First Steps

This is Tier 0 — the beginning. It assumes you have never programmed before, in any language. By the end of these five chapters you will have written, compiled, and run real Java: doing arithmetic, building text, reading what a person types, and printing answers back. Along the way you'll meet the idea that runs through the whole book — that Java checks your program twice, once when the compiler reads it and again when the JVM runs it — and you'll learn to read both kinds of message.

Five chapters, in order:

1. What Java Is & Running Code — what a program is, the compile-then-run loop (javac and the JVM), why public static void main looks the way it does, and println — the command that lets a program talk back.
2. Variables & Primitive Types — naming values so you can reuse them, static typing, and the eight built-in kinds of value: whole numbers, decimals, true/false, and single characters.
3. Numbers & Arithmetic — the operators, why integer division truncates, and why an int can silently wrap past its range to a negative number.
4. Strings, the Basics — building and reshaping text, why a String can never be changed, and the ==-vs-.equals trap that comparing text is full of.
5. Input & Output — formatted output with printf, reading typed input with Scanner, turning text into numbers, and writing a first interactive program.

Every code block with a ▶ Run button is live — click it, change the code, run it again. The fastest way to learn at this tier is to perturb working examples: change a number, swap a word, break something on purpose, and see what the compiler or the JVM says. You cannot harm anything; the worst case is an error message, and reading error messages is itself a skill these chapters build.