What is the diamond problem? Does it exist in Java? If so, how can it be avoided?


Taking a look at the graphic below helps in explaining the diamond problem.

In the diagram above, we have 2 classes B and C that derive from the same class – which would be class A in the diagram above. We also have class D that derives from both B and C by using multiple inheritance. You can see in the figure above that the classes essentially form the shape of a diamond – which is why this problem is called the diamond problem.

The problem with having an inheritance hierarchy like the one shown in the diagram above is that when we instantiate an object of class D, any calls to method definitions in class A will be ambiguous – because it’s not sure whether to call the version of the method derived from class B or class C.

Java does not have multiple inheritance

But, wait one second. Java does not have multiple inheritance! This means that Java is not at risk of suffering the consequences of the diamond problem. However, C++ does have multiple inheritance, and if you want to read more about the diamond problem in C++, check this out:
Diamond problem in C++.

Java does have interfaces

Java has interfaces which do allow it to mimic multiple inheritance. Although interfaces give us something similar to multiple inheritance, the implementation of those interfaces is singly (as opposed to multiple) inherited. This means that problems like the diamond problem – in which the compiler is confused as to which method to use – will not occur in Java.

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