Inheritance

public virtual class Vehicle {
    public String licensePlate;

    public void startEngine() {
        System.debug('Engine started.');
    }
}

public class DeliveryVan extends Vehicle {
    public Decimal cargoCapacityKg;
}

DeliveryVan van = new DeliveryVan();
van.licensePlate = 'CA 123-456'; // inherited from Vehicle
van.startEngine();               // inherited from Vehicle
van.cargoCapacityKg = 1200;      // defined on DeliveryVan itself

1. public virtual class Vehicle { — the parent class. virtual is what allows another class to extend it (covered fully in the next lesson).
2. public String licensePlate; — a field every Vehicle has.
3. (blank line)
4. public void startEngine() { — a method every Vehicle has.
5. System.debug('Engine started.'); — its behavior.
6. } — closes the method.
7. } — closes Vehicle.
8. (blank line)
9. public class DeliveryVan extends Vehicle { — extends Vehicle means DeliveryVan automatically gets everything Vehicle defines.
10. public Decimal cargoCapacityKg; — plus a field of its own.
11. } — closes DeliveryVan.
12. (blank line)
13. DeliveryVan van = new DeliveryVan(); — creates a van.
14. van.licensePlate = 'CA 123-456'; — inherited from Vehicle — never redeclared on DeliveryVan.
15. van.startEngine(); — also inherited from Vehicle.
16. van.cargoCapacityKg = 1200; — defined on DeliveryVan itself.

Inheritance models a genuine is-a relationship: a DeliveryVan is a Vehicle. If that sentence doesn't sound natural for two classes, they probably shouldn't inherit from each other — a Driver is not a Vehicle, for example, even though a driver operates one. That relationship belongs to a different pattern (composition, covered later in this module).

public virtual class Publication {
    public String title;

    public String getCitation() {
        return title;
    }
}

public class Magazine extends Publication {
    public Integer issueNumber;
}

public class Newspaper extends Publication {
    public Date publishDate;
}

1. public virtual class Publication { — the shared parent.
2. public String title; — a field both subclasses will inherit.
3. (blank line)
4. public String getCitation() { — a method both subclasses will inherit.
5. return title; — its behavior.
6. } — closes the method.
7. } — closes Publication.
8. (blank line)
9. public class Magazine extends Publication { — Magazine is a publication.
10. public Integer issueNumber; — plus its own specific data.
11. } — closes Magazine.
12. (blank line)
13. public class Newspaper extends Publication { — Newspaper is a publication too.
14. public Date publishDate; — with its own specific data.
15. } — closes Newspaper.

Both Magazine and Newspaper genuinely are publications, sharing a title and a citation method, while each adds its own specific data — exactly the shape inheritance is meant for.