Abstract Classes

What is an Abstract Class? An abstract class is a restricted class that cannot be used to create objects (it cannot be instantiated). It serves as a blueprint for other classes. It is declared with the abstract keyword.

What is an Abstract Method? An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon). abstract void myMethod();

Key Rules:

• If a class contains one or more abstract methods, then the class itself must be declared abstract.

• If a regular class extends an abstract class, it must implement all of the abstract methods of the superclass.

• An abstract class can have both abstract and non-abstract (concrete) methods.

When to Use Abstract Classes: Use an abstract class when you want to share code among several closely related classes. It establishes a strong 'is-a' relationship and allows you to provide a common base with some default implementation and some methods that subclasses must define themselves.

Abstract Class vs. Concrete Class: A regular (concrete) class has all its methods fully implemented. An abstract class can have a mix of implemented and unimplemented (abstract) methods.

abstract class GraphicObject {
    int x, y;

    void moveTo(int newX, int newY) {
        this.x = newX;
        this.y = newY;
    }

    // Abstract method - must be implemented by subclasses
    abstract void draw();
}

class Circle extends GraphicObject {
    @Override
    void draw() {
        System.out.println("Drawing a circle at (" + x + ", " + y + ")");
    }
}