为什么80%的码农都做不了架构师？>>> 
1.标签类
标签类是指在类中定义了一个变量，使用该变量的值来控制该做什么动作。
2.标签类（不推荐）
能够表示圆形或矩形的类：
public class Figure {
enum Shape {RECTANGLE, CIRCLE};
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();
}
}
}
标签类过于冗长、容易出错，并且效率低下。
3.类层次（推荐）
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;
}
}