多申明与数据类

• 1 多申明

   fun main(args: Array<String>) {
    val pair = Pair(1, "one")

    val (num, name) = pair

    println("num = $num, name = $name")
  }

   class Pair<K, V>(val first: K, val second: V) {
    operator fun component1(): K {
        return first
    }

    operator fun component2(): V {
        return second
    }
}

num = 1,name = one

• 2 数据类(data class)

data class User(val name: String, val id: Int)

fun getUser(): User {
return User("Arvin", 1)
}

fun main(args: Array<String>) {
val user = getUser()
println("name = ${user.name}, id = ${user.id}")

// or

val (name, id) = getUser()
println("name = $name, id = $id")

// or

println("name = ${getUser().component1()}, id = ${getUser().component2()}")
println("name = ${getUser().name}, id = ${getUser().id}")

}

> name = Arvin , id = 1
获取key-value值的三种写法


 - 3 Map-->HashMap

/kotlin标准库提供Map.Entry组件功能/

fun main(args: Array<String>) {
val map = hashMapOf<String, Int>()
map.put("one", 1)
map.put("two", 2)
map.put("three" ,3)
map.put("three" , 4)

for ((key, value) in map) {    //遍历集合里的key-value
    println("key = $key, value = $value")
}

}

> 迭代原因就是为了防止存在相同的key值，若发现两个hash值（key）相同时，HashMap的处理方式是用新value替换旧value，这里并没有处理key，这就解释了HashMap中没有两个相同的key。

- 4 自动生成功能(Autogenerated functions)
     数据级获取下一个功能，自动生成:
component functions, toString(), equals(), hashCode() and copy().

data class User(val name: String, val id: Int)

fun main(args: Array<String>) {
val user = User("Arvin ", 1)
println(user) // toString() 输出结果为 User(name = Arvin, id = 1)

val secondUser = User("Arvin", 1)
val thirdUser = User("Max", 3)
println(user.equals(secondUser))   //true
println("user == secondUser: ${user == secondUser}")   //user == secondUser: true
println("user == thirdUser: ${user == thirdUser}")   //user == thirdUser:false

// copy() function
println(user.copy())            //User(name = Arvin, id = 1)
println(user.copy("Max"))       //User(name = Max, id = 1)
println(user.copy(id = 2))      //copy后的参数是你想要改成的值
println(user.copy("Max", 2))

println(user.copy().hashcode())     //1969795863
println(user.hashcode())                //1969795863

}

> 注意 equals 与 == 的去别 tostring  copy hashcode的用法

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