Effective Java! Prefer Class Hierarchies to Tagged Classes

It seems like a lot of the previous chapters of Effective Java have shared some concerns with using all the capabilities that object-oriented programming offers us. There are indeed pitfalls with over using some of the functionality of these patterns so they should be used with prudence. Today, however, we look at a topic where object-oriented programming can come to the rescue.

Let’s start out by looking at the following class.

class Figure {
  enum Shape {CIRCLE, RECTANGLE}
  final Shape shape;
  double length;
  double width;
  double radius;
  Figure(double radius) {
    shape = Shape.CIRCLE;
    this.radius = radius;
  }
  Figure(double length, double width) {
    shape = Shape.RECTANGLE;
    this.length = length;
    this.width = width;
  }
  double area() {
    switch(shape) {
      case RECTANGLE:
        return length * width;
        break;
      case CIRCLE:
        return Math.PI * (radius * radius);
        break;
      default:
        throw new AssertionError(shape);
    }
  }
}

As you can see in the class above this is a single class that has a enum in it that is being used a flag to indicate whether the class is a Circle or a Rectangle. Presumably the reasoning (albeit extremely thin) was that they both have a area function. Outside of saving that one line everything else is worse about this class. Let's list some of…