Sealed classes are a powerful feature introduced in Java 15 as a preview, and solidified in Java 17. They allow you to define a class with a limited set of subclasses.

This concept tackles the common issue of uncontrolled inheritance, where any class can extend any other class, potentially leading to unmanageable code.

By sealing a class, you can ensure that only specific known subclasses can extend it, enhancing both maintainability and security in your codebase.

Let's dive into the world of sealed classes and explore their syntax, practical use cases, and nuances.

Sealed Class Basics

A sealed class is defined using the sealed keyword. This designation indicates that the class can only be subclassed by specified classes or interfaces. To declare a sealed class in Java, you also need to specify which classes are allowed to extend it using the permits clause.

Here's a basic example:

In this example, Shape is a sealed class that can only be extended by the Circle and Rectangle classes.

Why Use Sealed Classes?

Creating Sealed Classes

Let’s dig deeper into how you can create and utilize sealed classes effectively.

Basic Example of Sealed Class with Subclasses

Here’s a more detailed example illustrating how to define a sealed class and its subclasses:

In this example, Vehicle is a sealed class with Car and Truck as its permitted subclasses. Notice how both subclasses are declared as final. This means they cannot be further extended, which is a common practice in sealed class hierarchies to maintain control.

Permitted Subclasses

Permitted subclasses can be:

Here is how you might implement all three options:

The permits Clause

The permits clause clearly shows which classes can extend the sealed class. This explicit relationship is crucial for understanding the code structure.

Real-World Applications

Sealed classes shine in scenarios where you want to control the types of objects that can be created. Here are a few real-world applications:

Domain Modeling

In domain-driven design, sealed classes can represent a finite set of types. For example, consider an application that processes different payment methods:

By using sealed classes, you ensure that all payment types are predefined, reducing the risk of errors introduced by unexpected implementations.

State Machines

Sealed classes can also model state machines effectively. Each state can be a subclass that knows how to handle transitions:

This approach clarifies which states exist and prevents unauthorized states from being introduced.

Working with Sealed Interfaces

Just like classes, you can also create sealed interfaces. This can be useful in scenarios where you want to define a contract with a limited set of implementations.

Example of a Sealed Interface

Benefits of Sealed Interfaces

Edge Cases and Nuances

Working with sealed classes can bring some complexities. It’s essential to be aware of edge cases that can cause confusion or errors.

Nested Sealed Classes

You can have sealed classes nested within other classes. However, the outer class must also follow the rules of sealing if it’s declared as sealed.

Non-Subclassable Sealed Classes

If a sealed class is defined but has no permitted subclasses, it cannot be instantiated directly, which might lead to confusion.

Reflection and Sealed Classes

Using reflection with sealed classes can be tricky, as it may expose all subclasses, potentially violating the intent of sealing. When using reflection, ensure you maintain control over what is exposed.

Summary and Best Practices

Sealed classes and interfaces provide a robust way to create controlled hierarchies in your Java applications. Here are some best practices to keep in mind:

Sealed classes are a game-changer for managing class hierarchies effectively. They not only improve the design of your application but also enhance maintainability and readability.