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:
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:
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:
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:
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 |
Get optional value |
2 |
Check whether the optional type value is None. If so, return true, otherwisereturn false. |
3 |
Returns the number of elements, A (xroom1,…, xroomk), returns k |
4 |
Gets the specified option, starting with 0. That is, A (x, n < k), returns x _ (n < 1), 0 < n < k. |
5 |
Returns false if the element specified in the condition in the option existsand does not return true for None. |
6 |
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 |
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 |
If the option contains a value, it is passed to the function f for processing and returned, otherwise it returns None |
9 |
If the option contains values, each value is passed to the function f, otherwise it is not processed. |
10 |
If the option contains a value, returns the option value, otherwise returns the default value set. |
11 |
Returns true if the optional value is an instance of Some, otherwise returnsfalse. |
12 |
If the option contains a value, iterate over the optional value. Returns an empty iterator if the optional value is empty. |
13 |
If the option contains a value, return the Some processed by the function f,otherwise return None |
14 |
If an Option is None, the orElse method returns the value of the name parameter, otherwise, it returns the Option directly. |
15 |
Returns the option value if the option contains a value, otherwise returns null. |