feat: add function to get the total amount a domain has occured

src/Main.java

@@ -1,5 +1,47 @@
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
 public class Main {
 	public static void main(String[] args) {
-		System.out.println("Hello world!");
+		// Get the data from the file
+		List<String> data = ReadFile.open("test.txt");
+		
+		// Gets a map with the total occurrences for each domain
+		Map<String, Integer> domains = getDomainTotal(data);
+		
+		System.out.println(domains);
+	}
+
+	/**
+	 * Totals up the amount of times an email domain occured in the list. 
+	 * @param emails A list of emails
+	 * @return A map of email domains and the total amount of times they occured in the <code>emails</code> parameter
+	 */
+	private static Map<String, Integer> getDomainTotal(List<String> emails) {
+		// Create a map of all email address domains and counting them
+		Map<String, Integer> domains = new HashMap<>();
+		for (String email : emails) {
+			if (!EMail.validate(email)) {
+				throw new IllegalArgumentException("There was an invalid email in the file!");
+			}
+
+			String domain = EMail.getDomain(email);
+
+			// Add email domain to the list
+			if (!domains.containsKey(domain)) {
+				// Creates new entry in the map if the domain didn't exist before
+				domains.put(domain, 1);
+			} else {
+				// Updates the map with the new total for the domain
+				// Gets the old total and adds 1
+				Integer newTotal = domains.get(domain) + 1;
+
+				// Update domain value with new total
+				domains.replace(domain, newTotal);
+			}
+		}
+		
+		return domains;
 	}
 }