Summary of Java Sealed Classes

What are Sealed Classes?

• Definition: Sealed classes in Java are a feature that allows you to control which classes can extend or implement them.
• Purpose: They provide a way to restrict the inheritance of classes, helping to create a more controlled and predictable class hierarchy.

Key Concepts

• Sealed Class: A class that cannot be subclassed freely. You can only allow specific classes to extend it.
• Permitted Subclasses: Classes that are allowed to extend a sealed class must be explicitly defined within the sealed class declaration.
• Non-Sealed Classes: A class declared as non-sealed can extend a sealed class but does not have any restrictions on further subclasses.
• Final Classes: A final class cannot be subclassed at all.

How to Declare a Sealed Class

To define a sealed class, you use the sealed keyword followed by the permits clause, which lists the permitted subclasses.

Example:

public sealed class Shape permits Circle, Rectangle {
    // Class implementation
}

public final class Circle extends Shape {
    // Class implementation
}

public non-sealed class Rectangle extends Shape {
    // Class implementation
}

public class Triangle extends Rectangle {
    // Allowed since Rectangle is non-sealed
}

Benefits of Sealed Classes

• Control: You have full control over the subclassing of your classes.
• Safety: Reduces the risk of unexpected behavior from subclasses that weren't intended to be created.
• Design Clarity: Makes the class design clearer, as the relationships between classes are explicitly defined.

Conclusion

Sealed classes are a powerful feature in Java that helps developers enforce rules around class inheritance. By using sealed classes, you can create more robust and maintainable code while preventing unwanted extensions of your classes.