Abstraction & Abstract Classes

An abstract class can declare methods with no body, forcing every concrete subclass to provide its own implementation — and it can never be instantiated directly with new.

abstract class Shape {
    abstract double area(); // no body — subclasses must implement it

    void describe() {
        System.out.println("Area: " + area());
    }
}

class Circle extends Shape {
    double radius;

    Circle(double radius) {
        this.radius = radius;
    }

    @Override
    double area() {
        return Math.PI * radius * radius;
    }
}

Common mistakes

• Trying new Shape() directly — abstract classes exist to be extended, not instantiated.