 示例 data class Person(val name: String, val age: Int) val people = listOf( Person("Alice", 25), Person("Bob", 30), Person("Charlie", 25), Person("David", 30) ) // 按年龄分组 val byAge = people.groupBy { it.age } println(byAge) // {25=[Person(Alice, 25), Person(Charlie, 25)], 30=[Person(Bob, 30), Person(David, 30)]} // 按首字母分组 val byFirstLetter = people.groupBy { it.name.first() } println(byFirstLetter)

val numbers = listOf(3, 1, 4, 1, 5, 9, 2, 6) // sorted 示例 val sorted = numbers.sorted() println(sorted) // [1, 1, 2, 3, 4, 5, 6, 9] val sortedDesc = numbers.sortedDescending() println(sortedDesc) // [9, 6, 5, 4, 3, 2, 1, 1] // sortedBy 示例 data class Product(val name: String, val price: Double) val products = listOf( Product("Apple", 2.5), Product("Banana", 1.5), Product("Cherry", 5.0) ) val byPrice = products.sortedBy { it.price } println(byPrice) // [Banana, Apple, Cherry] // sortedWith 示例 - 多字段排序 val byPriceThenName = products.sortedWith( compareBy<Product> { it.price }.thenBy { it.name } )

// distinct 示例 val numbers = listOf(1, 2, 2, 3, 3, 3, 4) val unique = numbers.distinct() println(unique) // [1, 2, 3, 4] // distinctBy 示例 data class User(val id: Int, val name: String, val email: String) val users = listOf( User(1, "Alice", "alice@example.com"), User(2, "Bob", "bob@example.com"), User(3, "Alice", "alice2@example.com") ) // 按名字去重，保留第一个 val uniqueByName = users.distinctBy { it.name } println(uniqueByName) // [User(1, Alice, ...), User(2, Bob, ...)]

data class Product(val name: String, val price: Double) val products = listOf( Product("Apple", 2.5), Product("Banana", 1.5), Product("Cherry", 5.0) ) // minBy/maxBy 返回元素 val cheapest = products.minBy { it.price } println(cheapest) // Product(Banana, 1.5) val mostExpensive = products.maxBy { it.price } println(mostExpensive) // Product(Cherry, 5.0) // minOf/maxOf 返回属性值 val minPrice = products.minOf { it.price } println(minPrice) // 1.5 val maxPrice = products.maxOf { it.price } println(maxPrice) // 5.0

// associateBy 示例 - 以 id 为键 data class Student(val id: Int, val name: String, val score: Int) val students = listOf( Student(1, "Alice", 85), Student(2, "Bob", 90), Student(3, "Charlie", 78) ) val studentMap = students.associateBy { it.id } println(studentMap) // {1=Student(1, Alice, 85), 2=Student(2, Bob, 90), 3=Student(3, Charlie, 78)} // 通过 id 快速查找 val student = studentMap[2] println(student?.name) // Bob // associate 示例 - 自定义键值对 val nameToScore = students.associate { it.name to it.score } println(nameToScore) // {Alice=85, Bob=90, Charlie=78} // associateWith 示例 val names = listOf("Alice", "Bob", "Charlie") val nameLengths = names.associateWith { it.length } println(nameLengths) // {Alice=5, Bob=3, Charlie=7}

val numbers = listOf(1, 2, 3, 4, 5) // first/last 示例 println(numbers.first()) // 1 println(numbers.last()) // 5 // 带条件的 first println(numbers.first { it > 3 }) // 4 // firstOrNull 安全获取 val empty = emptyList<Int>() println(empty.firstOrNull()) // null // single 示例 - 集合必须只有一个元素 val single = listOf(42) println(single.single()) // 42 // singleOrNull 示例 println(listOf(42).singleOrNull()) // 42 println(listOf(1, 2).singleOrNull()) // null println(emptyList<Int>().singleOrNull()) // null

val numbers = listOf(1, 2, 3, 4, 5) // any 示例 val hasEven = numbers.any { it % 2 == 0 } println(hasEven) // true val hasNegative = numbers.any { it < 0 } println(hasNegative) // false // all 示例 val allPositive = numbers.all { it > 0 } println(allPositive) // true val allEven = numbers.all { it % 2 == 0 } println(allEven) // false // none 示例 val noNegative = numbers.none { it < 0 } println(noNegative) // true // count 示例 val evenCount = numbers.count { it % 2 == 0 } println(evenCount) // 2

val numbers = listOf(1, 2, 3, 4, 5) // 基本用法 val joined = numbers.joinToString() println(joined) // 1, 2, 3, 4, 5 // 自定义分隔符 val withDash = numbers.joinToString(" - ") println(withDash) // 1 - 2 - 3 - 4 - 5 // 完整参数 val formatted = numbers.joinToString( separator = ", ", // 分隔符 prefix = "[", // 前缀 postfix = "]", // 后缀 limit = 3, // 最多显示3个 truncated = "..." // 超出部分显示... ) println(formatted) // [1, 2, 3, ...] // 自定义转换 data class Person(val name: String, val age: Int) val people = listOf(Person("Alice", 25), Person("Bob", 30)) val names = people.joinToString( separator = " | ", transform = { it.name } ) println(names) // Alice | Bob

val numbers = listOf(1, 2, 3, 4, 5) // fold 示例 - 带初始值 val sum = numbers.fold(0) { accumulator, number -> accumulator + number } println(sum) // 15 // fold 用于字符串拼接 val joined = numbers.fold("Numbers: ") { acc, n -> "$acc$n " } println(joined) // Numbers: 1 2 3 4 5 // reduce 示例 - 不带初始值 val product = numbers.reduce { acc, n -> acc * n } println(product) // 120 (1*2*3*4*5) // runningFold - 返回中间结果 val runningSums = numbers.runningFold(0) { acc, n -> acc + n } println(runningSums) // [0, 1, 3, 6, 10, 15] // runningReduce val runningProducts = numbers.runningReduce { acc, n -> acc * n } println(runningProducts) // [1, 2, 6, 24, 120]

val numbers = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) // take 示例 val first3 = numbers.take(3) println(first3) // [1, 2, 3] val last3 = numbers.takeLast(3) println(last3) // [8, 9, 10] // takeWhile 示例 val lessThan5 = numbers.takeWhile { it < 5 } println(lessThan5) // [1, 2, 3, 4] // drop 示例 val skipFirst3 = numbers.drop(3) println(skipFirst3) // [4, 5, 6, 7, 8, 9, 10] // 组合使用 - 分页 val pageSize = 3 val page2 = numbers.drop(pageSize).take(pageSize) println(page2) // [4, 5, 6] (第2页)

// zip 示例 val numbers = listOf(1, 2, 3) val letters = listOf("a", "b", "c") val zipped = numbers.zip(letters) println(zipped) // [(1, a), (2, b), (3, c)] // 自定义合并方式 val combined = numbers.zip(letters) { n, l -> "$l$n" } println(combined) // [a1, b2, c3] // zipWithNext 示例 val adjacent = numbers.zipWithNext() println(adjacent) // [(1, 2), (2, 3)] // 用于计算差值 val diffs = numbers.zipWithNext { a, b -> b - a } println(diffs) // [1, 1] // unzip - 反向操作 val pairs = listOf(1 to "a", 2 to "b", 3 to "c") val (nums, strs) = pairs.unzip() println(nums) // [1, 2, 3] println(strs) // [a, b, c]

val numbers = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) // partition 示例 - 分成奇偶两组 val (evens, odds) = numbers.partition { it % 2 == 0 } println(evens) // [2, 4, 6, 8, 10] println(odds) // [1, 3, 5, 7, 9] // 实际应用 - 分离通过和未通过的学生 data class Student(val name: String, val score: Int) val students = listOf( Student("Alice", 85), Student("Bob", 45), Student("Charlie", 92), Student("David", 58), Student("Eve", 76) ) val (passed, failed) = students.partition { it.score >= 60 } println("通过的学生:") passed.forEach { println(" ${it.name}: ${it.score}") } println("未通过的学生:") failed.forEach { println(" ${it.name}: ${it.score}") }

data class Employee( val id: Int, val name: String, val department: String, val salary: Double, val yearsOfService: Int ) fun main() { val employees = listOf( Employee(1, "Alice", "Engineering", 80000.0, 5), Employee(2, "Bob", "Sales", 60000.0, 3), Employee(3, "Charlie", "Engineering", 90000.0, 7), Employee(4, "David", "HR", 50000.0, 2), Employee(5, "Eve", "Sales", 75000.0, 4), Employee(6, "Frank", "Engineering", 85000.0, 6) ) // 1. 找出工程部员工并按工资排序 val engineers = employees .filter { it.department == "Engineering" } .sortedByDescending { it.salary } println("工程部员工（按工资排序）: ${engineers.map { it.name }}") // 2. 按部门分组并计算平均工资 val avgSalaryByDept = employees .groupBy { it.department } .mapValues { (_, empList) -> empList.map { it.salary }.average() } println("各部门平均工资: $avgSalaryByDept") // 3. 找出工资最高的员工 val highestPaid = employees.maxBy { it.salary } println("工资最高: ${highestPaid.name} - $${highestPaid.salary}") // 4. 统计服务年限超过5年的员工 val seniorEmployees = employees.filter { it.yearsOfService > 5 } println("资深员工数量: ${seniorEmployees.count()}") // 5. 生成员工名单字符串 val nameList = employees .sortedBy { it.name } .joinToString( separator = ", ", prefix = "员工名单: ", transform = { it.name } ) println(nameList) // 6. 分区：高薪 vs 普通员工 val (highPaid, regular) = employees.partition { it.salary >= 70000 } println("高薪员工: ${highPaid.map { it.name }}") println("普通员工: ${regular.map { it.name }}") // 7. 计算总工资支出 val totalSalary = employees.sumOf { it.salary } println("总工资支出: $$totalSalary") // 8. 按服务年限分组 val byServiceYears = employees.groupBy { when { it.yearsOfService < 3 -> "Junior" it.yearsOfService < 6 -> "Mid" else -> "Senior" } } println("按年限分组: ${byServiceYears.mapValues { (_, v) -> v.map { it.name } }}") }