Nested HashMap

Part of the Logic & Flow section of Coddy's Java journey — lesson 19 of 59.

A nested HashMap is a HashMap whose values are themselves HashMaps. This lets you organize complex data hierarchically. For example, you can have an outer HashMap that maps categories (like "Electronics" or "Furniture") to inner HashMaps that hold products and their prices.

Create a nested HashMap: categories -> (items -> prices)

HashMap<String, HashMap<String, Integer>> inventory = new HashMap<>();

Add a category "Electronics" with an empty inner HashMap

inventory.put("Electronics", new HashMap<>());

Add an item "Laptop" with price 1200 into "Electronics"

inventory.get("Electronics").put("Laptop", 1200);

Check if the outer map is empty and print its size

System.out.println(inventory.isEmpty());
// Output: false
System.out.println(inventory.size()); 
// Output: 1

In this example, the outer map has one key ("Electronics") which maps to an inner HashMap containing the key "Laptop" and its value 1200.

Challenge

Easy

Create a method named <b>printNestedInventory</b> that takes a nested HashMap called inventory as input. In this inventory:

Keys are categories (e.g., "Electronics", "Furniture").
Values are inner HashMaps where keys are product names and values are their prices.

Your method should print the inventory in the following format:

For each category, print Category: <name>
If the category has products, print each one as Product: <name>, Price: <price> (with 2 spaces of indentation)
If the category has no products, print (No products) (with 2 spaces of indentation)

Example output:

Category: Electronics
  Product: Laptop, Price: 1200
  Product: Smartphone, Price: 800
Category: Furniture
  Product: Chair, Price: 50
  Product: Table, Price: 150
Category: EmptyCategory
  (No products)

Note: HashMap does not guarantee iteration order, so the order of categories and products in your output may differ from the example above — this is expected.

Note: You will need to import java.util.Map in addition to the other provided imports.

Cheat sheet

A nested HashMap is a HashMap whose values are themselves HashMaps, allowing hierarchical data organization.

Create a nested HashMap:

HashMap<String, HashMap<String, Integer>> inventory = new HashMap<>();

Add a category with an empty inner HashMap:

inventory.put("Electronics", new HashMap<>());

Add an item to the inner HashMap:

inventory.get("Electronics").put("Laptop", 1200);

Check if the outer map is empty and get its size:

System.out.println(inventory.isEmpty()); // false
System.out.println(inventory.size()); // 1

Try it yourself

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {
    public static void printNestedInventory(HashMap<String, HashMap<String, Integer>> inventory) {
        // Iterate over each category in the outer HashMap
        // For each category, print "Category: <name>"
        // If the inner map is empty, print "  (No products)"
        // Otherwise, iterate over each product and print "  Product: <name>, Price: <price>"
        // Write your code here
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String inventoryString = scanner.nextLine();

        // Convert JSON string to Nested HashMap
        Type inventoryType = new TypeToken<HashMap<String, HashMap<String, Integer>>>(){}.getType();
        HashMap<String, HashMap<String, Integer>> inventory = new Gson().fromJson(inventoryString, inventoryType);

        printNestedInventory(inventory);
    }
}