《类层次优于标签类》
这篇大概意思是说，不要把多种不同风格实例的类放到一个类里用标签区分，那样可读性很差。标签类过于冗余，容易出错，并且效率低下。
要用类的层次继承。层次清晰，不易出错，效率高。
不好的标签类示范：
// 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();
}
}
}
正确的方式是Class hierarchy，：
// 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; }
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;
}
double area() { return length * width; }
}
class Square extends Rectangle {
Square(double side) {
super(side, side);
}
}
