In object-oriented programming, abstract classes are used as a blueprint for derived classes. They serve as a foundation for creating objects and define common attributes and behaviors that the derived classes can inherit and override.

Here are some situations where using abstract classes in Java can be beneficial:

1. Creating a base class: Abstract classes are useful when you want to create a base class that provides a common interface and default implementation for a group of related classes. For example, if you are building a drawing application, you can create an abstract class called Shape that defines common methods like draw() and calculateArea(). Other classes like Circle and Rectangle can extend the Shape class and provide their own implementation of these methods.

2. Declaring common methods: Abstract classes allow you to declare methods that are common to multiple classes. This helps in enforcing consistency across the derived classes and ensures that certain methods are implemented. For instance, if you have a group of classes representing different types of vehicles, you can define an abstract class called Vehicle with methods like start(), stop(), and accelerate(), which all vehicle types should implement.

public abstract class Vehicle {
    public abstract void start();
    public abstract void stop();
    public abstract void accelerate();
}

Derived classes like Car and Motorcycle can extend Vehicle and provide their own implementation of these methods.

Using abstract classes allows you to create a hierarchy of classes with a clear structure and promotes code reusability. It also helps in achieving abstraction and modularity in your Java code.

Remember to use the abstract keyword when defining an abstract class and declare abstract methods using the same keyword. Abstract classes cannot be instantiated directly, so they are meant to be extended by concrete classes.

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