Java 15 introduced Sealed Classes, which allow you to control which classes can extend or implement a class or interface. This feature improves encapsulation, security, and maintainability in Java applications.
1. What Are Sealed Classes?
✅ Sealed classes restrict inheritance, so only specific subclasses can extend them.
✅ Prevent unwanted extensions, improving code security and maintainability.
✅ Useful for defining fixed hierarchies, like shapes (Circle, Square) or permissions (Admin, User).
2. How to Define a Sealed Class?
📌 Syntax of Sealed Classes
sealed class A permits B, C { } // A can ONLY be extended by B and C
final class B extends A { } // ✅ Allowed (Final means no further extension)
non-sealed class C extends A { } // ✅ Allowed (C can be extended further)✔ A “sealed” class explicitly declares which subclasses are permitted.
3. Example: Restricting Subclasses in a Sealed Hierarchy
📌 Problem: Only Circle and Square should be allowed to extend Shape.
🟢 Java 15+ Sealed Class Solution
sealed class Shape permits Circle, Square { } // Only Circle & Square can extend
final class Circle extends Shape { } // ✅ Allowed (final: No more extensions)
final class Square extends Shape { } // ✅ Allowed🚨 What’s NOT allowed?
class Triangle extends Shape { } // ❌ ERROR: Not permitted!✔ Prevents unauthorized subclasses!
4. Types of Subclasses Allowed in a Sealed Class
A subclass of a sealed class MUST be one of these types:
| Modifier | Behavior |
|---|---|
final | The subclass cannot be extended further. |
sealed | The subclass must define its own permitted subclasses. |
non-sealed | The subclass removes all restrictions, allowing anyone to extend it. |
📌 Example: Using final, sealed, and non-sealed
sealed class Vehicle permits Car, Bike { }
final class Car extends Vehicle { } // ✅ No more extensions
sealed class Bike extends Vehicle permits ElectricBike { } // ✅ Must specify its own subclasses
non-sealed class ElectricBike extends Bike { } // ✅ No restrictions, anyone can extend✔ Flexible but controlled inheritance!
5. Comparison: Before Java 15 vs. Java 15+ (sealed)
| Feature | Before Java 15 | Java 15+ (sealed) |
|---|---|---|
| Restricting subclasses | ❌ Not possible | ✅ Explicitly define permitted subclasses |
| Preventing unintended inheritance | ❌ Any class can extend another | ✅ Only permitted subclasses allowed |
| Security & Maintainability | ❌ Difficult to control | ✅ Safer and easier to maintain |
6. When Should You Use Sealed Classes?
🚀 Use sealed when:
✔ You want strict control over a class hierarchy (e.g., Shapes, User Roles).
✔ You want to prevent accidental extensions of a class.
✔ You want better security and maintainability in a large application.
🚨 Don’t use sealed if:
❌ You want to allow any class to extend the base class.
❌ You don’t need strict control over subclassing.
Lesson Reflection
- Why do Sealed Classes improve security and maintainability?
- When should you use
sealed,final, andnon-sealedin a class hierarchy? - What happens if a class tries to extend a sealed class without permission?
Answers to Reflection Questions on Sealed Classes (sealed, permits)
1️⃣ Why do Sealed Classes improve security and maintainability?
✅ They prevent unauthorized extensions, making the code more predictable and secure.
✅ They enforce a controlled hierarchy, ensuring only intended subclasses exist.
✅ They help with maintainability, as developers can clearly see which classes can extend a sealed class.
📌 Example: Preventing Unwanted Inheritance
sealed class Payment permits CreditCard, PayPal { }
final class CreditCard extends Payment { } // ✅ Allowed (Final: No further extensions)
final class PayPal extends Payment { } // ✅ Allowed🚨 What happens if another class tries to extend Payment?
class Bitcoin extends Payment { } // ❌ ERROR: Bitcoin is NOT permitted!✔ Prevents unauthorized subclasses, improving security.
2️⃣ When should you use sealed, final, and non-sealed in a class hierarchy?
| Modifier | Use When… | Example |
|---|---|---|
sealed | You want to restrict which classes can extend a base class. | sealed class Animal permits Dog, Cat { } |
final | You want to prevent further extension of a subclass. | final class Dog extends Animal { } |
non-sealed | You want to allow unrestricted inheritance after a class is already sealed. | non-sealed class Cat extends Animal { } |
📌 Example: Choosing sealed, final, and non-sealed Wisely
sealed class Animal permits Dog, Cat { } // Only Dog & Cat are allowed
final class Dog extends Animal { } // ✅ No more extensions
non-sealed class Cat extends Animal { } // ✅ Other classes can extend Cat✔ Use sealed for control, final to prevent further inheritance, and non-sealed for flexibility.
3️⃣ What happens if a class tries to extend a sealed class without permission?
🚨 The Java compiler throws an error!
If a class that is not listed in permits tries to extend a sealed class, Java prevents compilation.
📌 Example: Illegal Extension of a Sealed Class
sealed class Vehicle permits Car, Bike { }
final class Car extends Vehicle { } // ✅ Allowed
final class Bike extends Vehicle { } // ✅ Allowed
class Truck extends Vehicle { } // ❌ ERROR: Truck is NOT permitted!✅ Compiler Error Message:
error: class Truck is not allowed to extend sealed class Vehicle✔ This ensures that only approved subclasses exist, improving maintainability.
🔍 Key Takeaways
✔ Sealed classes restrict inheritance, preventing unintended extensions.
✔ Use sealed to control subclassing, final to prevent further extension, and non-sealed for flexibility.
✔ If an unauthorized class tries to extend a sealed class, the compiler throws an error.
The next Java 16+ feature: Records (record keyword for immutable data classes) 😊🚀
