Diamond Problem Interview Questions

The diamond problem is multiple class inheritance: two parents define the same method and the child does not know which one to run. Java and C# allow multiple interfaces but only one class parent, so that class diamond cannot happen. If two interfaces have default methods with the same signature, Java makes me override and pick. C# has explicit interface implementation for similar conflicts. Python allows multiple class inheritance and uses C3 MRO so the order is at least deterministic. I still would not design a diamond on purpose.

Two interfaces with the same default method force the class to override and pick. I can call I1.super.method(). Diamond of behavior, not fields. Default methods did not fully kill the diamond.

Virtual inheritance so there is one shared A base. Without it D has two A subobjects. I would still rather compose in application C++. For Java interviews I mention C++ chose a harder tool and Java chose a simpler rule.

Almost never in app code. If two interfaces conflict I override explicitly. Designing a diamond UML for a domain model is a smell. The problem is named so we avoid the shape.

Class MI copies two implementations and two states. Interface MI copies contracts, typically no fields. Java allows the second. The first is the dangerous one. The key word is state.