Intermediate 25 min read

Inheritance

By the end of this lesson, you'll be able to:

Write a subclass that extends a parent class using extends
Explain what a subclass automatically gets from its parent
Recognize inheritance as a way to model an "is-a" relationship

Prerequisites: "Encapsulation"

A parent class and a subclass

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

DeliveryVan extends Vehicle means every DeliveryVan automatically has everything Vehicle defines — licensePlate and startEngine() — plus its own cargoCapacityKg. The parent class must be marked virtual (covered in the next lesson) for another class to extend it at all.

The "is-a" test

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).

A real business example: Media (Publishing)

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;
}

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.

Exercise

Write a virtual class Employee with a public String name and a method describe() returning name. Write a subclass Manager extends Employee that adds a public Integer teamSize.

Show hint

public class Manager extends Employee { public Integer teamSize; }

APEX

Inheritance Quiz

1. What relationship does inheritance model?

An "is-a" relationship (a subclass is a kind of its parent) A "has-a" relationship No relationship at all A database relationship

My Notes

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Summary

Inheritance lets one class (a subclass) automatically get the fields and methods of another (its parent class), then add or change behavior of its own — modeling a genuine "is-a" relationship between things.

← Encapsulation Virtual and Override →