feat(build): add gradle build system

app/build.gradle.kts

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+/*
+ * This file was generated by the Gradle 'init' task.
+ *
+ * This generated file contains a sample Java application project to get you started.
+ * For more details on building Java & JVM projects, please refer to https://docs.gradle.org/8.10.2/userguide/building_java_projects.html in the Gradle documentation.
+ */
+
+plugins {
+    // Apply the application plugin to add support for building a CLI application in Java.
+    application
+}
+
+repositories {
+    // Use Maven Central for resolving dependencies.
+    mavenCentral()
+}
+
+dependencies {
+    // Use JUnit Jupiter for testing.
+    testImplementation(libs.junit.jupiter)
+
+    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
+
+    // This dependency is used by the application.
+    implementation(libs.guava)
+}
+
+// Apply a specific Java toolchain to ease working on different environments.
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(21)
+    }
+}
+
+application {
+    // Define the main class for the application.
+    mainClass = "Main"
+}
+
+tasks.named<Test>("test") {
+    // Use JUnit Platform for unit tests.
+    useJUnitPlatform()
+}

app/src/main/java/EMail.java

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+import java.util.regex.Pattern;
+
+public class EMail {
+	/**
+	 * Checks if the email is in a valid format
+	 * @param email The email address being checked
+	 * @return <code>true</code> if the email is correctly formatted, otherwise <code>false</code>
+	 */
+	public static boolean validate(String email) {
+		// Set up regex for email format
+		String emailRegex = "[^@ \\t\\r\\n]+@[^@ \\t\\r\\n]+\\.[^@ \\t\\r\\n]+";
+		Pattern pattern = Pattern.compile(emailRegex);
+		
+		// Checks and returns if the email matches the regex pattern
+		return pattern.matcher(email).matches();
+	}
+
+	/**
+	 * Retrieves the domain part of the email address
+	 * @param email A valid email address
+	 * @return The domain of the email address from the parameter
+	 */
+	public static String getDomain(String email) {
+		// Checks that the email address is valid, otherwise throw exception
+		if (!validate(email)) {
+			throw new IllegalArgumentException("The email address provided is not valid");
+		}
+		
+		// Returns the domain of the email address
+		return email.split("@")[1];
+	}
+}

app/src/main/java/Main.java

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+import java.util.*;
+
+public class Main {
+	public static void main(String[] args) {
+		// Get the data from the file
+		List<String> data = ReadFile.open("example.txt");
+		
+		// Gets a map with the total occurrences for each domain
+		Map<String, Integer> domains = getDomainTotal(data);
+
+		// Order the map by descending value of the value
+		List<Map.Entry<String, Integer>> sortedDomains = sortDomainMapDesc(domains);
+		
+		// Format the data to the format specified
+		String output = domainListToString(sortedDomains);
+		
+		System.out.println(output);
+	}
+
+	/**
+	 * 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;
+	}
+
+	/**
+	 * Sorts the domains in descending order, from most occurrences to the least occurrences of domains.
+	 * @param domains An unsorted map of domains with the amount that they occur
+	 * @return Sorted list of Map Entries, in descending order of occurrences
+	 */
+	private static List<Map.Entry<String, Integer>> sortDomainMapDesc(Map<String, Integer> domains) {
+		// Create a list from the map entries
+		List<Map.Entry<String, Integer>> sortedDomains = new ArrayList<>(domains.entrySet());
+		
+		// Sort the list in descending order
+		sortedDomains.sort(Map.Entry.comparingByValue(Comparator.reverseOrder()));
+		
+		return sortedDomains;
+	}
+
+	/**
+	 * Creates a string in a format to display the data from a list of domains and their occurrences.
+	 * @param domains A list of all the domains and their occurrences
+	 * @return A string with a specific format
+	 */
+	private static String domainListToString(List<Map.Entry<String, Integer>> domains) {
+		StringBuilder output = new StringBuilder();
+		for (Map.Entry<String, Integer> entry : domains) {
+			// Get the values of the entry
+			String domain = entry.getKey();
+			Integer occurrences = entry.getValue();
+			
+			output.append(domain).append(" ").append(occurrences).append("\n");
+		}
+		
+		// Remove unnecessary \n at the end of the output
+		output.deleteCharAt(output.length() - 1);
+		
+		return output.toString();
+	}
+}

app/src/main/java/ReadFile.java

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+import java.io.File;
+import java.io.FileNotFoundException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Scanner;
+
+public class ReadFile {
+	/**
+	 * Opens a file and returns a list of all the lines in the file
+	 * @param filename the name of the file with, only allowing a .txt extension
+	 * @return The contents of the file
+	 */
+	public static List<String> open(String filename) {
+		// Check that the filetype is correct, otherwise, throw IllegalArgumentException
+		if (!filename.endsWith(".txt")) {
+			throw new IllegalArgumentException("The filename is not a .txt format");
+		}
+
+		File file = new File(filename);
+		Scanner reader;
+		try {
+			// Open the file that was defined
+			reader = new Scanner(file);
+		} catch (FileNotFoundException e) {
+			throw new RuntimeException(e);
+		}
+
+		// List for all the separate lines on the database
+		List<String> data = new ArrayList<>();
+
+		// Go through all the lines in the file and append them to the list `data`
+		while (reader.hasNextLine()) {
+			String currentData = reader.nextLine();
+			data.add(currentData);
+		}
+
+		return data;
+	}
+}

app/src/test/java/EMailTest.java

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+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+@DisplayName("EMail class functions tests")
+class EMailTest {
+	
+	@Nested
+	@DisplayName("Testing EMail `validate()` function")
+	class ValidateTest {
+		
+		@Test
+		@DisplayName("Empty string")
+		void emptyString() {
+			assertFalse(EMail.validate(""));
+		}
+		
+		@Test
+		@DisplayName("Valid email")
+		void validEmail() {
+			assertTrue(EMail.validate("test@test.com"));
+			assertTrue(EMail.validate("test123@test.com"));
+			assertTrue(EMail.validate("123@123.com"));
+			assertTrue(EMail.validate("abc123abc123@abc123abc123.com"));
+			assertTrue(EMail.validate("123@test.no"));
+		}
+
+		@Test
+		@DisplayName("Invalid email")
+		void invalidEmail() {
+			assertFalse(EMail.validate("@test.com"));
+			assertFalse(EMail.validate("test@.com"));
+			assertFalse(EMail.validate("@test.com"));
+			assertFalse(EMail.validate("test.com"));
+			assertFalse(EMail.validate("test@test."));
+			assertFalse(EMail.validate("test@test"));
+			assertFalse(EMail.validate("test@testing@test.com"));
+		}
+		
+		@Test
+		@DisplayName("Email alias with +")
+		void aliasPlus() {
+			assertTrue(EMail.validate("test+alias@test.com"));
+			assertTrue(EMail.validate("test+123@test.com"));
+			assertTrue(EMail.validate("test+alias123@test.com"));
+		}
+
+		@Test
+		@DisplayName("Email alias with .")
+		void aliasDot() {
+			assertTrue(EMail.validate("te.st@test.com"));
+			assertTrue(EMail.validate("t.e.s.t@test.com"));
+		}
+	}
+
+	@Nested
+	@DisplayName("Testing EMail `getDomain()` function")
+	class GetDomainTest {
+		
+		@Test
+		@DisplayName("Throws with empty string")
+		void emptyString() {
+			assertThrowsExactly(IllegalArgumentException.class, () -> EMail.getDomain(""));
+		}
+		
+		@Test
+		@DisplayName("Throws with invalid emails")
+		void invalidEmail() {
+			assertThrowsExactly(IllegalArgumentException.class, () -> EMail.getDomain("@test.com"));
+			assertThrowsExactly(IllegalArgumentException.class, () -> EMail.getDomain("test@.com"));
+			assertThrowsExactly(IllegalArgumentException.class, () -> EMail.getDomain("@test.com"));
+			assertThrowsExactly(IllegalArgumentException.class, () -> EMail.getDomain("test.com"));
+			assertThrowsExactly(IllegalArgumentException.class, () -> EMail.getDomain("test@test."));
+			assertThrowsExactly(IllegalArgumentException.class, () -> EMail.getDomain("test@test"));
+			assertThrowsExactly(IllegalArgumentException.class, () -> EMail.getDomain("test@testing@test.com"));
+		}
+		
+		@Test
+		@DisplayName("Check the domain is correct for each email address")
+		void validEmail() {			
+			assertEquals("test.com", EMail.getDomain("test@test.com"));
+			assertEquals("test.com", EMail.getDomain("test123@test.com"));
+			assertEquals("123.com", EMail.getDomain("123@123.com"));
+			assertEquals("abc123abc123.com", EMail.getDomain("abc123abc123@abc123abc123.com"));
+			assertEquals("gmail.com", EMail.getDomain("test@gmail.com"));
+		}
+		
+		@Test
+		@DisplayName("Check the domains will not be affected by aliases")
+		void aliasEmail() {
+			assertEquals("test.com", EMail.getDomain("test+alias@test.com"));
+			assertEquals("test.com", EMail.getDomain("test+123@test.com"));
+			assertEquals("test.com", EMail.getDomain("test+alias123@test.com"));
+			assertEquals("test.com", EMail.getDomain("te.st@test.com"));
+			assertEquals("test.com", EMail.getDomain("t.e.s.t@test.com"));
+		}
+	}
+}