Scala has a rich set of collection library. Collections are containers of things. Those containers can be sequenced, linear sets of items like List, Tuple, Option, Map, etc. The collections may have an arbitrary number of elements or be bounded to zero or one element e.g., Option.Collections may be strict or lazy. Lazy collections have elements that may not consume memory until they are accessed, like Ranges. Additionally, collections may be mutable,the contents of the reference can change, or immutable (the thing that a reference refers to is never changed.
Examples of Collections:
1. List: Scala Lists are quite similar to arrays which means, all the elements of a list have the same type but there are two important differences. First, lists are immutable, which means elements of a list cannot be changed by assignment. Second, lists represent a linked list whereas arrays are flat.Example :
// List of Strings
val fruit: List[String] = List("apples", "oranges", "pears")
// List of Integers
val nums: List[Int] = List(1, 2, 3, 4)
// Empty List.
val empty: List[Nothing] = List()
// Two dimensional list
val dim: List[List[Int]] =
List(
List(1, 0, 0),
List(0, 1, 0),
List(0, 0, 1)
)
All operations on lists can be expressed in terms of the following three methods.
a. head b. tail c. isEmpty
2.Sets: Scala Set is a collection of pairwise different elements of the same type. In other words, a Set is a collection that contains no duplicate elements. There are two kinds of Sets, the immutable and the mutable. The difference between mutable and immutable objects is that when an object is immutable, the object itself can't be changed.
By default, Scala uses the immutable Set. If you want to use the mutable Set, you'll have to import scala.collection.mutable.Set class explicitly. If you want to use both mutable and immutable sets in the same collection, then you can continue to refer to the immutable Set as Set but you can refer to the mutable Set as mutable.Set.Syntax for defining scala sets:
// Empty set of integer type
var s : Set[Int] = Set()
// Set of integer type
var s : Set[Int] = Set(1,3,5,7)
or
var s = Set(1,3,5,7)
3. Maps: Scala map is a collection of key/value pairs. Any value can be retrieved based on its key. Keys are unique in the Map, but values need not be unique. Maps are also called Hash tables. There are two kinds of Maps, the immutable and the mutable. The difference between mutable and immutable objects is that when an object is immutable, the object itself can't be changed.
By default, Scala uses the immutable Map. If you want to use the mutable Set, you'll have to import scala.collection.mutable.Map class explicitly. If you want to use both mutable and immutable Maps in the same, then you can continue to refer to the immutable Map as Map but you can refer to the mutable set as mutable.Map.
The Following is the example statements to declare immutable Maps −Empty hash table whose keys are strings and values are integers:
var A:Map[Char,Int] = Map()
// A map with keys and values.
val colors = Map("red" -> "#FF0000", "azure" -> "#F0FFFF")
4. Tuples : Scala tuple combines a fixed number of items together so that they can be passed around as a whole. Unlike an array or list, a tuple can hold objects with different types but they are also immutable.
The following is an example of a tuple holding an integer, a string, and the console.
val t = (1, "hello", Console)
Which is syntactic sugar (short cut) for the following −
val t = new Tuple3(1, "hello", Console)
The actual type of a tuple depends upon the number and of elements it contains and the types of those elements. Thus, the type of (99, "Luftballons") is Tuple2[Int, String]. The type of ('u', 'r', "the", 1, 4, "me") is Tuple6[Char, Char, String, Int, Int, String]
5.Options: 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.
Option type is used frequently in Scala programs and you can compare this with the 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 follow the following program.
Example
object Demo {
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" ))
}
}
6.Iterators : An iterator is not a collection, but rather a way to access the elements of a collection one by one. The two basic operations on an iterator it are next and hasNext. A call to it.next() will return the next element of the iterator and advance the state of the iterator. You can find out whether there are more elements to return using Iterator's it.hasNext method.The most straightforward way to "step through" all the elements returned by an iterator is to use a while loop. Let us follow the following example program.
Example
object Demo {
def main(args: Array[String]) {
val it = Iterator("a", "number", "of", "words")
while (it.hasNext){
println(it.next())
}
}
}
