In Java object oriented design, what is the difference between an interface and an abstract class, and when would you use each?

Introduction / Context:
Interfaces and abstract classes are two core tools in Java for designing reusable, flexible, and testable object oriented systems. Both are used to represent abstractions, but they serve slightly different purposes. Interviewers often ask this comparison question to see whether you can choose the right construct when modeling contracts, common behavior, and shared state in your class hierarchies.

Given Data / Assumptions:

• We are using modern Java, where interfaces can have abstract methods, default methods, static methods, and constants.
• Abstract classes can contain abstract methods, concrete methods, fields, and constructors.
• A class can implement multiple interfaces but can extend only one class, abstract or concrete.
• We need to design APIs that separate what an object does from how it does it.

Concept / Approach:
An interface focuses on defining a contract: it specifies method signatures and possibly default behavior that implementing classes must provide or may reuse. It does not normally hold instance state. An abstract class, on the other hand, can carry both abstract methods and concrete methods along with instance fields and constructors. Abstract classes are ideal when you want to share common code, data, and helper behavior among related subclasses, while interfaces are better when you want to define capabilities that can be implemented by unrelated classes. Because Java supports implementing multiple interfaces but only single inheritance of classes, interfaces are also central to designing flexible type hierarchies.

Step-by-Step Solution:
Step 1: Note that neither interfaces nor abstract classes can be instantiated directly; both must be implemented or extended. Step 2: Recognize that interfaces define public abstract methods by default, plus optional default and static methods, but they do not store per instance fields. Step 3: Understand that abstract classes can declare fields, constructors, and both abstract and fully implemented methods, providing a partial implementation for subclasses. Step 4: Remember that a class may implement many interfaces, allowing multiple capabilities to be combined, but can only extend one superclass. Step 5: Conclude that interfaces are best for pure contracts and cross cutting capabilities, while abstract classes are best for sharing common code and state inside a family of related classes.

Verification / Alternative check:
Looking at Java standard libraries gives clear examples. The List interface defines operations like add and get without specifying how they are implemented, while abstract classes such as AbstractList provide partial implementations that ArrayList and LinkedList extend. Many unrelated classes implement interfaces like Serializable or Comparable to indicate capabilities, not inheritance relationships. These patterns illustrate the complementary roles of interfaces and abstract classes in real projects.

Why Other Options Are Wrong:
Option B is incorrect because interfaces cannot be instantiated directly, and abstract classes can definitely be subclassed. Option C is wrong because abstract classes are not restricted to exception handling, and interfaces are widely used beyond database access. Option D is clearly incorrect since interfaces and abstract classes differ in features, allowed members, and typical use cases and cannot always be swapped without design consequences.

Common Pitfalls:
A common pitfall is using abstract classes when an interface would be more flexible, leading to rigid inheritance structures. Another mistake is trying to put too much implementation logic into interfaces, which can blur the separation between contracts and concrete behavior. Developers should also avoid overusing default methods in interfaces to the point that interfaces become complex mini frameworks. Good design chooses interfaces for contracts and abstract classes for shared implementation, keeping both roles clear.

Final Answer:
In Java, an interface defines a pure contract that multiple classes can implement, while an abstract class can hold both abstract declarations and shared implementation and state that subclasses reuse.