Kotlin进阶之集合Map

Map是一种把键当做对象Key和值对象Value映射的集合

Kotlin的Map也分为可读的Map和可变的MutableMap

Map没有继承于Collection接口

Map

1、创建Map

Kotlin中的Map区分为可读的Map和可编辑的Map（MutableMap、HashMap、LinkedHashMap）

a、mapOf

创建一个只读空Map

方式1：

val map = mapOf<String,String>()

方式2：

val map = emptyMap<String, String>()

b、mapOf(pair: Pair): Map,>,>

调用的是LinkedHashMap的构造函数

1	val map = mapOf<String,String>("tom" to "123","jack" to "123","mary" to "123")

其他的创造Map都差不多，下面是其他一些方法创造Map

• mutableMapOf(): MutableMap<K, V> = LinkedHashMap()

• mutableMapOf(vararg pairs: Pair<K, V>): MutableMap<K, V> = LinkedHashMap()

• hashMapOf(): HashMap<K, V> = HashMap(),>

• linkedMapOf(): LinkedHashMap<K, V> = LinkedHashMap(),>

• linkedMapOf(vararg pairs: Pair<K, V>): LinkedHashMap<K, V> = LinkedHashMap(),>

• sortedMapOf(vararg pairs: Pair<K, V>): SortedMap<K, V> = SortedMap,>

2、访问Map元素

a、entries属性

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@Testfun run01() { val map = mapOf("tom" to "123", "jack" to "123", "mary" to "123") map.entries.forEach { println("key = " + it.key + " value = " + it.value) }}结果：key = tom value = 123key = jack value = 123key = mary value = 123

b、keys属性

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@Testfun run02() { val map = mapOf("tom" to "123", "jack" to "123", "mary" to "123") println(map.keys)}结果：[tom, jack, mary]

c、values属性

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@Testfun run03() { val map = mapOf("tom" to "123", "jack" to "123", "mary" to "123") println(map.values)}结果：[123, 123, 123]

e、size属性

获取map中键/值对的数目

f、get(key:K)

根据key来获取value，操作符是[]

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@Testfun run04() { val map = mapOf("tom" to "123", "jack" to "123", "mary" to "123") println(map["tom"])}结果：123

如果这个key不存在，则返回null

如果不想返回null，可以用map.getOrDefault("key","14")设置返回一个默认值

3、Map操作符函数

a、component1和component2

用来直接访问key和value

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@Testfun run01() { val map = mapOf("tom" to 18, "jack" to 25, "mary" to 19) map.entries.forEach { println("key = " + it.component1() + " value = " + it.component2()) }}结果：key = tom value = 18key = jack value = 25key = mary value = 19

b、getOrElse(key: K, defaultValue: () -> V): V

通过key获取值，当没有值可以设置默认值

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@Testfun run02() { val map = mapOf("tom" to 18, "jack" to 25, "mary" to 19) val x = map.getOrElse("tom", { 24 }) val y = map.getOrElse("aa", { 24 }) println(x) println(y)}

c、getOrPut(key: K, defaultValue: () -> V): V

如果不存在这个key，就添加这个key到map中，对应的value是defaultValue

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@Testfun run03() {大专栏  Kotlin进阶之集合Mapline"> val map = mutableMapOf("tom" to 18, "jack" to 25, "mary" to 19) map.getOrPut("cc", { 2222 }) println(map)}结果：{tom=18, jack=25, mary=19, cc=2222}

d、iterator()

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@Testfun run04() { val map = mutableMapOf("tom" to 18, "jack" to 25, "mary" to 19) for((key,value) in map.iterator()){ //iterator() 可以省略 println("key = $key value = $value") }}结果：key = tom value = 18key = jack value = 25key = mary value = 19

e、mapKeys(transform: (Map.Entry) -> R): Map,>,>

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@Testfun run04() { val map = mutableMapOf(1 to "a", 2 to "b", 3 to "c", -1 to "cc") val mMap = map.mapKeys { it.key * it.key } println(mMap)}结果：{1=cc, 4=b, 9=c}

1 的value通过转换被 -1的value覆盖了

mapValues用法差不多

f、filterKeys(predicate: (K) -> Boolean): Map,>

返回过滤出满足key判断条件的的元素组成的新的map

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@Testfun run05() { val map = mutableMapOf(1 to "a", 2 to "b", 3 to "c", -1 to "cc") val fmap = map.filterKeys { it % 2 == 0 } println(fmap)}结果：{2=b}

g、filterValues(predicate: (V) -> Boolean): Map,>

返回过滤出满足value判断条件的的元素组成的新的map

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@Testfun run06() { val map = mutableMapOf(1 to "a", 2 to "b", 3 to "c", -1 to "cc") val fmap = map.filterValues { it !== "c" } println(fmap)}

h、filter(predicate: (Map.Entry) -> Boolean): Map,>,>

返回过滤出满足Entry判断条件的元素组成的心的Map

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@Testfun run07() { val map = mutableMapOf(1 to "a", 2 to "b", 3 to "c", -1 to "cc") val fmap = map.filter { it.key == 1 && it.value == "a" } println(fmap)}结果：{1=a}

i、toMap(): Map,>

把持有Pait的Iterable集合转换成Map

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@Testfun run08() { val list = listOf(Pair(1,"a"),Pair(2,"b")) println(list) val map = list.toMap() println(map) }结果：[(1, a), (2, b)]{1=a, 2=b}

MutableMap<in K, in V>> Iterable<Pair<K, V>>.toMap(destination: M): M ====>val map = list.toMap(mutableMapOf())

j、toMutableMap(): MutableMap = LinkedHashMap(this),>

把一个只读的Mao转成可以编辑MutableMap

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@Testfun run09() { val map = mapOf(1 to "a", 2 to "b", 3 to "c", -1 to "cc") val mumap = map.toMutableMap() mumap[1] = "vv" println(mumap)}结果：{1=vv, 2=b, 3=c, -1=cc}

k、put、remove、clear略

来源：https://www.cnblogs.com/dajunjun/p/11694238.html