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In object-oriented programming, an abstract class is a special type of class that can't be instantiated. It's used solely as a base class for other classes. Abstract classes provide common features, methods, and properties that should be shared by all derived classes.

If a class is defined as abstract, there are some restrictions on how it can be used: 1. You cannot create an object directly from an abstract class. 2. Abstract classes can contain both abstract and non-abstract methods. Abstract methods don't have a method body and must be overridden in derived classes, while non-abstract methods have an implementation in the abstract class and can be overridden in derived classes optionally. 3. A class that inherits from an abstract class must implement all abstract methods declared in the abstract class, otherwise, the derived class should also be marked as abstract.

Despite the restrictions, abstract classes are useful in many scenarios. Some ways to work with abstract classes include: 1. Inherit from an abstract class: Create a new class that extends the abstract class and then use the derived class to create objects. 2. Override abstract methods: Implement the abstract methods declared in the abstract class within the derived class. 3. Use polymorphism: Abstract classes can be used to define a common interface for different derived classes, allowing you to write code that works with objects of various types through a single, unified interface. In conclusion, an abstract class can't be instantiated, meaning you can't create an object directly from it. It serves as a base class for other classes and provides a common interface and functionality for the derived classes. To work with an abstract class, you should inherit from it, override abstract methods, and utilize polymorphism.