EffectiveJava——类层次优于标签类

/**
* 类层次优先与标签类
* @author weishiyao
*
*/
// Tagged class - vastly inferior to a class hierarchy
public class Figure1{
enum Shape {
RECTANGLE,
CIRCLE
}// Tag field - the shape of this figure
final Shape shape;// These field are use only if shape if RECTANGLE
double length;
double width;// This field is use only if shape is CIRCLE
double radius;// Constructor for circle
public Figure1(double radius) {
shape = Shape.CIRCLE;
this.radius = radius;
}// Constructor for rectangle
public Figure1(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();
}
}
}

/**
* 类层次优于标签类
* @author weishiyao
*
*/
// Class hierarchy replacement for a tagged class
abstract class Figure2 {
abstract double area();
}class Circle extends Figure2 {
final double radius;Circle(double radius) {
this.radius = radius;
}double area() {
return Math.PI * (radius * radius);
}
}class Rectangle extends Figure2 {
final double length;
final double width;Rectangle(double length, double width) {
this.length = length;
this.width = width;
}
double area() {
return length * width;
}
}