Why instanceof Is the Devil.

Badness.java

public class Triangle {
  public int width;
  public int height;
}

public class Rectangle {
  public int width;
  public int height;
}

public class Circle {
  public int radius;
}

public class AreaReporter {
  public void printArea(Object shape) {
    double area = 0;
    if(shape instanceof Triangle) {
      area = 0.5 * ((Triangle)shape).width * ((Triangle)shape).height;
    } else if(shape instanceof Rectangle) {
      area = 1.0 * ((Rectangle)shape).width * ((Rectangle)shape).height;
    } else if(shape instanceof Circle) {
      area = Math.PI * ((Circle)shape).radius;
    }
    System.out.println("The area is: " + area);
  }
}

public class Main
  public static void main() {
    AreaReporter reporter = new AreaReporter();
    Circle x = new Circle();
    x.radius = 10;
    Triangle y = new Triangle();
    y.width = 10;
    y.height = 5;
    Rectangle z = new Rectangle();
    z.width = 10;
    z.height = 5;

    reporter.printArea(x);
    reporter.printArea(y);
    reporter.printArea(z);
  }
}

Betterness.java

public interface HasArea {
  double getArea();
}

public class Triangle implements HasArea {
  public int width;
  public int height;
  public double getArea() { return 0.5 * width * height; }
}

public class Rectangle implements HasArea {
  public int width;
  public int height;
  public double getArea() { return 1.0 * width * height; }
}

public class Circle implements HasArea {
  public int radius;
  public double getArea() { return Math.PI * radius; }
}

public class AreaReporter {
  public void printArea(HasArea shape) {
    System.out.println("The area is: " + shape.getArea());
  }
}

public class Main
  public static void main() {
    AreaReporter reporter = new AreaReporter();
    Circle x = new Circle();
    x.radius = 10;
    Triangle y = new Triangle();
    y.width = 10;
    y.height = 5;
    Rectangle z = new Rectangle();
    z.width = 10;
    z.height = 5;

    reporter.printArea(x);
    reporter.printArea(y);
    reporter.printArea(z);
  }
}