Core Java Interview Question:

What is the difference between abstract classes and interfaces in Java, and when would you choose to use one over the other?

In Java, abstract classes and interfaces are both used to achieve abstraction, but they have some key differences.

Abstract classes:

• Can have a mix of both abstract and concrete methods.
• Can have member variables.
• Can enforce certain methods to be overridden by subclasses using the "abstract" keyword.
• Supports single class inheritance, meaning a class can extend only one abstract class.

Interfaces:

• Can only have abstract methods and constants (public static final variables).
• Cannot have method implementations or member variables.
• Supports multiple interface inheritance, meaning a class can implement multiple interfaces.

When to choose abstract classes:

Use abstract classes when you need to define a common functionality among related classes and when you plan to have some default implementations in your base class.

When to choose interfaces:

Use interfaces when you want to provide a contract for a group of classes to implement, especially when these classes are unrelated and you want to achieve multiple inheritance.

By understanding the differences between abstract classes and interfaces in Java, you can make an informed decision on when to use each in your programming projects.