Map in Scala (Extended)

HashMap

HashMap is known as HashMap because it uses a technique called Hashing. Hashing is a technique of converting a large String to small String that represents same String. A shorter value helps in indexing and faster searches.

Some points on HashMap:

HashMap doesn’t allow duplicate keys but allows duplicate values. That means A single key can’t contain more than 1 value but more than 1 key can contain a single value.

HashMap allows null key also but only once and multiple null values.

This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time. It is roughly similar to HashTable but is unsynchronized.

Internal Structure of HashMap

Internally HashMap contains an array of Node and a node is represented as a class which contains 4 fields :

int hash, K key, V value & Node next

HashMap is preferrably used where you want the key values to be unique while storing information.

Initialize a HashMap with 3 elements

import scala.collection.mutable.HashMap

println("\nInitialize a HashMap with 3 elements")

var hashMap1: HashMap[String, String] = HashMap(("PD","Plain"),("SD","Strawberry"),("CD","Chocolate"))

//Output

Initialize a HashMap with 3 elements

hashMap1: scala.collection.mutable.HashMap[String,String] = Map(CD -> Chocolate, SD -> Strawberry, PD -> Plain)

initialize HashMap using key -> value notation

The code below shows how to initialize HashMap using key -> value notation.

import scala.collection.mutable.HashMap

println("\nInitialize HashMap using key -> value notation")

val hashMap2: HashMap[String, String] = HashMap("VD"-> "Vanilla", "GD" -> "Glazed")

//Output

Initialize HashMap using key -> value notation

hashMap2: scala.collection.mutable.HashMap[String,String] = Map(GD -> Glazed, VD -> Vanilla)

Access elements of HashMap by specific key

val hashMap2: HashMap[String, String] = HashMap("VD"-> "Vanilla", "GD" -> "Glazed")

hashMap2.get("VD").get

//Output:  Vanilla

Add elements to HashMap using +=

import scala.collection.mutable.HashMap

var hashMap1: HashMap[String, String] = HashMap(("PD","Plain"),("SD","Strawberry"))

hashMap1 += ("CD"->"Customer")

hashMap1: scala.collection.mutable.HashMap[String,String] = Map(CD -> Customer, SD -> Strawberry, PD -> Plain)

Add elements from a HashMap to an existing HashMap using ++=

hashMap1 ++= hashMap2

println(hashMap1)

Remove key and its value from HashMap using -=

hashMap1 -= "CD"

//Output

hashMap1: HashMap[String, String] = HashMap(("PD","Plain"),("SD","Strawberry"))

Initialize an empty HashMap

import scala.collection.mutable.HashMap

val emptyMap: HashMap[String,String] = HashMap[String,String]()

Note : Examples having duplicate value for single key using HashMap
import collection.mutable.{ HashMap, MultiMap, Set }
val nHashMap = new HashMap[Int, Set[String]] with MultiMap[Int, String]
nHashMap.addBinding(1,"a")
nHashMap.addBinding(1,"b")
nHashMap.addBinding(1,"c")
nHashMap.addBinding(2,"b")
for((k,v)<-nHashMap)
{
     println("Key value : "+k)
     println("Duplicate entry of key mapped with string : "+v)
 }
//Output

Key value : 2

Duplicate entry of key mapped with string : Set(b)

Key value : 1

Duplicate entry of key mapped with string : Set(c, a, b)

LinkedHashMap

LinkedHashMap is known as LinkedHashMap because it uses a technique called Hashing. Hashing is a technique of converting a large String to small String that represents same String. A shorter value helps in indexing and faster searches.

Some points on LinkedHashMap:

LinkedHashMap doesn’t allow duplicate keys but allows duplicate values. That means A single key can’t contain more than 1 value but more than 1 key can contain a single value.

LinkedHashMap allows null key also but only once and multiple null values.

This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time. It is roughly similar to HashTable but is unsynchronized.

Internal Structure of LinkedHashMap

Internally LinkedHashMap contains an array of Node and a node is represented as a class which contains 4 fields :

int hash, K key, V value & Node next

LinkedHashMap is preferrably used where you want the key values to be unique while storing information.

Initialize a LinkedHashMap with 3 elements

import scala.collection.mutable.LinkedHashMap

println("\nInitialize a LinkedHashMap with 3 elements")

var hashMap1: LinkedHashMap[String, String] = LinkedHashMap(("PD","Plain"),("SD","Strawberry"),("CD","Chocolate"))

//Output

Initialize a LinkedHashMap with 3 elements

hashMap1: scala.collection.mutable.LinkedHashMap[String,String] = Map(CD -> Chocolate, SD -> Strawberry, PD -> Plain)

initialize LinkedHashMap using key -> value notation

The code below shows how to initialize LinkedHashMap using key -> value notation.

import scala.collection.mutable.LinkedHashMap

println("\nInitialize LinkedHashMap using key -> value notation")

val hashMap2: LinkedHashMap[String, String] = LinkedHashMap("VD"-> "Vanilla", "GD" -> "Glazed")

//Output

Initialize LinkedHashMap using key -> value notation

hashMap2: scala.collection.mutable.LinkedHashMap[String,String] = Map(GD -> Glazed, VD -> Vanilla)

Access elements of LinkedHashMap by specific key

val hashMap2: LinkedHashMap[String, String] = LinkedHashMap("VD"-> "Vanilla", "GD" -> "Glazed")

hashMap2.get("VD").get

//Output:  Vanilla

Add elements to LinkedHashMap using +=

import scala.collection.mutable.LinkedHashMap

var hashMap1: LinkedHashMap[String, String] = LinkedHashMap(("PD","Plain"),("SD","Strawberry"))

hashMap1 += ("CD"->"Customer")

hashMap1: scala.collection.mutable.LinkedHashMap[String,String] = Map(CD -> Customer, SD -> Strawberry, PD -> Plain)

Add elements from a LinkedHashMap to an existing LinkedHashMap using ++=

hashMap1 ++= hashMap2

println(hashMap1)

Remove key and its value from LinkedHashMap using -=

hashMap1 -= "CD"

//Output

hashMap1: LinkedHashMap[String, String] = LinkedHashMap(("PD","Plain"),("SD","Strawberry"))

Initialize an empty LinkedHashMap

import scala.collection.mutable.LinkedHashMap

val emptyMap: LinkedHashMap[String,String] = LinkedHashMap[String,String]()

SortedMap

SortedMap uses a technique called Hashing. Hashing is a technique of converting a large Int to small Int that represents same String. A shorter value helps in indexing and faster searches.

import scala.collection.SortedMap

val grades = SortedMap(10->"aakash",4->"kumar",9->"Bihar",1->"piro")

grades

//Output

grades: scala.collection.SortedMap[Int,String] = Map(1 -> piro, 4 -> kumar, 9 -> Bihar, 10 -> aakash)

-Difference between HashMap & LinkedHashMap?

HashMap : - Result or output of hashmap are not in specified order.

LinkedHashMap : Result or output of Linkedhashmap are in specified order.