Sealed Classes (Finalized in Java 17)

Sealed classes allow you to restrict which classes can extend or implement them.

They provide controlled inheritance, making your domain model safer and more predictable.

Why Sealed Classes Were Introduced

Before Java 17, inheritance control options were limited:

final → no one can extend
public → anyone can extend

There was no middle ground.

Sealed classes introduce controlled extensibility.

Basic Syntax

public sealed class Shape permits Circle, Rectangle {
}

Subclasses must declare one of:

final
sealed
non-sealed

Example:

public final class Circle extends Shape {
}

public final class Rectangle extends Shape {
}

What Does permits Mean?

permits explicitly lists allowed subclasses.
Only the listed classes can extend the sealed class.

If another class tries:

public class Triangle extends Shape { } // ❌ Compilation error

It will fail.

Subclass Rules

A permitted subclass must declare one of:

1. final

public final class Circle extends Shape { }

No further inheritance allowed.

2. sealed

public sealed class Rectangle extends Shape permits Square {
}

Continues controlled inheritance.

3. non-sealed

public non-sealed class Circle extends Shape { }

Inheritance becomes unrestricted from here.

Rules & Restrictions

✔ Sealed class and permitted subclasses must be in the same module
✔ Or in the same package (if not using modules)
✔ Cannot extend outside permitted list ✔ Works with classes and interfaces

Sealed Interfaces Example

public sealed interface Payment permits UpiPayment, CardPayment {
}

public final class UpiPayment implements Payment { }

public final class CardPayment implements Payment { }

Sealed vs final vs abstract

Feature	final	abstract	sealed
Can extend?	❌ No	✔ Yes	✔ Restricted
Control subclasses	No	No	Yes
Domain modeling	Limited	Flexible	Controlled

Sealed classes, introduced in Java 17, allow controlled inheritance by explicitly defining which classes can extend them.