Kotlin学习笔记（二）——示例篇三

该篇继续根据Kotlin语言的Demo，来学习Kotlin的使用！本文继上一篇文章来继续学习的，如果有些不懂，请参考上一篇文章
参考链接地址：http://try.kotlinlang.org/#/Examples/Multi-declarations%20and%20Data%20classes/Multi-declarations/Multi-declarations.kt

示例一

/** * This example introduces a concept that we call mutli-declarations. * It creates multiple variable at once. Anything can be on the right-hand * side of a mutli-declaration, as long as the required number of component * functions can be called on it. * See http://kotlinlang.org/docs/reference/multi-declarations.html#multi-declarations */

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
    }
}

分析：上述代码中定义了一个类对象，关于如何建一个类和类的构造函数，请参考Data class文章。
另一个问题是关于代码val (num, name) = pair的使用，这么写代码表示什么意思呢？
根据参考文档http://kotlinlang.org/docs/reference/multi-declarations.html#example-returning-two-values-from-a-function，
看看我们需要从一个函数方法返回两个值，来看例子：

/**
*数据类对象
*/
data class Result(val result: Int, val status: Status)


fun function(...): Result {
    // computations

    return Result(result, status)
}

// Now, to use this function:
val (result, status) = function(...)

由于数据类对象自动生成componentN()函数，所以这里并没有进行生成！

NOTE: we could also use the standard class Pair and have function() return Pair

示例二：

/** * Data class gets component functions, one for each property declared * in the primary constructor, generated automatically, same for all the * other goodies common for data: toString(), equals(), hashCode() and copy(). * See http://kotlinlang.org/docs/reference/data-classes.html#data-classes */

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

fun getUser(): User {
    return User("Alex", 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()}")
}

分析： 每一个数据类对象，都会自动生成与其属性相对应的componnent方法。

示例三：

/** * Kotlin Standart Library provide component functions for Map.Entry */

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

    for ((key, value) in map) {
        println("key = $key, value = $value")
    }
}

分析：该代码创建了一个Map集合，并将Map集合的值打印出来！创建HashMap直接调用hashMapOf方法即可。遍历的时候使用 in 语法来遍历！（key,value）这只是一种形式，字符不限制于key和value，可以是（k,v)

示例四：

/** * Data class gets next functions, generated automatically: * component functions, toString(), equals(), hashCode() and copy(). * See http://kotlinlang.org/docs/reference/data-classes.html#data-classes */

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

fun main(args: Array<String>) {
    val user = User("Alex", 1)
    println(user) // toString()

    val secondUser = User("Alex", 1)
    val thirdUser = User("Max", 2)

    println("user == secondUser: ${user == secondUser}")
    println("user == thirdUser: ${user == thirdUser}")

    // copy() function
    println(user.copy())
    println(user.copy("Max"))
    println(user.copy(id = 2))
    println(user.copy("Max", 2))
}

分析：以上代码主要展现了copy的使用。copy用于复制对象，并在复制过程中可以改变对象的值！比如：user.copy(id=2)改变了user的id值！