Quiz yourself: Controlling the number of instances of a class

Which is the best approach: singletons, sealed classes, abstract classes, final classes, or enums?

Imagine that your design requires exactly four instances of a particular class.

Which approach best provides for this need? Choose one.

A. Singletons

B. Sealed classes

C. Abstract classes

D. Final classes

E. Enums

Answer. Option A is incorrect because singleton is not a Java keyword, nor is it a concept defined in the Java Language Specification. Singleton is actually the name of a design pattern and as a design pattern, it can be implemented in several different ways.

From the perspective of this question, the most important characteristic of a singleton is that it limits the number of instances of its type to one per JVM. The quiz question simply requires that the number of instances be fixed; it does not require that number to be one, and as such, an approach is necessary that allows the instance count to be greater than one. Consequently, option A is incorrect.

Option B is also incorrect. Sealed classes were added in Java 17, and the feature’s goal is to fix a set of types that exist as specializations of a particular type. For example, you could specify that a class RoadTransport is permitted to have only subtypes Car, Truck, Bus, Motorcycle, and Bicycle. Note that the sealed classes feature does not limit the number of instances of these classes that can exist.

In addition, option C is incorrect. Abstract classes are a mechanism to capture the common functionality that’s shared by several types in a single class, and they require developers to add specific additional functionality to subclasses. As a side note, an abstract class can be sealed, but as before, this does not have an impact on the number of instances you can create.

Option D is also incorrect. The final modifier, when applied to a class, means you cannot subclass that class. However, the modifier does not imply any control on the number of instances.

Option E is correct, because an enum type provides a clear, language-supported solution to control the number of instances of a class. There are some limitations on enum types; notably, the parent class of an enum is always java.lang.Enum, though an enum can implement interfaces of your choosing.

A second limitation is that an enum is always a final class and cannot be subclassed. This is important, since instances of a subclass are also instances of their parents, and if subclassing were possible, those subclasses would break the contract of a fixed number of instances.

It’s worth mentioning that a programmer can tightly control the construction of any class, and with it the number of instances of such a class, by marking all the constructors of that class as private and providing access to instances through a factory method. In fact, this idea is essential to the implementation of both Java’s enum type and the Singleton design pattern. However, because this approach is not an option for this question, it’s not a correct answer here.

Conclusion. The correct answer is option E.

Source: oracle.com