Object-Oriented Programming (OOP) is a programming paradigm centered around the concept of "objects", which encapsulate both state and behaviors. It has become a fundamental approach in software development, offering a more intuitive way to model complex systems, but despite its widespread adoption, OOP can present challenges: tight coupling, complex inheritance chains, and difficulties in modifying or extending code are some of the issues commonly associated with this paradigm. Tight coupling in Object-Oriented Programming (OOP) presents both technical and non-technical challenges that impact the development, maintenance, and scalability of software systems. From a technical perspective, tight coupling leads to a lack of modularization within the codebase. When classes or modules are tightly coupled, they become highly dependent on each other’s implementations, making it difficult to isolate and modify individual components without affecting the entire system. This increases the risk of introducing bugs or unintended side effects when making changes, as developers must navigate complex interdependencies that span across different parts of the codebase. As a result, the development process becomes more error-prone, time-consuming, and resource-intensive. Furthermore, tight coupling inhibits code reusability and extensibility. Since tightly coupled components are closely intertwined, they cannot be easily reused in other contexts or extended to accommodate new requirements without significant modifications. This limits the flexibility of the software system and hinders its ability to evolve over time. Developers may find themselves duplicating code or implementing workarounds to circumvent tight coupling, leading to code bloat, decreased maintainability, and reduced overall code quality. On a non-technical level, tight coupling can also have organizational and collaborative implications. Teams working on tightly coupled codebases may face challenges in coordinating their efforts and collaborating effectively. The complexity introduced by tight coupling can make it difficult for team members to understand and reason about each other’s code, leading to communication barriers and decreased productivity. Moreover, as the codebase grows larger and more intertwined, onboarding new team members becomes a daunting task, as they must familiarize themselves with the intricate relationships between various components. Addressing these challenges often involves implementing design patterns and best practices that promote modularity, reduce coupling, and simplify maintenance.

In this paper, we introduce Puzzle Pattern: an innovative approach that complements existing software design patterns for addressing challenges in Object-Oriented Programming (OOP). While solutions like Dependency Injection pattern[1], Observer pattern[2], Strategy pattern[2], and Composition over Inheritance[3] provide effective means to mitigate issues such as tight coupling, inheritance complexity, and encapsulation, the Puzzle Pattern offers a unique perspective on extreme modularity and code organization. Whereas traditional OOP often relies on concrete classes to represent functionality, the Puzzle Pattern advocates for writing code exclusively in interfaces. By doing so, it promotes a level of abstraction that goes beyond what other patterns typically offer. Rather than implementing functionality directly in classes, developers define interfaces that represent individual puzzle pieces of functionality. These interfaces serve as blueprints for concrete classes, which are then assembled through composition or dependency injection. This approach significantly reduces coupling by decoupling the implementation details from the interface definitions. It fosters a modular design where each puzzle piece can be developed, tested, and maintained independently. Moreover, the Puzzle Pattern promotes conceptual clarity by encouraging developers to focus on defining clear and cohesive interfaces, thus facilitating better communication and understanding among team members. By embracing extreme modularity and favoring interfaces over concrete classes, the Puzzle Pattern introduces a fresh perspective on software construction. It aligns well with the principles of Dependency Injection and Composition over Inheritance by promoting loose coupling and flexibility. Additionally, it complements patterns like the Observer pattern and Strategy pattern by providing a framework for organizing and assembling disparate pieces of functionality in a cohesive manner. The highlighted pattern presents numerous benefits within software development. Its focal point on extreme modularity by exclusively composing code in interfaces fosters a clearer division of responsibilities and simplifies upkeep. Leveraging multiple inheritance enables a class’s behavior to amalgamate from various interfaces, thereby augmenting adaptability in system architecture. Moreover, the pattern upholds the SOLID principles[4], championing a sturdy and expandable framework. Nevertheless, akin to any methodology, it brings forth its set of hurdles. Relying on multiple inheritance might introduce intricacy, necessitating vigilant management of shared states among interfaces. Furthermore, while striving for compatibility with conventional Object-Oriented methods, teams accustomed to more orthodox practices may encounter a learning curve.

The rest of this paper is structured as follows. After the introduction, in section 2, we want to provide a high-level overview of the current state of the art of software design patterns, giving an overview of the main OOP problems that these methodologies solve. In section 3, subsequently, we conceptualize the Puzzle Pattern by defining the concepts on which it is based and analyzing its conformity to the software development principles considered to be the most common best practices. Moreover, in section 4 a use case of application of the Puzzle Pattern is described, analyzing the design and structuring aspect of the source code, discussing the technical-functional aspects in detail. Finally, section 5 discusses the results of the case study, highlighting the advantages and disadvantages of the Puzzle Pattern compared to other design approaches.

Software design patterns play a crucial role in addressing issues such as tight coupling in Object-Oriented Programming (OOP) by providing proven solutions to common problems encountered in software development. These patterns encapsulate best practices, design principles, and architectural guidelines that promote modularity, flexibility, and maintainability in software systems. One of the primary ways design patterns mitigate tight coupling is by promoting loose coupling between components. Furthermore, design patterns facilitate the creation of flexible and extensible software architectures by promoting principles such as encapsulation, abstraction, and separation of concerns, providing a common language and set of solutions that facilitate communication and collaboration among developers. By following established patterns, developers can convey design decisions, communicate intentions, and share knowledge more effectively. This fosters a culture of best practices and design consistency within development teams, leading to higher code quality, improved maintainability, and enhanced productivity. The biggest impact in this field came from the Gang of Four (Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides) software patterns[1] in the 1990s. The GoF introduced 23 design patterns categorized into creational, structural, and behavioral patterns. For example, the Dependency Injection pattern decouples classes by externalizing their dependencies, allowing them to be provided from external sources rather than being instantiated internally. This reduces the direct dependencies between classes, making them more modular and easier to test, maintain, and extend, as dependencies can be replaced or mocked as needed. This pattern promotes the principle of "programming to interfaces, not implementations," thereby reducing coupling and enhancing flexibility. Similarly, the Observer pattern decouples subjects and observers, enabling them to communicate without being directly dependent on each other’s implementations. In this pattern, objects (observers) subscribe to changes in another object (subject) and are notified whenever there is a state change. By decoupling the observing objects from the subject, changes in one component do not directly impact others, promoting a more loosely coupled architecture. The Strategy pattern provides a way to encapsulate interchangeable algorithms within their own classes, allowing clients to switch between different algorithms dynamically without modifying client code. This promotes code reuse, modularity, and extensibility, as individual algorithms can be developed, tested, and maintained independently.

Beyond tight coupling, another challenge in OOP is managing inheritance hierarchies. While inheritance can facilitate code reuse and promote a hierarchical structure, it can also lead to overly complex and fragile designs, known as the "fragile base class problem". As systems evolve, changes in base classes can inadvertently affect derived classes, potentially introducing bugs and making maintenance difficult. To address this issue, the Composition over Inheritance principle advocates for favoring object composition over class inheritance, enabling developers to compose objects dynamically at runtime rather than relying on static inheritance relationships. This promotes a more modular and composable design, mitigating the risks associated with inheritance hierarchies and tight coupling. By composing objects dynamically at runtime rather than relying on static inheritance relationships, developers can achieve greater flexibility and resilience to change. This approach promotes a more modular and composable design, mitigating the risks associated with inheritance hierarchies. In terms of encapsulation, while OOP provides mechanisms such as access modifiers (e.g., public, private, protected), achieving true encapsulation can be challenging. Objects often have access to each other’s internal state, leading to tight coupling and potential unintended side effects. To mitigate this, the Principle of Least Privilege suggests restricting access to the bare minimum necessary for functionality, thereby reducing the risk of unintended interactions and enhancing encapsulation.

A puzzle consists of two main elements: a board and a set of pieces. The pieces are assembled on the board and together form a complex design. However, if we just look at the individual pieces, we will notice that each of them encloses only a small part of the picture. Considering this analogy, to which the pattern is named, we can associate the puzzle pieces with the interfaces, and the puzzle board with the concrete class. A concrete class, a puzzle board, takes on a definite meaning only when all the pieces are in place, but each piece is in its own way independent of the puzzle board itself. The idea behind the Puzzle Pattern is to develop code within interfaces (the puzzle pieces), and to assemble concrete classes (the puzzle boards) by implementing interfaces. Each interface is intended to implement a behavior, and to declare abstract all the requirements that need to be met in order to realize the developed behavior. The requirements are intended to be provided by the concrete class. In other words, each abstract method need only return the instance that can provide the functionality required in the concrete method. The behavior is given by the interface itself, which exploits the state implemented by the class that implements the interface itself. In the UML class diagram below, the class C is composed by the union of I and J: $C=I\cup J$. The behavior of class C is determined by a combination of interfaces I and J facilitated through the use of behavioral multiple inheritance. The only task required of class C is to ensure that the appropriate state is made available to both interfaces I and J.

The fundamental objectives of Puzzle Pattern are clearly outlined in order to guide the developer in achieving specific results and improvements over conventional approaches. Key objectives include modularity, flexibility, loose coupling, conceptual clarity, gradual project growth, and maintaining compatibility with the traditional OO approach. The primary goal is to promote extreme modularity in the software development process. By focusing on writing code exclusively in interfaces, the aim is to reduce the degree of dependency between concrete classes, thus making it easier to manage and maintain the system. Allowing flexible assembly of concrete classes by implementing interfaces provides a greatly enhanced level of adaptability. Developers can compose classes according to specific project requirements, allowing precise customization without having to modify the source code of existing classes. Reducing the degree of coupling between concrete classes is one of the central goals. Minimizing dependencies between system components improves maintainability, testability, and ease of software evolution over time. Improve conceptual clarity in software design. Focusing on writing code in interfaces attempts to more accurately reflect the conceptual structure of the system, facilitating understanding and collaboration among development team members. Fostering the scalability of the system is a key objective. The modular structure of the pattern should allow for smooth and gradual growth of the system, enabling smooth management of even continuously evolving projects. Ensuring compatibility with the traditional Object-Oriented approach is an important aspect. The proposed pattern should integrate smoothly with existing code written according to OO conventions, allowing a seamless transition. These goals have been outlined with the aim of addressing specific challenges encountered in software projects and introducing a paradigm that can contribute significantly to the improvement of software design and maintenance.

In the following section, as a result the application of the above Puzzle Pattern in the common case of CRUD[8] scenario implementation is demonstrated. The CRUD operations are a perfect example case for the application of the Puzzle Pattern. With "CRUD" operations here we intend the basic Create, Read, Update and Delete operations that are executed on the database side in order to update a logical entity state. The type of the database does not impact the pattern adoption. Let’s visualize an analogy between the CRUD scenario and the Puzzle Pattern main concepts: boards and pieces. In order to fulfill the implementation, we need a board and four pieces (create, read, update and delete). The board is the concrete class that connects all the pieces together, forming a complex entity, while the pieces are atomic functionalities. In order to describe the pattern application, we base our exposition on top of a simplistic Person Java class (Appendix A).

We start defining the CRUD puzzle pieces, whose aim is to handle the data-layer operations of the generic type "T". The first piece where we start from is C. C is responsible for the creation of a new entity on an underlying data-layer. Let’s assume that the "R" repository includes an "insert()" method. This method takes an instance of "T" as an argument and, upon the creation activity, returns the updated instance of "T" (appendix A.2). We define the "create()" method in "C" by calling "insert()" on the data-layer. Then, let’s define "R", that is responsible for the data-reading operations. Basically, we use "R" piece to fetch data from the data-layer. Assume that the generic type "T" possesses a method "getId()" that can return an object of type "ID". Let us also assume that the repository "R" possesses a method "find()" that receives as an argument an instance of "ID", returning "T" if present (appendix A.3). We define the "read" method in "R" by calling "insert()" on the data-layer. Let’s move along with U. Let’s also presume that the "R" repository features an "update()" method, wherein it takes an instance of "T" as a parameter. After the update operation, it returns the modified instance of "T" (appendix A.4), updating the "T" instance state on the data-layer. We define the "update" method in "U" by calling its homonym method on the data-layer. Finally, let’s assume that the "R" repository possesses a method "deleteById()" that receives as an argument an instance of "ID", softly deleting the related entity from the data layer (appendix A.5). We define the "delete" method in "D" by calling "deleteById()" on the data-layer. The Puzzle Pattern can now be applied by creating the CRUD class - the puzzle board. The action needed here is to assemble the board by implementing all the CRUD pieces mentioned before (appendix A.6). To cover the class with unit tests, it is sufficient to add a unit test for each interface functionality and add a unique one at the CRUD class level to verify that the returned repository instance conforms to expectations. After assembling the concrete CRUD class for the Person class with the "ID" of type Long, a similar process could be applied for another class C2 by implementing the same interfaces, provided that C2 also includes the "getId()" method returning a result of type Long. Moreover, it would be easily possible to make hybrid classes that implement only some of the CRUD functions (maybe just the create and delete).

The presented pattern offers several advantages in software development. Its emphasis on extreme modularity through writing code exclusively in interfaces promotes a clearer separation of concerns and facilitates maintenance. The use of multiple inheritance allows the behavior of a class to be defined by a combination of interfaces, enhancing flexibility in system design. Additionally, the pattern adheres to the SOLID principles, promoting a robust and scalable architecture. However, like any approach, it comes with its challenges. The reliance on multiple inheritance may introduce complexity, and developers need to carefully manage the state shared among interfaces. Furthermore, while the pattern aims for compatibility with the traditional Object-Oriented approach, there may be a learning curve for teams accustomed to more conventional practices. In summary, the pattern provides a promising avenue for modular and scalable software design, but its successful implementation requires careful consideration of its complexities and potential learning curve.

In closing, the proposed pattern represents an innovative response to software design challenges by synergistically integrating SOLID principles with a modular approach in the context of CRUD operations. Practical application of this pattern in the example of CRUD operations demonstrated its effectiveness in promoting modularity, flexibility, and conceptual clarity. Adherence to SOLID principles was evidenced through the clear separation of responsibilities in interfaces, enabling efficient management of CRUD operations. Extreme modularity was manifested in the way concrete classes are assembled through the implementation of atomic interfaces, ensuring conceptual clarity and gradual evolution of the system. The application of the pattern to CRUD operations has shown that greater flexibility in software design can be achieved, allowing specific, customized services to be built without affecting pre-existing code. This modular approach can be particularly advantageous in scenarios where system requirements can change rapidly, allowing rapid adaptability to new demands. The presented pattern emerges as a valuable resource for developers seeking to improve maintainability, flexibility, and conceptual clarity in their software projects. Its adherence to SOLID principles, combined with clear practical application in CRUD operations, positions it as a significant contribution to the field of modular and sustainable software development.