大家好，我是William李梓峰奢方，歡迎加入我的Kotlin學(xué)習(xí)之旅搔扁。
今天是我學(xué)習(xí) Kotlin 的第十三天，內(nèi)容是 Data Classes - 數(shù)據(jù)類蟋字。
官方文檔：
Data Classes - 數(shù)據(jù)類
We frequently create a class to do nothing but hold data. In such a class some standard functionality is often mechanically derivable from the data. In Kotlin, this is called a data class and is marked as data:
我們經(jīng)常創(chuàng)建一個(gè)類卻僅僅是為了裝載數(shù)據(jù)稿蹲。這種（萬惡的 DTO）類的唯一功能就是傳遞數(shù)據(jù)而已。在 Kotlin 里面鹊奖，這種類就叫數(shù)據(jù)類场绿，并用 data 標(biāo)識(shí)：
data class User(val name: String, val age: Int)
// 我更喜歡這樣：
data class User(
val name: String,
val age: Int
)
The compiler automatically derives the following members from all properties declared in the primary constructor:
編譯器自動(dòng)地處理所有在主要構(gòu)造器上聲明的形參，生成對(duì)應(yīng)的屬性嫉入，同時(shí)還會(huì)生成一些預(yù)設(shè)方法：
-
equals()/hashCode()pair, -
toString()of the form"User(name=John, age=42)", -
componentN()functions corresponding to the properties in their order of declaration, -
copy()function (see below).
包括
- equals() / hashCode() 這對(duì)方法，
- toString() 方法璧尸，返回 如 “User(name=John, age=42)” 的字符串咒林，
- componentN() 函數(shù)生成若干，按照那些屬性的聲明順序來生成爷光，這里又跳了垫竞，直接飛到 Destructuring Declarations，
- copy() 方法蛀序，見面欢瞪。
If any of these functions is explicitly defined in the class body or inherited from the base types, it will not be generated.
如果上述這些任何一個(gè)函數(shù)顯式地定義在一個(gè)類體中或從其他父類繼承過來，它則不會(huì)自動(dòng)生成了徐裸。（所以沒事就別繼承了遣鼓，也別手賤自己去定義，用默認(rèn)的挺好的）
To ensure consistency and meaningful behavior of the generated code, data classes have to fulfil the following requirements:
為了確保數(shù)據(jù)一致性和生成有意義的代碼重贺，數(shù)據(jù)類必須滿足以下要求：
The primary constructor needs to have at least one parameter;
All primary constructor parameters need to be marked as
valorvar;Data classes cannot be abstract, open, sealed or inner;
(before 1.1) Data classes may only implement interfaces.
主要構(gòu)造器至少要有一個(gè)形參
所有的主要構(gòu)造器形參都要用 val 或 var 標(biāo)明骑祟；
數(shù)據(jù)類不可以用 abstract, open, sealed 或 inner 修飾；
（在 Kotlin1.1 之前）數(shù)據(jù)類只能實(shí)現(xiàn)接口气笙。
Since 1.1, data classes may extend other classes (see Sealed classes for examples).
自從 1.1 版本以來次企，數(shù)據(jù)類都可以 extend（繼承或擴(kuò)展） 其他類。（例如潜圃，密封類缸棵，又跳了）
On the JVM, if the generated class needs to have a parameterless constructor, default values for all properties have to be specified
(see Constructors).
在 JVM 里面，如果已經(jīng)生成的類需要一個(gè)無參構(gòu)造器谭期，那么所有屬性的默認(rèn)都需要被指定（請(qǐng)看 構(gòu)造器堵第，跳回去了吧凉，如果所有字段都有默認(rèn)值，編譯器會(huì)自動(dòng)生成一個(gè)無參構(gòu)造器）
data class User(val name: String = "", val age: Int = 0)
Copying - 復(fù)制
It's often the case that we need to copy an object altering some of its properties, but keeping the rest unchanged.
我們經(jīng)常需要復(fù)制某個(gè)對(duì)象的部分字段型诚。而另外一部分字段不需要改變客燕。
This is what copy() function is generated for. For the User class above, its implementation would be as follows:
copy() 方法為此而生。例如上面的 User 類狰贯，就實(shí)現(xiàn)了一個(gè)：
// 系統(tǒng)默認(rèn)生成一個(gè) copy()
fun copy(name: String = this.name, age: Int = this.age) = User(name, age)
This allows us to write
我們可以這樣寫
val jack = User(name = "Jack", age = 1)
val olderJack = jack.copy(age = 2) // 直接改變 age 值也搓，而 name 不變
Data Classes and Destructuring Declarations - 數(shù)據(jù)類與解構(gòu)聲明
Component functions generated for data classes enable their use in destructuring declarations:
Component 方法 生成于數(shù)據(jù)類中，相當(dāng)于直接讓其能夠做 解構(gòu)聲明（又跳了）
val jane = User("Jane", 35)
val (name, age) = jane // 這里做了解構(gòu)聲明涵紊，有點(diǎn)像 bash shell 參數(shù)展開
println("$name, $age years of age") // prints "Jane, 35 years of age"
Standard Data Classes - 標(biāo)準(zhǔn)的數(shù)據(jù)類
The standard library provides Pair and Triple. In most cases, though, named data classes are a better design choice, because they make the code more readable by providing meaningful names for properties.
標(biāo)準(zhǔn)庫提供 Pair（一對(duì)） 和 Triple（三胞胎）傍妒。大多數(shù)情況下，即便如此摸柄，數(shù)據(jù)類的命名還是一個(gè)更好的設(shè)計(jì)選擇颤练，因?yàn)樗鼈兡軌蜃尨a可讀性更高一些。（Pair 和 Triple 都是 Kotlin 預(yù)設(shè)的類驱负，方便數(shù)據(jù)裝載嗦玖，這里官方更推薦自己寫）
