What does the 'final' keyword imply when applied to a class in Java?

When the 'final' keyword is applied to a class in Java, it signifies that the class cannot be subclassed. This means that no other class can extend the final class, preventing the creation of any subclasses that could alter or extend its behavior. This is particularly useful when designing classes that are meant to be immutable or when you want to ensure that certain functionalities remain unchanged.

By marking a class as final, the programmer can ensure the integrity of the class's implementation and maintain a consistent interface, which is critical in scenarios where stability and security are priorities. For instance, the Java standard library utilizes final classes (such as java.lang.String) for these reasons.

In contrast, other options suggest various behaviors that are not applicable when a class is declared as final, such as allowing inheritance or implying the existence of abstract methods or constructors, which are not inherently affected by the final keyword.