Explain the difference between abstract classes and interfaces in Java and when you would use each one.

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Abstract classes and interfaces in Java are used for achieving abstraction and encapsulation in object-oriented programming. The main difference between them is that abstract classes can have both abstract and non-abstract methods, while interfaces can only have abstract methods (methods without a body) and constants.

When deciding whether to use an abstract class or an interface, consider the following:

1. Use an abstract class when you want to provide a common base implementation for all subclasses and have some methods that are implemented and some that need to be overridden by subclasses.
2. Use an interface when you want to define a contract specifying what methods a class must implement without providing any implementation details. This allows for multiple inheritance of type while maintaining flexibility.

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