In Scala Collection API,

• :: and ::: are methods available in List class.
• #:: and #::: are methods available in STREAM class
• In List class, :: method is used to append an element to the beginning of the list.
  
  scala> VAR list1 = List(1,2,3,4)
  list1: List[Int] = List(1, 2, 3, 4)
   
  scala> list1 = 0 :: list1
  list1: List[Int] = List(0, 1, 2, 3, 4)
  
  
• In List class, ::: method is used to concatenate the elements of a given list in FRONT of this list.
  
  scala> var list1 = List(3,4,5)
  list1: List[Int] = List(3, 4, 5)
   
  scala> val list2 = List(1,2) ::: list1
  list2: List[Int] = List(1, 2, 0, 1, 2, 3, 4)
  
  
• In Stream class, #:: method is used to append a given element at beginning of the stream. Only this newly added element is evaluated and followed by lazily evaluated stream elements.
  
  scala> var s1 = Stream(1,2,3,4)
  s1: scala.collection.immutable.Stream[Int] = Stream(1, ?)
   
  scala> s1 = 0 #:: s1
  s1: scala.collection.immutable.Stream[Int] = Stream(0, ?)
  
  
• In Stream class, #::: method is used to concatenate a given stream at beginning of the stream. Only this newly added element is evaluated and followed by lazily evaluated stream elements.
  
  scala> var s1 = Stream(1,2,3,4)
  s1: scala.collection.immutable.Stream[Int] = Stream(1, ?)
   
  scala> val s2 = Stream(-1,0) #::: s1
  s2: scala.collection.immutable.Stream[Int] = Stream(-1, ?)
  
  
• :: method works as a cons OPERATOR for List class and #:: method words as a cons operator for Stream class. Here ‘cons’ stands for construct.
• ::: method works as a concatenation operator for List class and #::: method words as a concatenation operator for Stream class.

In Scala Collection API,