Explain the significance of the non-sealed keyword in Java.
Table of Contents
- Introduction
- Sealed Classes and Why
non-sealedMatters - How the
non-sealedKeyword Works - Practical Examples of
non-sealedKeyword - Conclusion
Introduction
The non-sealed keyword in Java was introduced with Java 17 as part of the enhancements to the object-oriented programming model. It works in conjunction with sealed classes, which were introduced to restrict which classes can extend or implement a given class or interface. The non-sealed keyword allows more flexibility in a controlled inheritance hierarchy by opening up a previously restricted class for extension by any class.
Understanding how non-sealed works within the context of sealed classes gives developers greater control over class hierarchies while balancing between tight control and flexibility.
Sealed Classes and Why non-sealed Matters
Sealed Classes in Java
Before diving into the non-sealed keyword, it's important to understand sealed classes. Sealed classes restrict which other classes or interfaces can extend them, helping developers design more predictable and controlled inheritance hierarchies. A sealed class explicitly declares which classes can extend it by using the permits clause.
Example of a sealed class:
In this example, only the Circle and Square classes are permitted to extend the Shape class.
Why non-sealed?
In some cases, you may want one subclass to continue restricting further extension (as sealed classes do), but another subclass to be open to any extensions. This is where the non-sealed keyword comes into play. By marking a subclass as non-sealed, you allow it to be extended freely, while other subclasses remain restricted.
The non-sealed keyword removes the restriction placed by sealed classes, creating more flexible inheritance hierarchies within a controlled structure.
How the non-sealed Keyword Works
The non-sealed keyword allows a class to be extended by any other class, even if it is part of a sealed hierarchy. This provides a balance where certain branches of a class hierarchy are restricted, while others remain open to extension.
Example of non-sealed Keyword Usage
In this example, Shape is sealed and only Circle, Square, and Triangle can extend it. However, Square is marked as non-sealed, meaning it can be extended by any other class, like ColoredSquare. Meanwhile, Circle remains sealed and cannot be extended further.
Practical Examples of non-sealed Keyword
Example 1: Allowing Specific Classes to Be Open for Extension
In a real-world scenario, you might have a class like Employee, where certain roles such as Manager should remain final, while others like Developer can be extended.
In this case, Manager is a final class, so it cannot be extended. However, Developer is marked as non-sealed, allowing other roles such as SeniorDeveloper to extend it freely.
Example 2: Mixing Sealed and Non-Sealed Hierarchies
In a banking system, you might want to seal high-level account types but allow variations in certain account types like savings or checking.
Here, BankAccount is sealed. CheckingAccount is non-sealed and can be extended by any other class, allowing further variations such as PremiumCheckingAccount. On the other hand, SavingsAccount is final, prohibiting any further extension.
Conclusion
The non-sealed keyword in Java adds flexibility to the strictness imposed by sealed classes, allowing you to open certain subclasses for further extension while keeping others restricted. It offers a controlled yet flexible approach to inheritance hierarchies, making it a useful tool for modern Java development. By strategically using non-sealed, developers can maintain a balance between strict design patterns and extendable systems.
With Java 17 and beyond, non-sealed enriches the ability to create more nuanced object-oriented designs, making it easier to control inheritance in a way that fits both security and flexibility requirements.