第 23 条：类层次优先与标签类

类层次优先与标签类.jpeg

标签类

// Tagged class - vastly inferior to a class hierarchy!
class Figure {
    enum Shape { RECTANGLE, CIRCLE };
    // Tag field - the shape of this figure
    final Shape shape;
    // These fields are used only if shape is RECTANGLE
    double length;
    double width;
    // This field is used only if shape is CIRCLE
    double radius;
    // Constructor for circle
    Figure(double radius) {
        shape = Shape.CIRCLE;
        this.radius = radius;
    }
    // Constructor for rectangle
    Figure(double length, double width){
        shape = Shape.RECTANGLE;
        this.length = length;
        this.width = width;
    }
    double area(){
        switch(shape) {
            case RECTANGLE:
                return length * width;
            case CIRCLE:
                return Math.PI * (radius * radius);
            default:
                throw new AssertionError(shape);
        }
    }
}

类层次

// Class hierarchy replacement for a tagged class
abstract class Figure {
    abstract double area();
}
class Circle extends Figure {
    final double radius;
    Circle(double radius) { this.radius = radius; }
    @Override double area() { return Math.PI * (radius*radius);}
}
class Rectangle extends Figure {
    final double length;
    final double width;
    Rectangle(double length, double width) {
        this.length = length;
        this.width = width;
    }
    @Override double area() { return length * width; }
}