Maven and Hello World

1. What is Maven

Maven is a build tool for Java. It solves three major problems every Java project faces:

Problem	Without Maven	With Maven
Managing libraries	Manually download each .jar file and copy it to the project	Declare the library name in pom.xml, Maven downloads it automatically
Building the project	Type long javac commands with dozens of parameters	mvn compile — a single command
Project structure	Everyone arranges folders however they like	A standard structure anyone can understand immediately

The standard Maven directory structure

my-app/
├── pom.xml                          ← project config and dependencies
└── src/
    ├── main/
    │   ├── java/
    │   │   └── com/example/
    │   │       └── App.java         ← main source code
    │   └── resources/               ← config files, SQL, XML
    └── test/
        └── java/
            └── com/example/
                └── AppTest.java     ← unit tests

Golden rule

Java source code goes in src/main/java. Tests go in src/test/java. Never mix them.

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2. Install Maven

WindowsmacOSLinux

Download

1. Go to https://maven.apache.org/download.cgi
2. Under Files, download the Binary zip archive (e.g. apache-maven-3.9.x-bin.zip).

Extract and configure

1. Extract the .zip file to C:\Program Files\Apache\maven (create the folder if it doesn't exist).
2. Set environment variables:
   • Press Windows + R, type sysdm.cpl, press Enter → Advanced tab → Environment Variables.
   • Under System variables, click New:
     • Variable name: MAVEN_HOME
     • Variable value: C:\Program Files\Apache\maven\apache-maven-3.9.x (match your version number)
   • Find the Path variable, click Edit → New, add: %MAVEN_HOME%\bin
   • Click OK on all dialogs.

3. Open a new Command Prompt and verify:

   mvn -version
   

   Example output:

   Apache Maven 3.9.6
   Maven home: C:\Program Files\Apache\maven\apache-maven-3.9.6
   Java version: 21.0.3, vendor: Eclipse Adoptium
   

brew install maven

Verify:

mvn -version

# Ubuntu / Debian
sudo apt update && sudo apt install -y maven

# Fedora
sudo dnf install -y maven

Verify:

mvn -version

Maven needs JAVA_HOME

Maven uses the JAVA_HOME variable to locate the JDK. If mvn -version reports a Java-related error, revisit the JAVA_HOME setup in the Install JDK 21 lesson.

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3. Create your first Maven project

Option 1 — Create from IDE (IntelliJ IDEA)

This is the easiest way to get started.

1. Open IntelliJ IDEA, click New Project.
2. Select New Project (not Maven archetype).
3. Fill in the details:
   • Name: hello-world
   • Location: choose where to save the project
   • Language: Java
   • Build system: Maven
   • JDK: 21
   • GroupId: com.example (your organization/name in reverse domain format)
   • ArtifactId: hello-world (project name, auto-filled from Name)
   • Version: 1.0-SNAPSHOT (keep the default)
4. Click Create.

IntelliJ creates the project with the full Maven directory structure and opens pom.xml.

Option 2 — Create from the terminal (Maven archetype)

mvn archetype:generate \
  -DgroupId=com.example \
  -DartifactId=hello-world \
  -DarchetypeArtifactId=maven-archetype-quickstart \
  -DarchetypeVersion=1.4 \
  -DinteractiveMode=false

This creates a hello-world/ directory with the standard Maven structure and a sample App.java.

cd hello-world

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4. Understanding pom.xml

pom.xml (Project Object Model) is the heart of a Maven project. Open it:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
             http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <!-- Unique project identifier -->
    <groupId>com.example</groupId>
    <artifactId>hello-world</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <!-- Java version used for compilation -->
        <maven.compiler.source>21</maven.compiler.source>
        <maven.compiler.target>21</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <dependencies>
        <!-- Add external libraries here -->
    </dependencies>
</project>

Tag	Meaning
groupId	Organization/person identifier, usually reverse domain (com.example)
artifactId	Project name (hello-world)
version	Current version (1.0-SNAPSHOT = under development)
maven.compiler.source	Java version used to write the code
maven.compiler.target	JVM version to run on
dependencies	List of external libraries required

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5. Write the Hello World program

Open (or create) App.java in src/main/java/com/example/:

package com.example;

public class App {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
        System.out.println("Java version: " + System.getProperty("java.version"));
    }
}

Line-by-line explanation

Line	Explanation
package com.example;	Declares the package — groups related classes. Must match the directory structure.
public class App	Class declaration. The class name must match the filename (App.java).
public static void main(String[] args)	Entry point — the JVM looks for this method to start execution.
System.out.println(...)	Prints a line to the screen with a trailing newline.

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6. Build and run

From IntelliJ IDEA

Click the green ▶ Run button next to the main method, or press Shift + F10.

The Run panel at the bottom shows:

Hello, World!
Java version: 21.0.3

From the terminal

From the project root directory (where pom.xml is located):

# Compile the entire project
mvn compile

# Run the program
mvn exec:java -Dexec.mainClass="com.example.App"

Or package it into a .jar and run it:

mvn package
java -cp target/hello-world-1.0-SNAPSHOT.jar com.example.App

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7. Important Maven commands

Command	Effect
mvn compile	Compile source code in src/main/java
mvn test	Compile and run all tests in src/test/java
mvn package	Compile, test, and package into a .jar in the target/ directory
mvn clean	Delete the target/ directory (old build output)
mvn clean package	Delete old build and produce a fresh one
mvn install	Build and install into the local Maven repository (~/.m2)

Maven build lifecycle

validate → compile → test → package → verify → install → deploy

When you run mvn package, Maven automatically runs all preceding phases (validate, compile, test) before reaching package.

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8. Adding external libraries (dependencies)

Example: adding Gson (Google's JSON library):

1. Find the dependency on Maven Central — the central repository for all Java libraries:
   https://mvnrepository.com/artifact/com.google.code.gson/gson

2. Copy the XML snippet and paste it into the <dependencies> section of pom.xml:

<dependencies>
    <dependency>
        <groupId>com.google.code.gson</groupId>
        <artifactId>gson</artifactId>
        <version>2.11.0</version>
    </dependency>
</dependencies>

1. IntelliJ detects the change and asks Load Maven Changes — click it to download the library.

From the terminal: run mvn compile — Maven downloads Gson into ~/.m2/repository/ and caches it.

1. Use Gson in your code:

import com.google.gson.Gson;

public class App {
    public static void main(String[] args) {
        Gson gson = new Gson();
        String json = gson.toJson("Hello, JSON!");
        System.out.println(json); // "Hello, JSON!"
    }
}

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9. Troubleshooting

mvn: command not found

Cause: Maven has not been added to PATH.

Fix: Re-check the MAVEN_HOME and Path variables in the installation section. Open a new terminal after making changes.

[ERROR] Source option 5 is no longer supported. Use 7 or later.

Cause: Maven is using a low default Java version that doesn't match the code.

Fix: Add to pom.xml:

<properties>
    <maven.compiler.source>21</maven.compiler.source>
    <maven.compiler.target>21</maven.compiler.target>
</properties>

BUILD FAILURE when running tests

Cause: One or more tests are failing. Maven won't package if tests don't pass.

Temporary fix (skip tests):

mvn package -DskipTests

Use -DskipTests only temporarily

Skipping tests is a debugging shortcut. In real projects, fix the tests properly.

Cannot resolve symbol in IntelliJ after adding a dependency

Fix: Click File → Invalidate Caches → Invalidate and Restart.

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10. Setup complete

Congratulations — you have completed the Environment Setup section! Here is what you now have:

• ✅ JDK 21 installed — java -version and javac -version both work
• ✅ IDE (IntelliJ / Eclipse / VS Code) installed — you can create and run a Java project
• ✅ Terminal navigation and running Java from the command line
• ✅ Git installed, name/email configured, GitHub connected via SSH
• ✅ Maven installed, project structure understood, program built and run

Next: Java Fundamentals — start learning the Java language.

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