Understanding 'Final' Classes in Java

Final classes in Java are a type of class that cannot be extended by any other classes. This unique characteristic makes them an invaluable tool in certain circumstances, and having a clear understanding of how they operate can help enhance your Java programming skills.

This concept in Java is defined by the keyword 'final'. When a class is declared as final, it tells the Java compiler that the class is complete and any attempts to subclass or extend it will result in a compilation error. This is distinctly different from a regular class in Java, which can freely be extended by other classes to inherit its properties and methods.

For instance, if we have a final class called 'FinalClass', and we attempt to extend it with another class 'ExtendedClass', the following code will throw an error:

final class FinalClass {
    // class code
}

class ExtendedClass extends FinalClass {
    // class code
}

The code above will result in a compile-time error, indicating that a final class cannot be extended.

It's important to note that a common misconception is that final classes cannot be instantiated. This is incorrect - final classes can be instantiated just like regular classes, but they simply cannot be extended.

Final classes are most often used to ensure the immutability and security of a class. For example, the String class in Java is final because it needs to be immutable, reliable, and secure. If the String class was not final, it could be extended and potentially modified in a way that compromises these attributes.

Keep in mind that while final classes cannot be extended, they can extend other classes. Also, they can contain both final and non-final methods. 'Final' applied to method means that method cannot be overridden by subclasses.

In conclusion, final classes in Java offer a means of security and control over class immutability. They should be used conscientiously, when it's desired to prevent a class from being subclassed. While they can limit the flexibility of a class hierarchy, they can also significantly enhance the robustness of your code if used correctly.