The Anatomy of a Class

A class works because data and behavior belong together. Width and height describe a rectangle, and only methods inside Rectangle should be trusted to use them correctly, so the class keeps the fields private and gives the rest of the program methods like getArea to ask questions about the object without touching its insides directly. That bundling is the whole idea of a class: state and the operations on that state, in one file.

Inside the file, three pieces stack up. At the top sit instance variables, declared once, outside any method, holding the object's state. Below them sits the constructor, a block that runs exactly once per object, at the moment of construction, to set those instance variables to starting values. Below that sit the methods, which run whenever called and can read or change the instance variables because every method in the class can see them. A Rectangle class holds width and height as instance variables, a constructor Rectangle(double w, double h) that assigns them, and a method getArea() that returns width times height.

Each object built from the class gets its own separate copy of the instance variables. Rectangle r1 = new Rectangle(3, 4) and Rectangle r2 = new Rectangle(5, 2) do not share width or height; r1.getArea() and r2.getArea() read different fields entirely. This separation is what makes instance variables different from local variables declared inside a method, which exist only for that one method call. It also sets up the next idea: passing objects to methods hands over a reference to this shared state, not a fresh copy of it.