Understanding Classes and Objects

Classes and objects sit at the very centre of how Java code is organised. A class is a blueprint describing a kind of thing, listing what data it holds and what actions it can perform, while an object is one real instance created from that blueprint, with its own actual values.

A class describes something

public class Car {
  String colour;
  int speed;
  void drive() {
    System.out.println("Driving");
  }
}

This class lists two properties, colour and speed, known as fields, and one action, drive, known as a method. On its own, a class defines only the shape of a car, not an actual one.

An object is a real one

Car myCar = new Car();
myCar.colour = "red";
myCar.drive();

The new keyword creates an actual object in memory following the Car blueprint, and myCar is a variable holding a reference to it. Setting myCar.colour only changes that particular object; any other Car object created separately keeps its own independent value.

Many objects from one class

Car car1 = new Car();
Car car2 = new Car();
car1.colour = "red";
car2.colour = "blue";

car1 and car2 are entirely separate objects; changing one never affects the other, even though both came from the same Car class.

Encapsulation: protecting fields with methods

Making fields private and only exposing them through getter and setter methods is a common practice called encapsulation. It lets a class control exactly how its data can be read or changed, which becomes valuable once a field needs validation:

public class Car {
  private int speed;
  public void setSpeed(int speed) {
    if (speed >= 0) this.speed = speed;
  }
  public int getSpeed() {
    return speed;
  }
}

Now speed can never be set to an invalid negative value, a rule that would be impossible to enforce if the field were simply public and freely editable from anywhere.

Modelling the real world

This pattern of classes and objects is how larger applications stay organised. A shopping application might have a Product class and an Order class, each describing its own data and behaviour, with individual Product and Order objects representing the actual items and purchases moving through the system at any given moment.

Why this matters

Almost every other Java concept, from constructors to inheritance to collections of objects, builds directly on this class-and-object foundation, which makes it worth being genuinely comfortable with before moving on to more advanced object-oriented ideas.