Null Pointer Exception

If you have programmed in the past, chances are you've seen this guy before...

Dang Exceptions

A Null Pointer Exception occurs when you attempt to perform some operation on a null value. In scala there is also a NoSuchElementException which occurs if you try to access data that isn't there.

Heres an example:

class Apples
case object RedApple extends Apples
case object GreenApple extends Apples

def removeGreenApples(basketOfApples:List[Apples]): List[Apples] =
  basketOfApples.filterNot(apple => apple == GreenApple)

def eatAnApple(basketOfApples:List[Apples]) =
  println(basketOfApples.head)

val basketOfApples = List(GreenApple, GreenApple, RedApple, GreenApple, GreenApple)
eatAnApple(removeGreenApples(basketOfApples))

RedApple

Heres Another:

case class UserAccount(userId: Int,
                       lastName: String,
                       firstName: String,
                       age: Int)

val users:List[UserAccount] = List(UserAccount(1, "Doe", "John", 18),
                                   UserAccount(2, "Johnson", "Steve", 22)
//How can we grab a specific user from this list?
//What if the user we're looking for doesnt exist?

What is an Option?

• Something or
• Nothing

Option[A]

Option[A] is simply a container for the type A. If the value of type A exists then Option[A] would be an instance of Some[A]. Otherwise is there is no value then Option[A] would be None. Lets look at some code!

val hello: Option[String] = Some("Hello World!")
val pleaseNoHelloWorld: Option[String] = None

case class UserAccount(userId: Int,
                         lastName: String,
                         firstName: String,
                         age: Option[Int])

def getUserById(users: List[UserAccount], userId: Int): Option[UserAccount] = {
  users.find(user => user.userId == userId)
}

val users:List[UserAccount] = List(UserAccount(1, "Doe", "John", Some(32)),
                                   UserAccount(2, "Johnson", "Steve", Some(30)),
                                   UserAccount(3, "Jon", "Doey", None))
val user1 = getUserById(users, 1)
println(user1)

Some(UserAccount(1,Doe,John,Some(32)))

None //if we change the call to getUserById(users, 4)

def displayAge(user: UserAccount) = user.age match {
  case Some(e) => println(e)
  case None    => println("No age provided")
}
val user1 = UserAccount(1, "Doe", "John", Some(35))
val user2 = UserAccount(2, "Smith", "Zach", None)
displayAge(user1)
displayAge(user2)

35
None

Enter default values

getOrElse[B >: A](default: => B): B

• If Some[A] exists get that, otherwise return the result of evaluating default
• If user1.age has Something get that
• If user1.age has Nothing get the String "No age provided"

  println(user1.age.getOrElse("No age provided"))

More Cool Stuff

• You can map over an option
• You can filter an option
• You can perform side effects when an Option value is present

Mapping over an option

val user1 = Some(UserAccount(1, "Doe", "John", Some(35)))
println(user1.map(_.age))

Some(Some(35))

val user1 = Some(UserAccount(1, "Doe", "John", Some(35)))
println(user1.flatMap(_.age))

Some(35)

Filtering!

val users:List[UserAccount] = List(UserAccount(1, "Doe", "John", Some(32)),
                                   UserAccount(2, "Johnson", "Steve", Some(30)),
                                   UserAccount(3, "Smith", "Tori", None))

println(getUserById(users,1).filter(_.age.isDefined))
println(getUserById(users,2).filter(_.age.isDefined))

Some(UserAccount(1,Doe,John,Some(32)))
None

Side Effects(if you need 'em)

val user1 = Some(UserAccount(1, "Doe", "John", Some(35)))
user1.foreach(e => println(e.age))

Some(35)

Basics

• Like Option, Either is a container type
• It takes two parameter types
• Either[A,B] can contain either an instance of A or B

Making a method return an Either is Easy

sealed trait ActionsAgainstUser
case object MuteUser extends ActionsAgainstUser

def preparePost(message: String):Either[String, ActionsAgainstUser] = {
  if (message.contains("damn")) Right(WarnUser)
  else Left(message)
}

preparePost will return one of two things

• Left[String]
• Right[ActionsAgainstUser]

Working with Either

val postAction = preparePost("How are you doing?")
postAction match {
  case Left(msg) => println(msg)
  case Right(MuteUser) => muteUser
}