Scala Option (Option)

Scala collections

Scala Option (Option) type used to represent a value is optional (for value or no value).

Option [T] is a type of container optional value T: If the value exists, Option [T] is a Some [T], if you do not exist, Option [T] is the object None.

Next we look at a piece of code:

// 虽然 Scala 可以不定义变量的类型，不过为了清楚些，我还是
// 把他显示的定义上了
 
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 one is a Key type is String, String type Value is a hash map, but not the same as his get () returns a named Option [String] category.

Scala using the Option [String] to tell you: "I will try to return a String, but it may not give you the String."

myMap was not key2 this data, get () method returns None.

Option two subcategories, one Some one is None, when he return Some of the time, on behalf of this function successfully gave you a String, and you can () function to get this through the String get, If he returns None, it represents no string can give you.

Another example:

object Test {
   def main(args: Array[String]) {
      val sites = Map("w3big" -> "www.w3big.com", "google" -> "www.google.com")
      
      println("sites.get( \"w3big\" ) : " +  sites.get( "w3big" )) // Some(www.w3big.com)
      println("sites.get( \"baidu\" ) : " +  sites.get( "baidu" ))  //  None
   }
}

Implementation of the above code, the output is:

$ scalac Test.scala 
$ scala Test
sites.get( "w3big" ) : Some(www.w3big.com)
sites.get( "baidu" ) : None

You can also output to match the value by pattern matching. Examples are as follows:

object Test {
   def main(args: Array[String]) {
      val sites = Map("w3big" -> "www.w3big.com", "google" -> "www.google.com")
      
      println("show(sites.get( \"w3big\")) : " +  
                                          show(sites.get( "w3big")) )
      println("show(sites.get( \"baidu\")) : " +  
                                          show(sites.get( "baidu")) )
   }
   
   def show(x: Option[String]) = x match {
      case Some(s) => s
      case None => "?"
   }
}

Implementation of the above code, the output is:

$ scalac Test.scala 
$ scala Test
show(sites.get( "w3big")) : www.w3big.com
show(sites.get( "baidu")) : ?

getOrElse () method

You can use getOrElse () method to get the tuple that exist or use the default values, examples are as follows:

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) )
   }
}

Implementation of the above code, the output is:

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

isEmpty () method

You can use the isEmpty () method to detect whether the tuple element to None, examples are as follows:

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 )
   }
}

Implementation of the above code, the output is:

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

Scala Option common method

The following table lists the Scala Option commonly used methods:

Scala collections