Why is `final` a bad word for classes in PHP packages?

• Understanding the final Keyword
• Arguments Against Using final in Packages
• Arguments for Using final

In object-oriented programming in PHP, the final keyword can be a polarizing topic among developers, especially when it comes to defining classes in reusable packages. While final can be useful for ensuring a class is not extended in a way that could break encapsulation or violate its intended usage, it can also introduce limitations that affect the flexibility and testability of a codebase. This article examines why final might be considered problematic for classes in PHP packages, highlighting the challenges and considerations from a software design perspective.

¶Understanding the final Keyword

In PHP, the final keyword can be used in two contexts: to prevent a class from being extended (inherited from), and to prevent a method from being overridden. When a class is defined as final, it cannot be subclassed:

final class MyFinalClass {
    // Class implementation
}

Attempting to extend this class would result in a fatal error.

¶Arguments Against Using final in Packages

1. Inhibits Extensibility

One of the core principles of object-oriented design is the ability to extend existing code without modifying it. This is particularly important in distributed packages where the source code isn't directly controlled by the end user. When a package developer marks a class as final, they prevent other developers from extending these classes to add or modify functionality, potentially limiting the package's usefulness in scenarios not foreseen by its original authors.

2. Complicates Testing

Testing can become more challenging when a class is marked as final. For instance, developers often use inheritance to mock or override methods of a class during unit testing. By preventing extension, final can force developers to use more complex or less intuitive methods for testing the behavior of classes that interact with the final classes.

3. Discourages Contribution

In the open-source community, where contributions and forks are common, final classes can be seen as a barrier. If developers feel that they cannot easily extend a package to fit their needs, they may choose to fork the repository or look for alternative solutions, potentially fragmenting the community and diluting the pool of contributors.

4. Evolution and Refactoring

Software needs evolve over time, and what might seem like a good use of final today may not hold in the future. Developers maintaining a package might find themselves needing to refactor or remove the final keyword to accommodate new use cases, which could break backward compatibility.

¶Arguments for Using final

Despite these challenges, there are valid reasons why package developers might choose to use final:

1. Protecting the Integrity of the Code

final ensures that the core functionality of a class cannot be altered through inheritance, protecting the intended behavior of the class against modifications that could introduce bugs or security vulnerabilities.

2. Encouraging Composition Over Inheritance

Many software design experts advocate for composition over inheritance as a more flexible and less error-prone approach to extending functionality. By marking classes as final, developers encourage users to use composition instead of inheritance, potentially leading to better software design patterns.

3. Performance Considerations

In some cases, using final can lead to minor performance optimizations. PHP engines can make certain assumptions about final classes and methods that allow for more efficient execution.

The use of final in PHP classes within packages is a nuanced decision that involves weighing the benefits of protecting class integrity against the potential downsides of reduced flexibility and extensibility. Package developers should consider the specific needs and usage scenarios of their libraries, possibly providing extensive justifications in their documentation when they decide to use final. Meanwhile, consumers of these packages should be aware of these limitations and plan their architecture and testing strategies accordingly. In the end, whether to use final should be guided by a careful consideration of both design principles and practical implications.