Maps

• A map is an object that maps keys and values

• Cannot contain the same keys

• similar to a dictionary in Python and a Key-Value pair in JS

• Keys 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 LinkedHashMap

Ordering

1. HashMap

• Check if Empty: isEmpty()
• Remove a particular key: .remove()
• Does Not Maintain Insertion Order
• Holds A Value Depending on A Key
• Only Holds Unique Elements
• Lookup & Insertion: O(1)
• Only allowed to store 1 Null Key
• Allowed to store multiple null values

2. TreeMap

• Key Order
• Only Holds Unique Elements
• Lookup & Insertion: O(log(n))
• Cannot Store Null as a key
• Allowed to store multiple null values
• Maintains Ascending Orders

3. LinkedHasMap:

• Only Holds Unique Elements
• Allowed to store only one null key
• Allowed to store multiple null values.
• Maintains Insertion Order
• FIFO

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
}

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=newLinkedHashMap<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);
	}

}