Classes & Objects: Overview

Welcome to Chapter 3, where Java becomes object-oriented. Control Flow gave you decisions, loops, arrays, and methods — the procedural building blocks. Now the unit of design changes: instead of methods acting on loose data, you bundle data and behavior together into classes, and build objects from them. The chapter ends with the lesson everything else rests on — the reference model — which turns the "surprises" of earlier chapters into predictable consequences.

There are four lessons, in this order:

1. Defining Classes & Creating Objects — fields and methods bundled into a blueprint, new, constructors, this, and independent instances.
2. Encapsulation & Access Modifiers — private, getters/setters that enforce invariants, the four access levels, and immutability by design.
3. static vs Instance — class-level shared state vs per-object state, why a static method has no this, static final constants, and static blocks.
4. References, Equality & the Object Model — the flagship: stack vs heap, values vs references, == vs .equals (with the Integer cache and String pool), and null.

Every code block with a ▶ Run button is live. In this chapter, the most valuable habit you can build is to draw the references: whenever an example aliases two variables or compares two objects, sketch which variables point at which heap objects before you predict the output. The reference-model lesson gives you that picture explicitly.