A Philosophy of Software Design

by John Ousterhout

parent class and each of its subclasses. Class instance variables in the parent class are often accessed by both the parent and child classes; this results in information leakage between the classes in the inheritance hierarchy and makes it hard to modify one class in the hierarchy without looking at the others. For example, a developer making changes to the parent class may need to examine all of the subclasses to ensure that the changes don’t break anything. Similarly, if a subclass overrides a method in the parent class, the developer of the subclass may need to examine the implementation in the parent. In the worst case, programmers will need complete knowledge of the entire class hierarchy underneath the parent class in order to make changes to any of the classes. Class hierarchies that use implementation inheritance extensively tend to have high complexity. Thus, implementation inheritance should be used with caution. Before using implementation inheritance, consider whether an approach based on composition can provide the same benefits.