Kotlin 中的数据操作

2023-06-06

Kotlin 提供了丰富的数据处理功能，包括数据的映射、过滤、查找、排序、求和、字符连接、转换、循环等。这些功能可以帮助开发者更加方便地处理数据，而不用关心具体的实现过程，开发者可以将更多的关注放在业务逻辑本身。此外，这些功能还能极大简化代码，提升代码的可读性。下面是一些数据操作的常用模版代码：

1. 映射操作

// 将集合转换为 MAP

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val map: Map<Long, Member> = data.associateBy { it.id }
val map: Map<Long, String> = data.associate { it.id to it.name }

// 将集合中的元素按照某个条件分组，返回 MAP

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val map = data.groupBy { it.id }
map.forEach {
    print("${it.key} - ${it.value.count()}")
}

2. 数据过滤

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val users: List<Member> = data.filter { it.id >= 2 }.map { it }
val names: List<String> = data.filter { it.id >= 2 }.map { it.name }

3. 查找数据

// 验证是否包含 ID 为 2 的用户，如包含返回 true

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val boolean = data.any { it.id == 2 }

// 查找 ID 为 2 的用户下标，如未找到返回 -1

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val index = data.indexOfFirst { id == it.id }

// 查找 ID 为 2 的用户值，如未找到返回 null

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val member: Member? = data.find { it.id == 2 }

// 查找 ID 值最大的用户

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val member: Member = data.maxBy { it.id }

4. 排序操作

// 将数据集合反转

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

data.reverse()

// 根据方法处理结果进行降序排序

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val list = data.sortedByDescending { it.id }

list.forEach {
    print("${it.id} - ${it.name}")
}

5. 数据求和

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val sum = data.sumOf { it.id }

6. 字符连接

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val value: String = data.joinToString(separator = ",") { it.id.toString() }

7. 类型转换

class Member(var id: Int, var name: String)
class Person(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

val persons = data.filter { it.id > 1 }.map(fun(it: Member): Person {
    return Person(it.id, it.name)
})

8. 数据循环

class Member(var id: Int, var name: String)

val data = mutableListOf(
    Member(1, "Tom"),
    Member(2, "Tim"),
    Member(3, "Tom")
)

data.forEach {
    print(it.name)
}

data.forEachIndexed { index, member ->
    print(index.toString())
    print(member.name)
}