Multiple inheritance

We inevitably had to mention multiple inheritance in the previous sections due to the fact that we can mix multiple traits and they can all have their own implementations of the methods. Multiple inheritance is not only a powerful technique, but also a dangerous one, and some languages such as Java have decided to not even allow it. As we already saw, Scala allows this, but with some limitations. In this subsection, we will present the problems of multiple inheritance and show how Scala deals with them.

The diamond problem

Multiple inheritance suffers from the diamond problem.

Let's have a look at the following diagram:

Here, both B and C extend A and then D extends B and C. Some ambiguities might arise from this. Let's say that there was a method that was originally defined in A, but both B and C override it. What would happen if D calls this method? Which one will it exactly call?

All the preceding questions make things ambiguous and this could lead to mistakes...