Map of Map in Java

In Java, Map is an interface that maps keys to values. Sometimes it is required to implement Map of Map (nested Map).

Nested Map is used in many cases, such as storing students' names with their Ids of different courses. In this case, we create a Map having a key, i.e., course name and value, i.e., another Map having a key, i.e., Id and value, i.e., the student's name.

We should use the following steps to create a Map of Map in Java:

Take the total number of students from the user for each course.
Fill in the student's information of all courses by taking input from the user.
Fill the main Map by adding the course name as a key and user filled Map as a value.
Within for loop, we first convert all the Map's keys into an ArrayList of type Object.
After that, we get the object from the list by using the get() method and get value from the Map corresponding to this object for printing value.

Let's implement the logic for creating Map of Map to store data.

MapOfMapExample.java

import java.util.HashMap;
import java.util.Map;
import java.util.*;
//create class MapOfMapExample to create a MapOfMap Example
public class MapOfMapExample {
	// main() method start
    	public static void main(String[] args) {		
		// declare variables
		int size1 = 0;
		int size2 = 0;
		// create a Map for BCA students that will store students Id and Name
        		Map<Object,String> bcaStudents = new HashMap<Object, String>();
       	 	// create a Map for MCA students that will store students Id and Name
        		Map<Object,String> mcaStudents = new HashMap<Object, String>();
        		// create Scanner class object to take input from user
		Scanner sc = new Scanner(System.in);
		System.out.println("Enter total number of BCA students.");
		size1 = Integer.parseInt(sc.nextLine());
		System.out.println("Enter total number of MCA students.");
		size2 = Integer.parseInt(sc.nextLine());
		// fill bcaStudents Map by taking input from user        
		for(int i = 1; i <= size1; i++){
			int id = 100;
			String name = "";
			System.out.println("Enter name of " + i + "st student of BCA:");
			name = sc.nextLine();
			System.out.println("Student Id = " + (id+i) + "    Student Name = "+name);
			id = id + i;
			bcaStudents.put(Integer.valueOf(id), name);
		}
		// fill mcaStudents Map by taking input from user        
		for(int i = 1; i <= size2; i++){
			int id = 100;
			String name = "";
			System.out.println("Enter name of " + i + "st student of MCA:");
			name = sc.nextLine();
			System.out.println("Student Id = " + (id+i) + "   Student Name = "+name);
			id = id + i;
			mcaStudents.put(Integer.valueOf(id), name);			
		}
		// create a Map to store data of BCA and MCA students 
		Map<String, Object> students = new HashMap<String, Object>();
		students.put("BCA", bcaStudents);
		students.put("MCA", mcaStudents);
		// print students Map
        		System.out.println("Map of Map:   " + students); 
        		// print elements of students Map
        		for (int i = 0; i < students.size(); i++){
        			ArrayList<Object> data = new ArrayList<Object>(students.keySet());
        			Object obj = data.get(i);
        			System.out.println("Course: " + obj + " Students: " + students.get(obj));
        		}
		// close Scanner class object
		sc.close();            
}
}

Output: