In Java object oriented programming, what is the “diamond problem” in multiple inheritance and why is it considered an ambiguity issue?

Introduction / Context:
The “diamond problem” is a classic object oriented programming issue, most often discussed in the context of multiple inheritance. In Java interviews, this topic is important because it explains why Java does not allow multiple inheritance of classes, but still allows multiple inheritance of interfaces. Understanding this concept helps you understand Java design decisions and how method resolution works when you have complex inheritance hierarchies.

Given Data / Assumptions:

• We are working in an object oriented programming environment.
• There is multiple inheritance of classes in the general concept, even though Java itself does not support it for classes.
• A single class can conceptually inherit from two parent classes that share a common ancestor.
• The same method name and signature can exist in more than one parent class.

Concept / Approach:
The diamond problem gets its name from the shape of the inheritance diagram. One base class is at the top, two intermediate classes inherit from it, and then a single class inherits from both of those intermediate classes. This creates a diamond shape. The problem appears when all of these classes define the same method. The compiler or runtime then faces ambiguity: which implementation should be chosen when the bottom class calls that method? Java avoids this ambiguity for classes by disallowing multiple inheritance of classes, while still providing interfaces and default methods with clear rules for conflict resolution.

Step-by-Step Solution:
1. Imagine a base class A that defines a method display(). 2. Two subclasses B and C extend A and both override display(). 3. Now imagine a class D that extends both B and C (this is allowed in some languages, but not in Java for classes). 4. When D calls display(), the question arises: should it use B's implementation or C's implementation? 5. This conflict is the diamond problem: an ambiguity in method resolution caused by multiple inheritance of classes.

Verification / Alternative check:
Some languages such as C++ handle the diamond problem with features like virtual inheritance, which ensure that only one base class instance is shared. Java instead chooses a simpler model: it completely disallows multiple inheritance of classes so that a class cannot extend more than one concrete or abstract class. For interfaces with default methods, Java defines explicit rules: if there is a conflict, the implementing class must override the method and choose the desired behavior, which removes ambiguity.

Why Other Options Are Wrong:
Option B is wrong because the diamond problem is not a memory leak or garbage collection issue. Option C is wrong because a deadlock between threads is a concurrency problem, not an inheritance ambiguity. Option D is incorrect because having many interfaces may make design complex, but it is not what the diamond problem specifically describes.

Common Pitfalls:
A common mistake is to think that Java completely avoids all diamond situations. While Java avoids the classic multiple inheritance diamond for classes, similar patterns can appear with interfaces that have default methods. Another pitfall is confusing the diamond problem with runtime errors such as null pointer exceptions or with performance issues. The diamond problem is purely about method resolution ambiguity in inheritance hierarchies.

Final Answer:
It is an ambiguity that occurs when a class inherits the same method from two different parent classes in a multiple inheritance hierarchy.