(转)Scala中的Some和Option

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Scala Option[T] is a container for zero or one element of a given type. An Option[T] can be either Some[T] or None object, which represents a missing value. For instance, the get method of Scala's Map produces Some(value) if a value corresponding to a given key has been found, or None if the given key is not defined in the Map. The Option type is used frequently in Scala programs and you can compare this to null value available in Java which indicate no value. For example, the get method of java.util.HashMap returns either a value stored in the HashMap, or null if no value was found.

Let's say we have a method that retrieves a record from the database based on a primary key:

def findPerson(key: Int): Option[Person]

The method will return Some[Person] if the record is found but None if the record is not found. Let us see a real example:

object Test {
   def main(args: Array[String]) {
      val capitals = Map("France" -> "Paris", "Japan" -> "Tokyo")
      
      println("capitals.get( \"France\" ) : " +  capitals.get( "France" ))
      println("capitals.get( \"India\" ) : " +  capitals.get( "India" ))
   }
}

When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
capitals.get( "France" ) : Some(Paris)
capitals.get( "India" ) : None

C:/>

The most common way to take optional values apart is through a pattern match. For instance:

object Test {
   def main(args: Array[String]) {
      val capitals = Map("France" -> "Paris", "Japan" -> "Tokyo")
      
      println("show(capitals.get( \"Japan\")) : " +  
                                          show(capitals.get( "Japan")) )
      println("show(capitals.get( \"India\")) : " +  
                                          show(capitals.get( "India")) )
   }
   
   def show(x: Option[String]) = x match {
      case Some(s) => s
      case None => "?"
   }
}

When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
show(capitals.get( "Japan")) : Tokyo
show(capitals.get( "India")) : ?

C:/>

Using getOrElse() Method:

Following is the example of showing how to use getOrElse() to access a value or a default when no value is present:

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) )
   }
}

When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
a.getOrElse(0): 5
b.getOrElse(10): 10

C:/>

Using isEmpty() Method:

Following is the example of showing how to use isEmpty() to check if the option is None or not:

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 )
   }
}

When the above code is compiled and executed, it produces the following result:

C:/>scalac Test.scala
C:/>scala Test
a.isEmpty: false
b.isEmpty: true

C:/>

Scala Option Methods:

Following are the important methods which you can use while playing with Options. For a complete list of methods available, please check official documentation of Scala.

SN	Methods with Description
1	def get: A Returns the option's value.
2	def isEmpty: Boolean Returns true if the option is None, false otherwise.
3	def productArity: Int The size of this product. For a product A(x_1, ..., x_k), returns k
4	def productElement(n: Int): Any The nth element of this product, 0-based. In other words, for a product A(x_1, ..., x_k), returns x_(n+1) where 0 < n < k.
5	def exists(p: (A) => Boolean): Boolean Returns true if this option is nonempty and the predicate p returns true when applied to this Option's value. Otherwise, returns false.
6	def filter(p: (A) => Boolean): Option[A] Returns this Option if it is nonempty and applying the predicate p to this Option's value returns true. Otherwise, return None.
7	def filterNot(p: (A) => Boolean): Option[A] Returns this Option if it is nonempty and applying the predicate p to this Option's value returns false. Otherwise, return None.
8	def flatMap[B](f: (A) => Option[B]): Option[B] Returns the result of applying f to this Option's value if this Option is nonempty. Returns None if this Option is empty.
9	def foreach[U](f: (A) => U): Unit Apply the given procedure f to the option's value, if it is nonempty. Otherwise, do nothing.
10	def getOrElse[B >: A](default: => B): B Returns the option's value if the option is nonempty, otherwise return the result of evaluating default.
11	def isDefined: Boolean Returns true if the option is an instance of Some, false otherwise.
12	def iterator: Iterator[A] Returns a singleton iterator returning the Option's value if it is nonempty, or an empty iterator if the option is empty.
13	def map[B](f: (A) => B): Option[B] Returns a Some containing the result of applying f to this Option's value if this Option is nonempty. Otherwise return None.
14	def orElse[B >: A](alternative: => Option[B]): Option[B] Returns this Option if it is nonempty, otherwise return the result of evaluating alternative.
15	def orNull Returns the option's value if it is nonempty, or null if it is empty.