How to Solve Parallel Inheritance Hierarchies when we try to reuse code through inheritance

Question

Recently in a job interview, they ask me "how to solve Parallel Inheritance Hierarchies when we try to reuse code through inheritance". I thought on Aggregation or Composition, but i was a little confused on making an example based on that.

So I decided to leave it pending to deepen concepts later, but after investigating it did not end up forming a precise answer to that question, could someone explain me a solution or an example to this?

Answer 1

The c2 wiki has a page on parallel inheritance hierarchies where ChaoKuoLin lists four possible solutions. I paraphrase them here, along with some context for each. See the original page for a full explanation including advantages, disadvantages, and examples.

1. Keep the parallel hierarchies.
   • When the hierarchies have separate responsibilities and each contains many methods.
   • When maximum flexibility is required.
2. Keep one of the hierarchies and collapse the other into a class.
   • When one of the hierarchies can be reduced to a single class, for example by moving some methods to the other hierarchy.
   • When one of the hierarchies contains few methods.
3. Collapse the two hierarchies into one.
   • When the hierarchies have similar responsibilities and each contains few methods.
4. Keep a partial parallel hierarchy with the rest collapsed.
   • When you want a middle ground among the previous solutions.

Another solution suggested in the wiki is Mix In and it is also suggested in How to solve parallel Inheritance in UI case?