A map is an object that maps keys and values

A map cannot contain the same keys

A map is similar to a dictionary in Python and a Key-Value pair in JS

Each key must be unique within a map

Maps are very important to know when dealing with Abstraction in OOP

Java has three types of maps: HashMap , TreeMap and and LinkedHashMap

Ordering:

    1- HashMap:Key Order
    2- TreeMap: Key Order
    3- LinkedHasMap: Reverse Insertion Order FIFO

HashMap:

```
  Key Order
```

import java.util.*;
public class HashMap
{
          public static void main(String args[])
          {
              //HashMap Declaration
              //HashMap<DT of Key, DT of Value> nameOfHashMap= new HashMap<DT of Key, DT of Value>();
              HashMap<Integer,String> myHashMap=new HashMap<Integer,String>();//Creating HashMap.
              
              myHashMap.put(2,"Papaya"); //Putting elements in Map.
              myHashMap.put(3,"Mango");
              myHashMap.put(1,"Apple");
              myHashMap.put(4,"Lemon");
              
              System.out.println(myHashMap);
          }
          
           //Output: {1=Apple, 2=Papaya, 3=Mango, 4=Lemon} meaning Key Order
}

TreeMap

import java.util.*;
public class Main
{
        public static void main(String args[])
        {
            //Tree Declaration
            //TreeMap<DT of Key, DT of Value> nameOfTreeMap= new HashMap<DT of Key, DT of Value>();
            TreeMap<Integer,String> myTreeMap=new TreeMap<Integer,String>();//Creating HashMap.
            
            myTreeMap.put(2,"Papaya"); //Putting elements in Map.
            myTreeMap.put(3,"Mango");
            myTreeMap.put(1,"Apple");
            myTreeMap.put(4,"Lemon");
            
            System.out.println(myTreeMap);
        }
        
         //Output: {1=Apple, 2=Papaya, 3=Mango, 4=Lemon}
         //meaning Key Order
}


    

LinkedHashMap

    Ordered by Insertion FIFO

import java.util.*;
public class Main
{
          public static void main(String args[])
          {
              //LHM Declaration
              //LinkedHashMap<DT of Key, DT of Value> nameOfLinkedHashMap= new LinkedHashMap<DT of Key, DT of Value>();
              LinkedHashMap<String,String> myLHashMap=new LinkedHashMap<String,String>();//Creating Linked HashMap.
              
              myLHashMap.put("MW","Calculus3"); //Putting elements in Map.
              myLHashMap.put("MWF","OrgCh1");
              myLHashMap.put("T","DS");
              myLHashMap.put("F","Music");
              
              System.out.println(myLHashMap);
          }
          
           //Output: {MW=Calculus3, MWF=OrgCh1, T=DS, F=Music}
           //meaning Reverse Insertion Order FIFO
}