Classes and objects

A class is a blueprint for a kind of thing. It defines what data that thing has, stored in fields, and what it can do, defined by methods. For a robot, a class might include fields like a name and a batteryLevel, plus methods like reportStatus() or move(). The class itself is not a robot; it is the plan for making robots.

An object is a real instance created from a class, usually with new in Java. Each object gets its own copy of the fields. If you create three robots from one Robot class, you get three independent objects. Lowering one robot's battery does not change the others, because their fields live in separate objects.

class Robot {
  String name;
  int batteryLevel;

  Robot(String name, int batteryLevel) {
    this.name = name;
    this.batteryLevel = batteryLevel;
  }

  void reportStatus() {
    System.out.println(name + " battery: " + batteryLevel + "%");
  }
}

Robot r1 = new Robot("Bolt", 80);
Robot r2 = new Robot("Nova", 80);
r1.batteryLevel -= 30;  // only r1 changes

Encapsulation is the idea that an object should control its own state. In Java, you can make fields private so outside code cannot change them directly. Access goes through methods like a getter or a setter, which can enforce rules ("batteryLevel cannot go below 0 or above 100"). This prevents invalid states.

In the figure below, edit the class blueprint, stamp out robot objects, change one robot's battery to see the others unchanged, then mark a field private to see direct access blocked while the method still works.

Building robots from the blueprint

The objects are real state: each card's fields live in its own object, updates change only that object, and the private-mode refusal is an actual guard in the code. The class blueprint panel is a description of the class, not source code being compiled or run.