Virtual and Override

public virtual class Vehicle {
    public virtual String getDescription() {
        return 'A generic vehicle';
    }
}

public class DeliveryVan extends Vehicle {
    public override String getDescription() {
        return 'A delivery van';
    }
}

Vehicle van = new DeliveryVan();
System.debug(van.getDescription()); // "A delivery van"

1. public virtual class Vehicle { — the parent class.
2. public virtual String getDescription() { — virtual opts this specific method into being replaceable by subclasses. Without it, DeliveryVan would not be allowed to change it at all — Apex methods are non-overridable by default.
3. return 'A generic vehicle'; — the default behavior.
4. } — closes the method.
5. } — closes Vehicle.
6. (blank line)
7. public class DeliveryVan extends Vehicle { — the subclass.
8. public override String getDescription() { — override replaces the parent's version with this one.
9. return 'A delivery van'; — the new behavior.
10. } — closes the method.
11. } — closes DeliveryVan.
12. (blank line)
13. Vehicle van = new DeliveryVan(); — a DeliveryVan stored in a Vehicle-typed variable.
14. System.debug(van.getDescription()); — logs "A delivery van" — the actual object's overridden version runs, not the Vehicle type's original.

public class DeliveryVan extends Vehicle {
    public override String getDescription() {
        return super.getDescription() + ' (delivery configuration)';
    }
}

// "A generic vehicle (delivery configuration)"

1. public class DeliveryVan extends Vehicle { — the subclass.
2. public override String getDescription() { — the override.
3. return super.getDescription() + ' (delivery configuration)'; — super.getDescription() calls the parent's original version from inside the override, then builds on it rather than fully replacing it.
4. } — closes the method.
5. } — closes DeliveryVan.
6. (blank line)
7. // "A generic vehicle (delivery configuration)" — the resulting output: the parent's text plus the addition.

public virtual class Equipment {
    public virtual Decimal getDailyRentalRate() {
        return 500;
    }
}

public class Crane extends Equipment {
    public override Decimal getDailyRentalRate() {
        return super.getDailyRentalRate() * 4; // cranes cost more
    }
}

1. public virtual class Equipment { — the parent class.
2. public virtual Decimal getDailyRentalRate() { — a sensible default rate, marked changeable.
3. return 500; — the default value.
4. } — closes the method.
5. } — closes Equipment.
6. (blank line)
7. public class Crane extends Equipment { — a specific equipment type.
8. public override Decimal getDailyRentalRate() { — overrides the default.
9. return super.getDailyRentalRate() * 4; — builds on the base rate via super rather than duplicating the 500, since cranes cost more.
10. } — closes the method.
11. } — closes Crane.