<aside> Abstract classes are the foundation of object-oriented design, serving as templates for other classes while never being instantiated themselves.

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📚 Understanding Abstract Classes

An abstract class serves as a base class in class hierarchies. It cannot be instantiated directly and is meant to be inherited by other classes.

📊 UML Representation

In UML diagrams, abstract classes are distinguished from regular classes with specific notation:

• 🖋️ The class name is written in italics
• 🏷️ The {abstract} keyword may be shown above or below the class name
• 📝 Abstract methods are also shown in italics
• 🔄 The inheritance relationship is shown with an arrow pointing from the subclass to the abstract class

<aside> 💡 Example: In a UML class diagram, our Shape class would appear as Shape with CalculateArea() in italics, while concrete methods like DisplayInfo() would be in regular font.

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✨ Key Characteristics

• 🚫 Cannot be instantiated using the new keyword
• 🔄 Can contain both abstract and concrete methods
• 🎯 Used to define common characteristics of related classes
• 🏗️ Provides a base for inheritance hierarchies

💡 Basic Example: Shape Hierarchy

public abstract class Shape
{
    public string Color { get; set; }
    
    // Abstract method - must be implemented by derived classes
    public abstract double CalculateArea();
    
    // Concrete method - available to all derived classes
    public virtual void DisplayInfo()
    {
        Console.WriteLine($"This is a {Color} shape.");
    }
}

// Concrete class inheriting from Shape
public class Circle : Shape
{
    public double Radius { get; set; }
    
    // Must implement the abstract method
    public override double CalculateArea()
    {
        return Math.PI * Radius * Radius;
    }
}

🚗 Extended Example: Vehicle System

<aside> This example demonstrates how abstract classes can be used to create a hierarchy of vehicle types.

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