Key Differences Between Abstract Classes and Interfaces in Java

Abstract classes and interfaces are fundamental concepts in Java OOP. The main differences between the two are:

1. Definition: Abstract classes can have both abstract and concrete methods, while interfaces only have abstract methods that must be implemented by the classes that implement the interface.
2. Inheritance: A class can extend only one abstract class, but it can implement multiple interfaces.
3. Accessibility: Abstract classes can have both public and protected access modifiers, while interfaces are always public.
4. Constructors: Abstract classes can have constructors, whereas interfaces cannot have constructors.

When to Use Abstract Classes or Interfaces in a Project?

Choosing between abstract classes and interfaces in a Java project depends on the specific requirements and design considerations.

Use abstract classes when:

• The classes need to share common methods or fields.
• You want to define default behavior that subclasses can override.
• You want to restrict access and ensure a certain level of implementation.

Use interfaces when:

• There are unrelated classes that need to implement common methods.
• You want to achieve multiple inheritances as a class can implement multiple interfaces.
• You want to define a contract for behavior without specifying the implementation.

Ultimately, the decision to use abstract classes or interfaces in a project comes down to the design requirements and the specific use case.