Scala 中带有可选字段的案例类
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/11982474/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Case Classes with optional fields in Scala
提问by jeypijeypi
For example, I have this case class:
例如，我有这个案例类：
case class Student (firstName : String, lastName : String)
If I use this case class, is it possible that supplying data to the fields inside the case class are optional? For example, I'll do this:
如果我使用这个案例类，是否有可能向案例类中的字段提供数据是可选的？例如，我会这样做：
val student = new Student(firstName = "Foo")
Thanks!
谢谢！
回答by om-nom-nom
You were close:
你很接近：
case class Student (firstName : String = "John", lastName : String = "Doe")
val student = Student(firstName = "Foo")
Another possibility is partially applied function:
另一种可能性是部分应用函数：
case class Student (firstName : String, lastName : String)
val someJohn = Student("John", _: String)
//someJohn: String => Student = <function1>
val johnDoe = someJohn("Doe")
//johnDoe: Student = Student(John,Doe)
And to be complete, you can create some default object and then change some field:
为了完整起见，您可以创建一些默认对象，然后更改一些字段：
val johnDeere = johnDoe.copy(lastName="Deere")
//johnDeer: Student = Student(John,Deere)
回答by tgr
If you just want to miss the second parameter without a default information, I suggest you to use an Option.
如果您只想错过没有默认信息的第二个参数，我建议您使用Option.
case class Student(firstName: String, lastName: Option[String] = None)
Now you might create instances this way:
现在您可以通过以下方式创建实例：
Student("Foo")
Student("Foo", None) // equal to the one above
Student("Foo", Some("Bar")) // neccesary to add a lastName
To make it usable as you wanted it, I will add an implicit:
为了让它可以随心所欲地使用，我将添加一个隐含的：
object Student {
implicit def string2Option(s: String) = Some(s)
}
Now you are able to call it those ways:
现在你可以这样称呼它：
import Student._
Student("Foo")
Student("Foo", None)
Student("Foo", Some("Bar"))
Student("Foo", "Bar")
回答by akauppi
I would see two ways this is normally done.
我会看到通常这样做的两种方式。
1. default parameters
1.默认参数
case class Student (firstName : String, lastName : String = "")
Student("jeypijeypi") # Student(jeypijeypi,)
2. alternative constructors
2. 替代构造函数
case class Student (firstName : String, lastName : String)
object Student {
def apply(firstName: String) = new Student(firstName,"")
}
Student("jeypijeypi") # Student(jeypijeypi,)
Which one is better depends slightly on the circumstances. The latter gives you more freedom: you can make any parameter(s) optional, or even change their order (not recommended). Default parameters need always to be at the end of the parameter list, I think. You can also combine these two ways.
哪个更好取决于具体情况。后者为您提供了更多的自由：您可以将任何参数设为可选，甚至更改它们的顺序（不推荐）。我认为，默认参数需要始终位于参数列表的末尾。您也可以将这两种方式结合起来。
Note: within the alternative constructors you need newto point the compiler to the actual constructor. Normally newis not used with case classes.
注意：在替代构造函数中，您需要new将编译器指向实际的构造函数。通常new不与案例类一起使用。
