Java Overview

Write once, run anywhere. Java powers Android, enterprise backends, financial systems, and cloud-native microservices. The #1 language for large-scale backend engineering — with 30 years of ecosystem maturity and modern features from Java 21.

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🗺️ Learning Path

🌱 Foundations

15 labs from Hello World to a full inventory system. OOP, generics, streams, concurrency, NIO.2.

⚙️ Practitioner

Spring Boot REST APIs, JPA/Hibernate, JUnit 5, Docker, Maven/Gradle, design patterns.

🚀 Advanced

Microservices, reactive programming, Kafka, Redis, GraalVM native image, observability.

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📋 Foundations — 15 Labs

Labs 1–5: Basics

#	Lab	Topics
1	Hello World & JVM	compile, JVM, classpath, primitives
2	Variables & Primitives	int/double/char/boolean, casting, overflow
3	Strings & StringBuilder	immutability, methods, formatting, regex
4	Arrays	1D/2D arrays, sorting, Arrays utility
5	Control Flow & Recursion	switch expressions, loops, binary search

Labs 6–9: OOP

Labs 10–15: Modern Java

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⚡ Java 21 Highlights

All labs use Java 21 LTS — the most feature-rich Java release ever. Key modern features used throughout:

Feature	Since	Used In
Records	Java 16	Lab 6, 9, 12, 15
Sealed interfaces	Java 17	Lab 7, 15
Pattern matching instanceof	Java 16	Lab 5, 7
Switch expressions	Java 14	Lab 5, 7, 8
Text blocks	Java 15	Lab 5
var type inference	Java 10	Lab 12
Virtual threads (Loom)	Java 21	Lab 14
Record patterns	Java 21	Lab 7, 12

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🐳 Docker Quick Start

All labs run inside Docker — no local Java install needed. Install Docker first, then pull the lab image.

1. Install Docker

Ubuntu / Debian

# Remove old versions
sudo apt remove docker docker-engine docker.io containerd runc 2>/dev/null

# Install via official script (quickest)
curl -fsSL https://get.docker.com | sh

# Add your user to the docker group (no sudo needed)
sudo usermod -aG docker $USER

# Log out and back in, then verify
docker --version

macOS

Windows

RHEL / Fedora / CentOS

After adding your user to the docker group on Linux, log out and back in (or run newgrp docker) for the change to take effect.

2. Pull the Java Lab Image

# Pull the pre-built Java 21 lab image from Docker Hub
docker pull zchencow/innozverse-java:latest

# Verify the image is ready
docker run --rm zchencow/innozverse-java:latest java --version

Expected output:

openjdk 21.0.10 2026-01-20 LTS
OpenJDK Runtime Environment Temurin-21.0.10+7 (build 21.0.10+7-LTS)
OpenJDK 64-Bit Server VM (build 21.0.10+7-LTS, mixed mode, sharing)

3. Run Your First Java Program

# One-liner Hello World
docker run --rm zchencow/innozverse-java:latest \
  sh -c 'echo "class H{public static void main(String[]a){System.out.println(\"Hello, Java 21!\");}}" > H.java && javac H.java && java H'

# Compile and run a local file
docker run --rm -v $(pwd):/app -w /app zchencow/innozverse-java:latest \
  sh -c "javac Main.java && java Main"

# Interactive REPL (JShell)
docker run --rm -it zchencow/innozverse-java:latest jshell

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📊 Java vs the Ecosystem

Use Case	Java Tool
REST APIs	Spring Boot 3
Persistence	JPA + Hibernate / Spring Data
Build	Maven / Gradle
Testing	JUnit 5 + Mockito
Containerization	Docker + Jib
Observability	Micrometer + Prometheus
Native binaries	GraalVM

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🔗 Resources

• Oracle Java 21 Docs

• Effective Java, 3rd Ed. — Joshua Bloch

• Spring Boot Reference

• Baeldung Java Tutorials

• OpenJDK JEPs