Java 17 Sealed Classes Explained with Examples

Java 17 introduced a new feature called sealed classes, which allows developers to restrict the classes that can extend or implement a particular class or interface. In this tutorial, we will explore the concept of sealed classes in Java 17, their benefits, and how to use them with code examples.

Introduction to Sealed Classes

Sealed classes are a new feature in Java 17 that allows developers to control the classes that can extend or implement a particular class or interface. This feature is useful when you want to restrict the classes that can extend a particular class or implement an interface. For example, you can use sealed classes to create a hierarchy of classes where only specific classes can extend a particular class.

Before diving into the details of sealed classes, it’s essential to have a good understanding of Java Algorithms and Java programming concepts.

Benefits of Sealed Classes

Sealed classes provide several benefits, including:

• Improved code security: By restricting the classes that can extend or implement a particular class or interface, you can prevent unauthorized access to sensitive data.
• Better code organization: Sealed classes allow you to create a hierarchy of classes where only specific classes can extend a particular class, making it easier to organize and maintain your code.
• Reduced errors: By limiting the classes that can extend or implement a particular class or interface, you can reduce the risk of errors and bugs in your code.

Declaring Sealed Classes

To declare a sealed class, you use the sealed keyword followed by the class name. For example:

public sealed class Vehicle {
    // class body
}

You can also declare a sealed interface using the sealed keyword. For example:

public sealed interface Printable {
    // interface body
}

Extending Sealed Classes

To extend a sealed class, you use the extends keyword followed by the name of the sealed class. For example:

public final class Car extends Vehicle {
    // class body
}

You can also extend a sealed interface using the implements keyword. For example:

public final class Document implements Printable {
    // class body
}

Permitted Subclasses

A sealed class can have one or more permitted subclasses, which are declared using the permits keyword. For example:

public sealed class Vehicle permits Car, Truck, Motorcycle {
    // class body
}

In this example, the Vehicle class has three permitted subclasses: Car, Truck, and Motorcycle.

Common Mistakes

When working with sealed classes, there are several common mistakes to avoid, including:

• Forgetting to declare the sealed keyword when declaring a sealed class or interface.
• Forgetting to declare the permits keyword when declaring permitted subclasses.
• Trying to extend a sealed class or implement a sealed interface without being a permitted subclass.

For more information on SOLID Design Principles in Java, which include the use of sealed classes, check out our tutorial.

Conclusion

In conclusion, sealed classes are a powerful feature in Java 17 that allows developers to control the classes that can extend or implement a particular class or interface. By using sealed classes, you can improve code security, better organize your code, and reduce errors. For more information on Java programming and related topics, check out our Java Interview Questions and Mastering SQL tutorials.