The blog post "Inheritance and Subtyping" argues that inheritance and subtyping are distinct concepts often conflated, leading to inflexible and brittle code. Inheritance, a mechanism for code reuse, creates a tight coupling between classes, whereas subtyping, focused on behavioral compatibility, allows substitutability. The author advocates for composition over inheritance, suggesting interfaces and delegation as preferred alternatives for achieving polymorphism and code reuse. This approach promotes looser coupling, increased flexibility, and easier maintainability, ultimately leading to more robust and adaptable software design.
Nicolas Frankel's blog post, "On Inheritance," delves into the nuanced relationship between inheritance and subtyping, arguing that while often conflated, they are distinct concepts that should be treated as such. Frankel begins by establishing the common understanding of inheritance as a mechanism for code reuse, where a subclass inherits properties and methods from its superclass, thereby avoiding redundant code. He acknowledges the convenience this provides, especially in object-oriented programming. However, he proceeds to dissect the potential pitfalls of overusing inheritance for this purpose.
The core argument revolves around the Liskov Substitution Principle (LSP), a cornerstone of solid object-oriented design. The LSP dictates that objects of a subtype should be substitutable for objects of their supertype without altering the correctness of the program. Frankel emphasizes that adherence to the LSP is crucial for maintaining predictable and reliable software. He illustrates how inheritance solely for code reuse can easily violate the LSP, leading to unexpected behavior and difficult-to-maintain code. Examples are provided demonstrating how seemingly innocuous modifications in a subclass, driven by the desire to reuse code, can inadvertently break the contract established by the superclass, thereby violating the LSP.
Frankel then introduces the concept of subtyping, contrasting it with inheritance. He posits that subtyping is fundamentally about establishing an "is-a" relationship, where the subtype truly represents a specialized version of the supertype, adhering fully to the supertype's contract. This implies that any behavior expected of the supertype is also guaranteed by the subtype. He argues that while inheritance can be a mechanism for achieving subtyping, it's not the only mechanism, nor does it guarantee subtyping. Other methods, such as composition and interface implementation, can achieve subtyping without the tight coupling inherent in inheritance.
The blog post further explores the implications of violating the LSP, highlighting the fragility and brittleness it introduces into a codebase. Changes in one part of the system can ripple through seemingly unrelated parts due to the tight coupling created by improper use of inheritance. This fragility hinders maintainability and extensibility, increasing the risk of introducing bugs and making future modifications more complex and time-consuming.
Frankel concludes by advocating for a more thoughtful approach to inheritance, urging developers to prioritize subtyping and adherence to the LSP over the mere convenience of code reuse. He suggests considering composition and interfaces as alternative mechanisms for achieving code reuse and polymorphism while maintaining the integrity of the type system. The post ultimately champions a design philosophy where inheritance is reserved for situations where a true "is-a" relationship exists, thereby ensuring robust, maintainable, and predictable software.
Summary of Comments ( 17 )
https://news.ycombinator.com/item?id=42870216
Hacker News users generally agree with the author's premise that inheritance is often misused, especially when subtyping isn't the goal. Several commenters point out that composition and interfaces are generally preferable, offering greater flexibility and avoiding the tight coupling inherent in inheritance. One commenter highlights the "fragile base class problem," where changes in a parent class can unexpectedly break child classes. Others discuss the nuances of Liskov Substitution Principle and how it relates to proper inheritance usage. One user specifically calls out Java's overuse of inheritance, citing the infamous
AbstractSingletonProxyFactoryBean. A few dissenting opinions mention that inheritance can be a useful tool when used judiciously, especially in domains like game development where hierarchical relationships are naturally occurring.The Hacker News post titled "Inheritance and Subtyping," linking to Nicolas Fränkel's blog post on the same topic, has generated a moderate discussion with several insightful comments. Many of the comments revolve around the nuances of inheritance, subtyping, and composition, and their practical implications in software design.
One compelling comment thread delves into the Liskov Substitution Principle (LSP) and its importance in ensuring that inheritance is used correctly. Commenters discuss the challenges of adhering to the LSP in real-world scenarios, particularly when dealing with mutable objects and side effects. They highlight the importance of careful consideration when overriding methods and the potential pitfalls of violating the principle, such as unexpected behavior and difficult-to-debug issues. The discussion also touches on the trade-offs between inheritance and composition, suggesting that composition can be a more robust and flexible approach in many cases.
Another commenter emphasizes the distinction between "is-a" and "has-a" relationships, which are often conflated when discussing inheritance. They argue that inheritance should strictly represent an "is-a" relationship, where the subclass is a true subtype of the superclass, fulfilling all the contracts of the parent class. Using inheritance for code reuse when a "has-a" relationship is more appropriate can lead to brittle and difficult-to-maintain code.
Several comments explore the idea that inheritance is often misused or overused, leading to complex and tightly coupled systems. They advocate for favoring composition over inheritance as a general principle, arguing that composition provides greater flexibility, modularity, and testability. This perspective aligns with the blog post's argument that inheritance should be used judiciously and only when it truly represents a subtype relationship.
One commenter offers a practical example of the challenges of inheritance, describing a scenario where a subclass needs to modify the behavior of a superclass method in a way that violates the LSP. They discuss potential workarounds and highlight the complexity that can arise from such situations.
There's also a discussion on the role of interfaces and abstract classes in facilitating polymorphism and decoupling. Commenters suggest that interfaces, by defining contracts without implementation details, can be a more effective way to achieve polymorphism than inheritance. Abstract classes can be useful for providing default implementations, but should still be used with caution.
Overall, the comments on the Hacker News post reflect a nuanced understanding of the complexities of inheritance and subtyping. They offer valuable insights into the challenges and best practices associated with these concepts, and provide a platform for a thoughtful discussion on their appropriate use in software design. They generally agree with the author's premise that inheritance should be used sparingly and with careful consideration of the LSP and the "is-a" relationship.