I have started learning Scala and going through the code (case classes and other stuff) in my project.
I see a case class defined like this:
case class Test(firstName: Option[String] = None, lastName: Option[String])
I have few questions on the above case class:
What is the difference between Option[String] = None and Option[String]?
What is the correct usage, when should we use Option[String] = None and Option[String]?
The "regex" for a parameter is something like this: name: type (= defaultValue) - Note that the default value part is optional.
Thus firstName: Option[String] = None means the parameter is named firstName and it is of type Option[String] and its default value is a None (also remember the Option type is used to explain that a value may exist or not. And that None means it doesn't exist).
Both are correct depending on the context, in this case, this class can be used like:
Test(lastName = None)
// res: Test = Test(None,None) - A person without names.
Test(lastName = Some("Mejia"))
// res: Test = Test(None,Some(Mejia)) - A person with just its last name, which it is "Mejia".
Test(firstName = Some("Luis"), lastName = Some("Mejia"))
// res: Test = Test(Some(Luis),Some(Mejia)) - A person whose first name is "Luis" and its last name is "Mejia".
Test(firstName = Some("Luis"), lastName = None)
// res: Test = Test(Some(Luis),None) - A person with just its first name, which it is "Luis".
Note that I always have to specify its last name because it doesn't have a default value.
Related
I want to parse province to case class, it throws mismatch
scala.MatchError: Some(USA) (of class scala.Some)
val result = EntityUtils.toString(entity,"UTF-8")
val address = JsonParser.parse(result).extract[Address]
val value.province = Option(address.province)
val value.city = Option(address.city)
case class Access(
device: String,
deviceType: String,
os: String,
event: String,
net: String,
channel: String,
uid: String,
nu: Int,
ip: String,
time: Long,
version: String,
province: Option[String],
city: Option[String],
product: Option[Product]
)
This:
val value.province = Option(address.province)
val value.city = Option(address.city)
doesn't do what you think it does. It tries to treat value.province and value.city as extractors (which don't match the type, thus scala.MatchError exception). It doesn't mutate value as I believe you intended (because value apparently doesn't have such setters).
Since value is (apparently) Access case class, it is immutable and you can only obtain an updated copy:
val value2 = value.copy(
province = Option(address.province),
city = Option(address.city)
)
Assuming the starting point:
val province: Option[String] = ???
You can get the string with simple pattern matching:
province match {
case Some(stringValue) => JsonParser.parse(stringValue).extract[Province] //use parser to go from string to a case class
case None => .. //maybe provide a default value, depends on your context
}
Note: Without knowing what extract[T] returns it's hard to recommend a follow-up
I have a case class like below:
case class Class1(field1: String,
field2: Option[String] = None,
var var1: Option[String] = None,
var var2: Option[Boolean] = None,
var var3: Option[Double] = None
)
The list of variables is a bit longer. Now I want to convert all variables, which are inside the class, into a string. Say Option[] must be omitted and also Boolean, Double and Number must be converted to string type. My first approach was:
def anyOptionalToString(class1Dataset: Dataset[Class1]): DataFrame = {
val ds1 = class1Dataset.map { class1 =>
(
class1.field1,
class1.field2.getOrElse(""),
class1.var1.getOrElse(""),
class1.var2.getOrElse(false),
class1.var3.getOrElse(-1.0)
)
}
Is there a way to cast them without calling every field?
Speak in a kind of loop or something similar?
What I would do, is creating a new Seq containing the defaults you want to have. Let's say:
val defaults = Seq("", "", "", false, -1)
Now, we can use the productIterator to iterate over the existing elements, and choose whether we want to use the existing value, or the default:
val c1 = Class1("f1", Some("f2"), None, Some(true), Some(3))
c1.productIterator.zip(defaults.iterator).map {
case (None, default) => default
case (Some(value), _) => value
case (value, _) => value
}.map(_.toString)
The resulting type of the code above is Iterator[String]. Code run can be found at Scastie.
I have a Option employee object. From employee I want to get the name, department, address, number, age or anything else from it if it exists but if None the name, department, and everything else I want to set to "".
I would like to just do like in Java:
if (employee.isDefined) {
val name = employee.get.getEmployeName
val department = employee.get.getDepartment
val address = employee.get.getAddress
val number = employee.get.getNumber
val age = employee.get.getAge
} else {
val name, department, address, number, age = ""
}
but I learned it does not work like that. It looks like I would need another employee object and set the values like and then access it later:
if (employee.isDefined) {
emp.setName(employee.get.getEmployeName)
emp.setDepartment(employee.get.getDepartment)
...
} else {
emp.setName("")
emp.setDepartment("")
...
}
I also experimented with tuples?
val employeeInfo = employee match {
case Some(emp) => (employee.getEmployeName, employee.getDepartment, employee.getAddress,
employee.getNumber, employee.getAge)
case None => ("", "", "", "", "")
}
val name = employeeInfo._1
val department = employeeInfo._2
val address = employeeInfo._3
...
Are these methods okay? Or are there any better ways to do this? Thanks for the help
.getOrElse() is the usual means of extracting a value from an Option while specifying a default if the option is None.
In your case, however, it is the container of many values that might be None. For that I'd recommend .fold().
case class Employee(empName : String
,dept : String
,addr : String
,num : String
,age : String)
val employee: Option[Employee] =
Some(Employee("Jo","mkt","21A","55","44"))
//or None
val name = employee.fold("")(_.empName)
val department = employee.fold("")(_.dept)
val address = employee.fold("")(_.addr)
val number = employee.fold("")(_.num)
val age = employee.fold("")(_.age)
But I have to agree with the comments from #sinanspd, your overall design is questionable at best.
This is how I would tackle this specific operation:
val (name, department, address, number, age) =
employee.fold(("", "", "", "", "")) { e =>
(e.getEmployeName, e.getDepartment, e.getAddress, e.getNumber, e.getAge)
}
But as suggested in the comments, it is worth looking at the overall design. For example it may be better to keep the values optional:
val employeeData: Option[(String, String, String, String, String)] =
employee.map{ e =>
(e.getEmployeName, e.getDepartment, e.getAddress, e.getNumber, e.getAge)
}
This allows you to tell whether a value is "" because employee was None or because the value in the Employee object was "". And you would probably define a different class to represent this restricted set of employee data to make the code cleaner and clearer.
So, the problem is in the title, but here are the details.
I have two case classes:
case class JourneyGroup(id: Option[Int] = None,
key: UUID,
name: String,
data: Option[JsValue],
accountId: Int,
createdAt: DateTime = DateTime.now,
createdById: Int,
updatedAt: Option[DateTime] = None,
updatedById: Option[Int] = None,
deletedAt: Option[DateTime] = None,
deletedById: Option[Int] = None)
and
case class JourneyGroupApi(id: Option[Int] = None,
key: UUID,
name: String,
data: Option[JsValue],
accountId: Int,
createdAt: DateTime = DateTime.now,
createdById: Int,
updatedAt: Option[DateTime] = None,
updatedById: Option[Int] = None,
deletedAt: Option[DateTime] = None,
deletedById: Option[Int] = None,
parties: Seq[Party] = Seq.empty[Party])
Background: the reason for having these two separate classes is the fact that slick does not support collections, and I do need collections of related objects that I build manually. Bottom line, I could not make it work with a single class.
What I need is an easy way to convert from one to another.
At this point, to unblock myself, I created a manual conversion:
def toJourneyGroupApi(parties: Seq[Party]): JourneyGroupApi = JourneyGroupApi(
id = id,
key = key,
name = name,
data = data,
accountId = accountId,
createdAt = createdAt,
createdById = createdById,
updatedAt = updatedAt,
updatedById = updatedById,
deletedAt = deletedAt,
deletedById = deletedById,
parties = parties
)
Which is working, but extremely ugly and requires a lot of maintenance.
One thing that I tried doing is:
convert the source object to tuple
Add an element to that tuple using shapeless
and build a target object from resulting tuple
import shapeless._
import syntax.std.tuple._
val groupApi = (JourneyGroup.unapply(group).get :+ Seq.empty[Party])(JourneyGroupApi.tupled)
But, this thing is claiming, that the result of :+ is not tuple, even though in console:
Party.unapply(p).get :+ Seq.empty[Participant]
res0: (Option[Int], model.Parties.Type.Value, Int, Int, org.joda.time.DateTime, Int, Option[org.joda.time.DateTime], Option[Int], Option[org.joda.time.DateTime], Option[Int], Seq[model.Participant]) = (None,client,123,234,2016-11-12T03:55:24.006-08:00,987,None,None,None,None,List())
What am I doing wrong? Maybe there is another way of achieving this.
Could you consider Composition?
case class JourneyGroup(
...
)
case class JourneyGroupApi(
journeyGroup: JourneyGroup=JourneyGroup(),
parties: Seq[Party] = Seq()
)
Converting a journeyGroup would just be something like JourneyGroupApi(journeyGroup,parties) and "converting" a journeyGroupApi would be a matter of accessing journeyGroupApi.journeyGroup. You could perhaps come up with names that worked better for this case. Not sure if this approach would fit the rest of your code. In particular referencing journeyGroup attributes in a journeyGroupApi will be one extra level, e.g. journeyGroupApi.journeyGroup.accountId. (This could potentially be mitigated by "shortcut" definitions on journeyGroupApi like lazy val accountId = journeyGroup.accountId.)
Inheritance might also be an approach to consider with a base case class of JourneyGroup then a normal class (not case class) that extends it with parties as the extra attribute. This option is discussed further in this SO thread.
For example, I have this case class:
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:
val student = new Student(firstName = "Foo")
Thanks!
If you just want to miss the second parameter without a default information, I suggest you to use an Option.
case class Student(firstName: String, lastName: Option[String] = None)
Now you might create instances this way:
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:
object Student {
implicit def string2Option(s: String) = Some(s)
}
Now you are able to call it those ways:
import Student._
Student("Foo")
Student("Foo", None)
Student("Foo", Some("Bar"))
Student("Foo", "Bar")
You were close:
case class Student (firstName : String = "John", lastName : String = "Doe")
val student = Student(firstName = "Foo")
Another possibility is partially applied function:
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:
val johnDeere = johnDoe.copy(lastName="Deere")
//johnDeer: Student = Student(John,Deere)
I would see two ways this is normally done.
1. default parameters
case class Student (firstName : String, lastName : String = "")
Student("jeypijeypi") # Student(jeypijeypi,)
2. alternative constructors
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.
Note: within the alternative constructors you need new to point the compiler to the actual constructor. Normally new is not used with case classes.