An abstract class is a class that cannot be instantiated directly; it must be subclassed by another class. It can contain both abstract methods (methods without implementations) and non-abstract methods (methods with implementations). The primary purpose of an abstract class is to provide a common interface and shared functionality for its subclasses, enforcing a contract that subclasses must adhere to by implementing the abstract methods.

1. Cannot be Instantiated: You cannot create an object of an abstract class directly. Instead, you must create a subclass that implements its abstract methods.
2. Abstract Methods: These are declared without a body and must be implemented by subclasses. For example:```javapublic abstract void draw();```
3. Non-Abstract Methods: These methods have a complete implementation and can be inherited by subclasses. For example:```javapublic void display() {System.out.println("This is a shape.");}```
4. Constructors: Abstract classes can have constructors, which are called when an instance of a subclass is created.

• Code Reusability: Abstract classes allow you to define common behavior that can be reused across multiple subclasses, reducing code duplication.
• Enforcing a Contract: By defining abstract methods, you ensure that all subclasses implement these methods, providing a consistent interface.
• Polymorphism: Abstract classes enable polymorphic behavior, allowing you to treat different subclasses as instances of the abstract class.

Here’s a simple example of an abstract class in Java:```javaabstract class Shape {// Abstract methodpublic abstract double area();

}```In this example, any subclass of `Shape` must implement the `area()` method, ensuring that all shapes have a way to calculate their area.

Abstract classes are a fundamental concept in Java that facilitate code organization and design. They allow developers to define a common interface and shared functionality for related classes, promoting code reuse and ensuring that subclasses adhere to a specific contract. Understanding how to effectively use abstract classes is essential for designing robust and maintainable Java applications.For more detailed information, you can refer to sources like GeeksforGeeks, W3Schools, and Baeldung.

An abstract class is used primarily to provide a common structure and default behavior for its subclasses. It serves as a blueprint, allowing subclasses to inherit methods and attributes while ensuring they implement certain essential methods. This promotes code reusability and maintainability, as subclasses can override or extend the abstract class's methods to fit their specific needs. Additionally, abstract classes cannot be instantiated directly, which enforces a level of abstraction in the design.