Encapsulation

The private balance + public getBalance() pattern from Module 10's Access Modifiers lesson has a name: encapsulation. It's the first of the four core object-oriented principles this module covers — encapsulation, inheritance, polymorphism, and (via composition) code reuse without inheritance.

public class DeliveryVehicle {
    private Decimal fuelLevel;

    public DeliveryVehicle(Decimal startingFuel) {
        this.fuelLevel = startingFuel;
    }

    public void refuel(Decimal amount) {
        if (amount > 0) {
            fuelLevel += amount;
        }
    }

    public Decimal getFuelLevel() {
        return fuelLevel;
    }
}

1. public class DeliveryVehicle { — the class.
2. private Decimal fuelLevel; — private means nothing outside this class can touch fuelLevel directly.
3. (blank line)
4. public DeliveryVehicle(Decimal startingFuel) { — the constructor.
5. this.fuelLevel = startingFuel; — sets the starting fuel through controlled code, not a direct outside assignment.
6. } — closes the constructor.
7. (blank line)
8. public void refuel(Decimal amount) { — the only way outside code can change fuelLevel upward.
9. if (amount > 0) { — enforces a business rule: only positive amounts count.
10. fuelLevel += amount; — adds the fuel.
11. } — closes the if.
12. } — closes refuel().
13. (blank line)
14. public Decimal getFuelLevel() { — a controlled, read-only window onto the value.
15. return fuelLevel; — returns it.
16. } — closes the method.
17. } — closes the class.

fuelLevel can never go directly negative or be corrupted from outside — every change is forced through refuel(), which enforces rules like "only positive amounts count." A fleet system with hundreds of vehicles stays trustworthy because no code anywhere can bypass this.

public class DeliveryVehicle {
    public Decimal fuelLevel; // no protection at all
}

DeliveryVehicle van = new DeliveryVehicle();
van.fuelLevel = -9999; // perfectly legal, completely nonsensical

1. public class DeliveryVehicle { — the class.
2. public Decimal fuelLevel; — a plain public field, with no protection at all.
3. } — closes the class.
4. (blank line)
5. DeliveryVehicle van = new DeliveryVehicle(); — creates a vehicle.
6. van.fuelLevel = -9999; — perfectly legal Apex, completely nonsensical in reality — with a public field, literally any code anywhere in the org can set fuelLevel to anything, including impossible values, and there's no single place to add a rule later.