HashMap – In Java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;

public class Program2 {

    public static void main(String[] args) {

        HashMap<String, String> country_capital =
                new HashMap<String, String>();

        country_capital.put("India", "Delhi");
        country_capital.put("USA", "Washington DC");
        country_capital.put("China", "Beijing");
        country_capital.put("Russia", "Moscow");

        System.out.println(country_capital);

    }
}

🧾 Output:

{China=Bejing, Russia=Moscow, USA=Washington DC, India=Delhi}

💡 What is a HashMap?

A HashMap stores data in key-value pairs, and allows:

• Fast lookup

• No duplicate keys

• One null key and multiple null values (if needed)

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✅ Key Operations:

Operation	Example
Add a key-value pair	put("France", "Paris")
Get value by key	get("India") → Delhi
Check if key exists	containsKey("USA")
Remove a pair	remove("Russia")
Get all keys or values	keySet(), values()
Loop through key-value pairs	see below 👇

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🔁 Looping through the HashMap:

for (Map.Entry<String, String> entry : country_capital.entrySet()) {

🧠 When to Use a HashMap:

• To store and quickly access data using a unique key.

• Great for mapping IDs, usernames, names with details, etc.