What is the difference between an abstract class and an interface in Java? When would you use one over the other?

What is the difference between an abstract class and an interface in Java?

An abstract class in Java is a class that can contain abstract methods (methods without a body) as well as concrete methods (methods with a body). It can also have member variables. Abstract classes cannot be instantiated on their own and can only be used as base classes for other classes. Interfaces, on the other hand, are like blueprints for classes, defining methods that implementing classes must include. Interfaces do not have member variables, and all methods are abstract by default.

The main difference between an abstract class and an interface is that a class can only extend one abstract class but can implement multiple interfaces. This allows for more flexibility in Java's class hierarchy.

When would you use one over the other?

Use an abstract class when you want to provide a common base implementation for a group of related classes. Abstract classes are useful when there is shared functionality among different classes and when you want to define default behavior that can be overridden by subclasses.

Use an interface when you want to define a contract for a class to implement certain methods. Interfaces are useful for defining relationships between unrelated classes and for defining specific behaviors that must be implemented by implementing classes.

In general, if you need to implement multiple "is-a" relationships, use abstract classes, and if you need to implement behavior that is not related to the class hierarchy, use interfaces in Java.