Java for Mere Mortals

Java is the language behind banks, Android apps, and half the servers on earth: write once, compile to bytecode, run on the Java Virtual Machine anywhere. This guide covers the language itself — installing a JDK, your first program, types, methods, classes and objects, interfaces, exceptions, and the collections you'll use daily.

No CS degree assumed. If you've written any program in any language, you're ready.

What this guide covers

• What Java is — the JVM, bytecode, why Java is still everywhere, and which version to pick
• Installing Java — a JDK on Linux, macOS, or Windows, plus jshell for experiments
• Your first program — one file, compiled and run two ways, every line explained
• Variables and types — primitives, strings, var, final, and integer division's classic trap
• Control flow — if, modern switch, the three loops, break/continue
• Methods — defining, calling, return values, overloading, static
• Classes and objects — fields, constructors, new, and encapsulation
• Interfaces and inheritance — extends, implements, abstract classes
• Exceptions — try/catch, checked vs unchecked, try-with-resources, reading stack traces
• Collections — List, Map, generics, and autoboxing
• Troubleshooting — symptom → fix, nine ways

What it doesn't cover

Build tools (Maven, Gradle), web frameworks (Spring), Android development, and the module system. This guide gets you writing Java — the advanced topography comes later.

How to use this guide

Code samples were compiled and run with OpenJDK 21; everything here works on any LTS from 17 up. Commands are meant to be run — Check: blocks show what success looks like. Version-sensitive facts (like which LTS is current) were verified in October 2026 and link their sources; treat them as "check again in a year."

Validation notes

• Every Java sample in these chapters was compiled and executed with OpenJDK 21.0.12; outputs shown are the actual outputs.
• apt package names (openjdk-21-jdk, openjdk-25-jdk) were checked against Debian documentation current as of October 2026.
• Homebrew cask and Windows installer steps follow the vendors' own instructions and were not run here.