android入门学习

Appearance

kotlin进阶2 ​

使用集合 ​

1.数组 使用arrayOf()函数声明数组 ​

java
val rockPlanets = arrayOf<String>("Mercury", "Venus", "Earth", "Mars")
val gasPlanets = arrayOf("Jupiter", "Saturn", "Uranus", "Neptune")

val solarSystem = rockPlanets + gasPlanets
  • 访问数组的元素和设置数组的值

array name [ index ]

java
println(solarSystem[0])
println(solarSystem[1])
println(solarSystem[2])
println(solarSystem[3])

solarSystem[3] = "Little Earth"

2.列表 ​

  • list定义属性和方法相当于只读

  • MutableList扩展list接口，可以新增，移除元素

java
fun main() {
    val solarSystem = listOf("Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune")

    println(solarSystem.size) 

    // 访问
    println(solarSystem[2])
    // 访问
    println(solarSystem.get(3))
}
  • 迭代列表
java
for (planet in solarSystem) {
    println(planet)
}
  • 添加数组
java
val solarSystem = mutableListOf("Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune")
solarSystem.add("Pluto")
solarSystem.add(3, "Theia")
  • 移除数组
java
solarSystem.removeAt(9)

solarSystem.remove("Future Moon")
  • 其他访问数组
java
println(solarSystem.contains("Pluto"))

println("Future Moon" in solarSystem)

3.Sets ​

set集合没有明确的顺序而且不允许值重复

  • set比list快，特备是大的集合

  • set比list使用更多内存

java
val solarSystem = mutableSetOf("Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune")

println(solarSystem.size)
solarSystem.add("Pluto")

println(solarSystem.contains("Pluto"))
solarSystem.remove("Pluto")

4.Map集合 ​

map的是key是唯一的

  • 定义

mutableMapOf<key type, value type>()

java
val solarSystem = mutableMapOf(
    "Mercury" to 0,
    "Venus" to 0,
    "Earth" to 1,
    "Mars" to 2,
    "Jupiter" to 79,
    "Saturn" to 82,
    "Uranus" to 27,
    "Neptune" to 14
)

println(solarSystem.size)
solarSystem["Pluto"] = 5
println(solarSystem.get("Theia"))
solarSystem.remove("Pluto")

solarSystem["Jupiter"] = 78
solarSystem.put("Theia", 78)
println(solarSystem["Jupiter"])

伴随集合的高阶函数 ​

  • forEach()和字符串模板
java
class Cookie(
    val name: String,
    val softBaked: Boolean,
    val hasFilling: Boolean,
    val price: Double
)

val cookies = listOf(
    Cookie(
        name = "Chocolate Chip",
        softBaked = false,
        hasFilling = false,
        price = 1.69
    ),
    Cookie(
        name = "Banana Walnut", 
        softBaked = true, 
        hasFilling = false, 
        price = 1.49
    ),
    Cookie(
        name = "Vanilla Creme",
        softBaked = false,
        hasFilling = true,
        price = 1.59
    ),
    Cookie(
        name = "Chocolate Peanut Butter",
        softBaked = false,
        hasFilling = true,
        price = 1.49
    ),
    Cookie(
        name = "Snickerdoodle",
        softBaked = true,
        hasFilling = false,
        price = 1.39
    ),
    Cookie(
        name = "Blueberry Tart",
        softBaked = true,
        hasFilling = true,
        price = 1.79
    ),
    Cookie(
        name = "Sugar and Sprinkles",
        softBaked = false,
        hasFilling = false,
        price = 1.39
    )
)

fun main() {
	cookies.forEach {
	    println("Menu item: ${it.name}")
	}
}
  • map() map函数把一个集合变成一个新的集合
java
val fullMenu = cookies.map {
    "${it.name} - $${it.price}"
}

println("Full menu:")
fullMenu.forEach {
    println(it)
}

// Full menu:
// Chocolate Chip - $1.69
// Banana Walnut - $1.49
// Vanilla Creme - $1.59
// Chocolate Peanut Butter - $1.49
// Snickerdoodle - $1.39
// Blueberry Tart - $1.79
// Sugar and Sprinkles - $1.39
  • filter() 创建一个子集合
java
val softBakedMenu = cookies.filter {
    it.softBaked
}

println("Soft cookies:")
softBakedMenu.forEach {
    println("${it.name} - $${it.price}")
}

// Soft cookies:
// Banana Walnut - $1.49
// Snickerdoodle - $1.39
// Blueberry Tart - $1.79
  • groupBy()
java
val groupedMenu = cookies.groupBy { it.softBaked }

val softBakedMenu = groupedMenu[true] ?: listOf()
val crunchyMenu = groupedMenu[false] ?: listOf()

println("Soft cookies:")
softBakedMenu.forEach {
    println("${it.name} - $${it.price}")
}
println("Crunchy cookies:")
crunchyMenu.forEach {
    println("${it.name} - $${it.price}")
}

// Soft cookies:
// Banana Walnut - $1.49
// Snickerdoodle - $1.39
// Blueberry Tart - $1.79
// Crunchy cookies:
// Chocolate Chip - $1.69
// Vanilla Creme - $1.59
// Chocolate Peanut Butter - $1.49
// Sugar and Sprinkles - $1.39
  • fold()

fold()函数用来从一个集合里面生成单独的值

java
val totalPrice = cookies.fold(0.0) {total, cookie ->
    total + cookie.price
}

println("Total price: $${totalPrice}")
  • sortedBy()
java
val alphabeticalMenu = cookies.sortedBy {
    it.name
}

println("Alphabetical menu:")
alphabeticalMenu.forEach {
    println(it.name)
}
// 正序和倒叙
val numbers = listOf(5, 2, 10, 1, 8)

// 正序排序
val sortedNumbers = numbers.sortedBy { it }

// 倒序排序
val sortedDescendingNumbers = numbers.sortedByDescending { it }

println("正序排序: $sortedNumbers") // 输出: 正序排序: [1, 2, 5, 8, 10]
println("倒序排序: $sortedDescendingNumbers") // 输出: 倒序排序: [10, 8, 5, 2, 1]