Lab 01: Hello World & Java Basics

🎯 Objective

Write, compile, and run your first Java program using the command line.

📚 Background

Java is a compiled, statically-typed language. Source code (.java) is compiled by javac into bytecode (.class), then executed by the Java Virtual Machine (JVM) via java. This compile-once, run-anywhere model is Java's core promise.

⏱️ Estimated Time

20 minutes

📋 Prerequisites

• Docker installed

• Access to innozverse-java:latest Docker image

🛠️ Tools Used

• Docker (innozverse-java:latest)

• javac (Java compiler)

• java (JVM runtime)

🔬 Lab Instructions

Step 1: Verify the Java environment

docker run --rm innozverse-java:latest java -version

💡 You should see the Java version (17+). The JVM is pre-installed in the Docker image.

Step 2: Create your first Java source file

cat > /tmp/HelloWorld.java << 'EOF'
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
        System.out.println("Welcome to Java programming!");
    }
}
EOF

💡 In Java, the filename must match the public class name exactly (case-sensitive).

Step 3: Understand the anatomy of a Java program

public class HelloWorld {          // Class declaration — matches filename
    public static void main(String[] args) {  // Entry point — JVM calls this
        System.out.println("Hello, World!");  // Print to stdout with newline
    }
}

• public — accessible from anywhere

• static — belongs to the class, not an instance

• void — returns nothing

• String[] args — command-line arguments array

Step 4: Compile the Java source file

docker run --rm -v /tmp:/tmp innozverse-java:latest javac /tmp/HelloWorld.java

💡 This produces /tmp/HelloWorld.class — the bytecode file executed by the JVM.

Step 5: Verify the class file was created

ls -la /tmp/HelloWorld.class

Step 6: Run the compiled program

docker run --rm -v /tmp:/tmp innozverse-java:latest java -cp /tmp HelloWorld

📸 Verified Output:

Hello, World!
Welcome to Java programming!

Step 7: Pass command-line arguments

cat > /tmp/HelloArgs.java << 'EOF'
public class HelloArgs {
    public static void main(String[] args) {
        if (args.length > 0) {
            System.out.println("Hello, " + args[0] + "!");
        } else {
            System.out.println("Hello, stranger!");
        }
    }
}
EOF
docker run --rm -v /tmp:/tmp innozverse-java:latest sh -c "javac /tmp/HelloArgs.java && java -cp /tmp HelloArgs InnoZverse"

📸 Verified Output:

Hello, InnoZverse!

Step 8: Compile and run in one command

docker run --rm -v /tmp:/tmp innozverse-java:latest sh -c "cd /tmp && javac HelloWorld.java && java HelloWorld"

💡 From Java 11+, you can also run single-file programs directly: java HelloWorld.java (no separate compile step needed for simple programs).

✅ Verification

Run the full verification:

cat > /tmp/Lab.java << 'JEOF'
public class Lab {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
        System.out.println("Java version: " + System.getProperty("java.version"));
    }
}
JEOF
docker run --rm -v /tmp:/tmp innozverse-java:latest sh -c "cd /tmp && javac Lab.java && java Lab"

📸 Verified Output:

Hello, World!
Java version: 17.x.x

🚨 Common Mistakes

• Filename mismatch: public class Foo must be in Foo.java — not foo.java or FOO.java

• Missing semicolons: Every statement in Java ends with ;

• Running without compiling: You must javac before java

• Wrong classpath: Use -cp /tmp when the .class file is in /tmp

📝 Summary

You've written, compiled, and run your first Java program. Key takeaways:

• Java source → .java files

• Compiled bytecode → .class files

• Run with java ClassName (no .class extension)

• Every Java program needs a main method as its entry point

🔗 Further Reading

• Java Language Specification

• JVM Architecture Overview

• OpenJDK Documentation