Abstract Classes vs. Interfaces in Java

In Java, abstract classes and interfaces are both used to define the structure of classes, but they have some key differences.

Abstract Classes:

• Can have both abstract methods (methods without a body) and concrete methods.
• Can have instance variables.
• Can provide a partial implementation of a class.

Interfaces:

• Can only have abstract methods and constants (variables that are static and final).
• Cannot have instance variables.
• Define a contract for classes to implement certain methods.

When to use abstract classes:

• When you want to provide a common base implementation for all subclasses.
• When you have some concrete methods that can be shared among multiple classes.

When to use interfaces:

• When you want multiple inheritance, as Java does not support multiple inheritance with classes.
• When you want to define a contract for classes to implement specific methods.

Remember, you can also have interfaces with default methods in Java 8 and later versions, blurring the lines between interfaces and abstract classes.