8.34. Scala Option

The Scala Option (option) type is used to indicate that a value is optional (with or without value).

Option[T] is a container of optional values of type T: if the value exists Option[T] . It’s just one. Some[T] , if it does not exist Option[T] is the object None .

Next, let’s look at a piece of code:

// Although Scala may not define the type of variables, for clarity, I still
// Define what he displays

val myMap: Map[String, String] = Map("key1" -> "value")
val value1: Option[String] = myMap.get("key1")
val value2: Option[String] = myMap.get("key2")

println(value1) // Some("value1")
println(value2) // None

In the above code, myMap is a type of ‘Key’ that is String , and Value is a type of hash map that is String . However, the difference is that its get() returns a category called Option[String] .

Scala usage Option[String] to tell you: I’ll find a way to send one back String , but maybe not, but there may not be a String for you.

There is no key2 data in myMap , but the ‘get()’ method returns’ None ‘

Option has two subcategories, one is Some , another one is None when he sent it back Some represents that this function successfully gives you a String and you can pass through get() , this function gets that. String if he returns None , it means there is no string to give you

Another example:

8.34.1. Example #

object Test {
   def main(args: Array[String]) {
      val sites = Map("runoob" -> "www.runoob.com", "google" ->
"www.google.com")

      println("sites.get( \\"runoob\\" ) : " +  sites.get( "runoob" ))
// Some(www.runoob.com)
      println("sites.get( \\"baidu\\" ) : " +  sites.get( "baidu" ))  //
 None
   }
}

Execute the above code, and the output is as follows:

$ scalac Test.scala
$ scala Test
sites.get( "runoob" ) : Some(www.runoob.com)
sites.get( "baidu" ) : None

You can also output matching values through pattern matching. Examples are as follows:

8.34.2. Example #

object Test {
   def main(args: Array[String]) {
      val sites = Map("runoob" -> "www.runoob.com", "google" ->
"www.google.com")

      println("show(sites.get( \\"runoob\\")) : " +
                                          show(sites.get( "runoob")) )
      println("show(sites.get( \\"baidu\\")) : " +
                                          show(sites.get( "baidu")) )
   }

   def show(x: Option[String]) = x match {
      case Some(s) => s
      case None => "?"
   }
}

Execute the above code, and the output is as follows:

$ scalac Test.scala
$ scala Test
show(sites.get( "runoob")) : www.runoob.com
show(sites.get( "baidu")) : ?

`getOrElse()` method #

You can use getOrElse() method to get the elements that exist in the tuple or use their default values, as an example:

8.34.3. Example #

object Test {
   def main(args: Array[String]) {
      val a:Option[Int] = Some(5)
      val b:Option[Int] = None

      println("a.getOrElse(0): " + a.getOrElse(0) )
      println("b.getOrElse(10): " + b.getOrElse(10) )
   }
}

Execute the above code, and the output is as follows:

$ scalac Test.scala
$ scala Test
a.getOrElse(0): 5
b.getOrElse(10): 10

`isEmpty()` method #

You can use isEmpty() method to detect whether the element in the tuple is None , the example is as follows:

8.34.4. Example #

object Test {
   def main(args: Array[String]) {
      val a:Option[Int] = Some(5)
      val b:Option[Int] = None

      println("a.isEmpty: " + a.isEmpty )
      println("b.isEmpty: " + b.isEmpty )
   }
}

Execute the above code, and the output is as follows:

$ scalac Test.scala
$ scala Test
a.isEmpty: false
b.isEmpty: true

Common methods of Scala Option #

The following table lists the Scala Option common methods:

Serial number	Method and description
1	`def get: A` Get optional value
2	`def isEmpty: Boolean` Check whether the optional type value is None. If so, return true, otherwisereturn false.
3	`def productArity: Int` Returns the number of elements, A (xroom1,…, xroomk), returns k
4	`def productElement(n: Int): Any` Gets the specified option, starting with 0. That is, A (x, n < k), returns x _ (n < 1), 0 < n < k.
5	`def exists(p: (A) => Boolean): Boolean` Returns false if the element specified in the condition in the option existsand does not return true for None.
6	`def filter(p: (A) => Boolean): Option[A]` If the option contains a value and the conditional function passed to filterreturns true, filter returns the Some instance. Otherwise, the return valueis None.
7	`def filterNot(p: (A) => Boolean): Option[A]` If the option contains a value and the conditional function passed to filterreturns false, filter returns the Some instance. Otherwise, the return value is None.
8	`def flatMap[B](f: (A) => Option[B]): Option[B]` If the option contains a value, it is passed to the function f for processing and returned, otherwise it returns None
9	`def foreach[U](f: (A) => U): Unit` If the option contains values, each value is passed to the function f, otherwise it is not processed.
10	`def getOrElse[B >: A](default: => B): B` If the option contains a value, returns the option value, otherwise returns the default value set.
11	`def isDefined: Boolean` Returns true if the optional value is an instance of Some, otherwise returnsfalse.
12	`def iterator: Iterator[A]` If the option contains a value, iterate over the optional value. Returns an empty iterator if the optional value is empty.
13	`def map[B](f: (A) => B): Option[B]` If the option contains a value, return the Some processed by the function f,otherwise return None
14	`def orElse[B >: A](alternative: => Option[B]): Option[B]` If an Option is None, the orElse method returns the value of the name parameter, otherwise, it returns the Option directly.
15	`def orNull` Returns the option value if the option contains a value, otherwise returns null.

8.34. Scala Option

8.34.1. Example #

8.34.2. Example #

getOrElse() method #

8.34.3. Example #

isEmpty() method #

8.34.4. Example #

Common methods of Scala Option #

`getOrElse()` method #

`isEmpty()` method #