In Java design, when is it more appropriate to use an abstract class and when is it better to use an interface to model behavior and shared code?

Introduction / Context:
This question explores a classic design decision in Java: choosing between abstract classes and interfaces. Both mechanisms support abstraction and polymorphism, but they are used in slightly different situations. Understanding when to favor one over the other is a frequent Java interview theme and helps you design clean, maintainable hierarchies in real world applications.

Given Data / Assumptions:

- We are working with modern Java, where interfaces can contain default and static methods in addition to abstract methods.
- Abstract classes can have fields, constructors, concrete methods, and abstract methods.
- A subclass can extend only one class but can implement multiple interfaces.
- The question is asking for general design guidance, not rigid rules.

Concept / Approach:
An abstract class represents an incomplete base type that captures shared state and behavior for a family of closely related subclasses. It is ideal when you have strong is a relationships and significant code that should be reused. An interface represents a contract or capability that any class can implement, regardless of its place in the inheritance tree. Interfaces are ideal for describing roles, capabilities, and APIs that can cut across otherwise unrelated class hierarchies. Choosing between them depends mainly on the need to share implementation and state versus the need to specify a pure contract that many classes can adopt.

Step-by-Step Solution:
Step 1: Identify whether your types share common fields and common implementation code that should live in one place. If yes, an abstract class is a strong candidate. Step 2: Consider whether your design requires a class to inherit from some other base class for technical or framework reasons. Since Java supports single inheritance of classes, this limits use of abstract classes. Step 3: If you want to describe a capability such as Serializable, Comparable, or Runnable that can be added to many unrelated classes, an interface is more appropriate, because a class can implement many interfaces. Step 4: Check option A and see that it correctly states that abstract classes are good when you share state and implementation among related classes, while interfaces are best to define contracts for many unrelated implementations. Step 5: Compare with other options that misuse abstract classes or interfaces, or that assert incorrect statements about deprecation.

Verification / Alternative check:
Look at common Java library examples. The AbstractList and AbstractMap classes provide partial implementations and shared fields for list and map implementations; these are abstract classes. On the other hand, interfaces like List, Map, Runnable, and Comparable define contracts that many unrelated classes can implement. This real world pattern supports the design guidance described above and matches option A.

Why Other Options Are Wrong:
Option B is wrong because abstract classes are not limited to static utility methods, and interfaces in modern Java are not just collections of constants. Option C is incorrect because interfaces do not have constructors and abstract classes do not support multiple inheritance of implementation. Option D is clearly wrong because interfaces are heavily used in modern Java and are not deprecated at all.

Common Pitfalls:
Developers sometimes start with an abstract class when they only need a contract, which then prevents classes from extending other useful base classes. Another pitfall is to turn interfaces into god interfaces with too many responsibilities. In modern Java, careless use of default methods in interfaces can also make designs harder to reason about. Keeping the roles clear helps: prefer interfaces for contracts and capabilities, and abstract classes for shared implementation within a narrow family of types.

Final Answer:
Use an abstract class when related subclasses need to share state and common implementation code, and use an interface when you want to define a reusable contract or capability that many unrelated classes can implement.