The "diamond problem" $($sometimes referred to as the "Deadly Diamond of Death"$[6])$ is an ambiguity that arises when two classes B and C inherit from A$,$ and class D inherits from both B and C$.$ If there is a method in A that B and C have overridden, and D does not override it$,$ then which version of the method does D inherit: that of B$,$ or that of C$?$

For example, in the context of GUI software development, a class Button may inherit from both classes Rectangle $($for appearance$)$ and Clickable $($for functionality$/$input handling$),$ and classes Rectangle and Clickable both inherit from the Object class. Now if the equals method is called for a Button object and there is no such method in the Button class but there is an overridden equals method in Rectangle or Clickable $($or both$),$ which method should be eventually called?

It is called the "diamond problem" because of the shape of the class inheritance diagram in this situation. In this case, class A is at the top, both B and C separately beneath it$,$ and D joins the two together at the bottom to form a diamond shape.