Abstract Class

Java Abstract Class — Summary

Definition

• A class declared with the abstract keyword.
• Cannot be instantiated directly.
• Can contain abstract methods (no body) and concrete methods (with body).

Key Points

1. Purpose

   • To provide a base class with common fields/methods while enforcing subclasses to implement certain methods.

2. Abstract Methods

   • Declared without a body:

     abstract void draw();

   • Must be implemented by the first concrete subclass.

3. Concrete Methods

   • Abstract classes can also have fully implemented methods.

4. Constructors & Fields

   • Can have constructors, instance variables, and static members.

5. Inheritance Rules

   • A subclass must either:
     • Implement all abstract methods, or
     • Be declared abstract itself.
   • Supports single inheritance (unlike interfaces which can be multiple).

6. Difference from Interface

   • Abstract class: can have state (fields), constructors, and both abstract & concrete methods.
   • Interface: cannot have instance fields (only constants), but can have default/static methods (Java 8+).

Example

abstract class Shape {    
    String color;
    // Abstract method
    abstract double area();
    // Concrete method
    void setColor(String color) {        
        this.color = color;    
    }
}

class Circle extends Shape {    
    double radius;    
    Circle(double radius) {        
        this.radius = radius;    
    }    
    
    @Override double area() {        
        return Math.PI * radius * radius;    
    }
}

public class Main {    
    public static void main(String[] args) {        
    Shape s = new Circle(5);        
    s.setColor("Red");        
    System.out.println("Area: " + s.area());    
    }
}

✅ When to use:

• You have a base class with shared code but also methods that must be implemented by subclasses.