Abstraction in Java Explained

// ── Abstract class: shared-base abstraction ────────────────────────────
abstract class Shape {
    private final String color;

    Shape(String color) { this.color = color; }

    public String getColor() { return color; }  // concrete: shared by all shapes

    public abstract double area();              // abstract: each shape calculates differently

    public void describe() {                    // template method: uses the abstract hook
        System.out.println(color + " shape, area = " + area());
    }
}

class Circle extends Shape {
    private final double radius;
    Circle(String color, double radius) { super(color); this.radius = radius; }

    @Override
    public double area() { return Math.PI * radius * radius; }  // Circle's version
}

class Rectangle extends Shape {
    private final double width, height;
    Rectangle(String color, double w, double h) { super(color); this.width = w; this.height = h; }

    @Override
    public double area() { return width * height; }             // Rectangle's version
}

// ── Interface: shared contract ─────────────────────────────────────
interface Drawable {
    void draw();  // type contract — any class can implement this
}

// ── Caller only knows the Shape contract ────────────────────────────
public class AbstractionDemo {
    public static void main(String[] args) {
        Shape[] shapes = {
            new Circle("red", 5),
            new Rectangle("blue", 4, 6)
        };

        for (Shape s : shapes) {
            s.describe();  // no idea which concrete class — abstraction at work
        }
        // Output:
        // red shape, area = 78.53981633974483
        // blue shape, area = 24.0

        // Everyday abstraction — depend on the interface, not the implementation:
        java.util.List<String> names = new java.util.ArrayList<>();
        names = new java.util.LinkedList<>();  // choose another implementation when the required behavior and construction allow it
    }
}