Intermediate 25 min read

Abstract Classes

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

Explain what an abstract class is and why it can't be instantiated directly
Write an abstract method that every subclass must implement
Distinguish an abstract class from a regular virtual class

Prerequisites: "Virtual and Override"

A class you can never create directly

public abstract class Shape {
    public abstract Decimal calculateArea();
}

// Shape s = new Shape(); // compile error — abstract classes cannot be instantiated

Shape on its own has no clear area formula — "calculate the area of a generic shape" doesn't mean anything until you know what kind of shape it is. abstract enforces that: you can never create a bare Shape, only specific subclasses of it.

abstract methods have no body — subclasses must supply one

public class Circle extends Shape {
    public Decimal radius;

    public override Decimal calculateArea() {
        return Math.PI * radius * radius;
    }
}

public class Rectangle extends Shape {
    public Decimal width;
    public Decimal height;

    public override Decimal calculateArea() {
        return width * height;
    }
}

calculateArea() on Shape has no { } body at all — it's a contract, not an implementation. Any class that extends Shape is required to provide its own calculateArea(), or it fails to compile.

Abstract class vs regular virtual class

A virtual class can be instantiated on its own and may have methods a subclass chooses to override. An abstract class cannot be instantiated on its own, and any abstract methods it declares must be overridden by every subclass. Reach for abstract specifically when the parent's version genuinely shouldn't exist standalone.

Exercise

Write an abstract class PaymentMethod with an abstract method Decimal processFee(Decimal amount). Write a subclass CreditCard that implements processFee as amount * 0.03.

Show hint

public abstract Decimal processFee(Decimal amount); then override it in CreditCard.

APEX

Abstract Classes Quiz

1. Can you write new Shape() directly if Shape is an abstract class?

No — abstract classes can never be instantiated directly Yes, always Only inside the class itself Only if it has no methods

My Notes

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Summary

An abstract class defines a shared shape that every subclass must follow but can never be created directly itself — useful when "generic version" genuinely makes no sense on its own.

← Virtual and Override Interfaces →