Java Basics

A small Java 21 learning project that verifies my local Java toolchain and includes a command-line IPv4 subnet calculator.

This repository verifies that a Java 21 workflow works correctly with IntelliJ IDEA, Eclipse Temurin, Git and GitHub on my Intel Mac and Windows ThinkPad setup.

Purpose

This is not intended to be a large Java application.

It is a compact baseline project for:

• verifying the local Java toolchain
• practicing Java fundamentals
• testing IntelliJ IDEA with Eclipse Temurin 21 LTS
• keeping IDE metadata and build output out of Git
• maintaining a clean Git/GitHub workflow for Java projects
• documenting small command-line Java exercises

Java is a foundational programming skill in my current learning path. My main focus remains data and process analysis, SQL, Python, BI and Microsoft-oriented data tooling.

Tested Environment

The project is used across my current developer setup:

• iMac Retina 4K, 21.5-inch, Late 2015
• Intel x86_64
• macOS Sonoma via OpenCore Legacy Patcher
• Lenovo ThinkPad X1 Carbon Gen 9
• Windows 11 Enterprise
• IntelliJ IDEA via JetBrains Toolbox
• Eclipse Temurin 21 LTS
• Java 21
• Git / GitHub
• Claude Code as an optional project assistant in the terminal

This repository also documents that the Java 21 workflow works on a legacy Intel Mac setup and on a Windows ThinkPad used as a mirrored learning and development machine.

Repository Structure

java-basics/
├── src/
│   ├── Main.java
│   └── SubnetCalculator.java
├── .gitignore
├── LICENSE
└── README.md

Local IntelliJ IDEA files, build output and local assistant notes are intentionally excluded from Git:

.idea/
out/
*.iml
*.class
CLAUDE.md

Source Files

File	Purpose
src/Main.java	Minimal Java 21 toolchain check and simple entry point
src/SubnetCalculator.java	Command-line IPv4 subnet calculator with explicit input validation

Run in IntelliJ IDEA

Open the project in IntelliJ IDEA and run one of the following entry points:

• Main.main()
• SubnetCalculator.main()

For SubnetCalculator.main(), use the following program argument:

192.168.10.42/24

Expected output:

Input IP:          192.168.10.42
CIDR prefix:       /24
Subnet mask:       255.255.255.0
Wildcard mask:     0.0.0.255
Network address:   192.168.10.0
Broadcast address: 192.168.10.255
Total addresses:   256
Usable hosts:      254
First usable host: 192.168.10.1
Last usable host:  192.168.10.254
Note:              Standard subnet with network and broadcast addresses excluded.

Run from Terminal

Compile the source files:

javac -d out src/*.java

Run Main:

java -cp out Main

Run the IPv4 subnet calculator:

java -cp out SubnetCalculator 192.168.10.42/24

The same commands also work in zsh on macOS.

IPv4 Subnet Calculator

SubnetCalculator accepts input in this format:

IPv4/CIDR

Examples:

192.168.10.42/24
10.0.0.5/30
0.0.0.0/0
255.255.255.255/32

The calculator prints:

• normalized subnet mask
• wildcard mask
• network address
• broadcast address
• total address count
• usable host count
• first usable host
• last usable host
• a short note for special CIDR cases

The input validation rejects common invalid forms, for example:

• missing CIDR prefix
• more than one / separator
• empty IPv4 octets
• non-decimal characters
• leading zeros in IPv4 octets
• octets outside 0-255
• prefixes outside 0-32

What This Demonstrates

This repository demonstrates a working Java 21 baseline setup using:

• Eclipse Temurin 21 LTS as the project JDK
• IntelliJ IDEA as the primary Java IDE
• a simple src/ based project structure
• a small command-line IPv4 subnet calculator
• Java records for structured result data
• IPv4 parsing and validation
• CIDR prefix handling
• subnet mask and wildcard mask calculation
• network and broadcast address calculation
• basic bitwise operations in Java
• local build output excluded from Git
• Git and GitHub for version control

Development Workflow

Before working on the repository:

git status -sb
git pull --ff-only

After making changes:

git status -sb
git diff

Only intentional source or documentation changes should be committed.

Next Steps

Possible future additions:

• package structure for a more realistic Java layout
• basic Java class and object examples
• additional small command-line exercises
• simple file input/output examples
• a small CSV parser
• basic unit tests later on

Notes

This repository is intentionally small.

Its purpose is to document and verify a clean Java 21 development setup before building larger Java exercises or tools.

No IDE metadata, build output or machine-specific files are committed.