Quotes About Subclassing

For the sake of future evolution, it's better to disallow subclassing.

When objects first became popular, subclassing seemed like a magic pill. First, subclasses were used for classification—a Train was a subclass of Vehicle regardless of whether they shared any implementation. In time, some people saw that since what inheritance did was share implementation, it could most effectively be used to factor out common bits of implementation. Quickly, though, the limitations of subclassing became apparent.