Scala: Emptiness

 

There are multiple constructs to represent Emptiness in Scala.

a.   null

b.   Null

c.    Nothing

d.   Nil

e.   None

f.     Unit

 

null

It is same as null in Java. All reference types in Scala can be assigned with null, but not the value types.

 

Example

var x : String = null

scala> var x : String = null
var x: String = null

scala> if(x == null) println("x is null") else println("x is not null")
x is null

 

You can’t assign a null to value type variables.

scala> var y : Int = null
                     ^
       error: an expression of type Null is ineligible for implicit conversion

 

 

Null

Null is a trait, means it is a type not a value. It is a subtype of every type except those of value classes.

 

Null is the type of null. Nothingness for the references can be specified using Null type.

 

Just to confirm, Null (with capital N) is a type whereas null (with small n) is a value.

 

Nothing

Nothing is a trait, means it is a type not a value.

 

Example

scala> var emptyMap = Map()
var emptyMap: scala.collection.immutable.Map[Nothing,Nothing] = Map()

When you create an empty collection, the type of the collection is Nothing.

 

Nothing is a subtype of both AnyVal and AnyRef.

Nil

Nil is a special value associated with List class. Nil is a singleton object of type List[Nothing].

scala> Nil
val res0: scala.collection.immutable.Nil.type = List()

 

Why Nil?

Lists in Scala are implemented as LinkedList. Nil is used to represent end of the list.

 

None

None is a special value associated with an Option collection. Option collection is used to represent presence or absence of a value. It is similar to java.util.Optional class.

scala> def div(a: Int, b: Int) : Option[Int]
     | = {
     |     if(b == 0) None
     |     else Option(a / b)
     | }
def div(a: Int, b: Int): Option[Int]

 

If b is 0, the div function return None, else it return Option[Int]

 

Unit

Unit is similar to void in Java.

scala> def sayHello(): Unit= {println("Hello!!!!")}
def sayHello(): Unit

scala> sayHello()
Hello!!!!

 

 

 

 

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