+---------------------------+
|         Shape             |  <-- 抽象类
+---------------------------+
|                           |
| + getArea(): double       |  <-- 抽象方法
+---------------------------+
          ▲
          |  继承
+----------+----------+-----------+
|  Circle   | Rectangle | Triangle |
+-----------+----------+-----------+
| - radius: double | - width: double  | - base: double   |
| + Circle(r: double) | + Rectangle(w: double, h: double) | + Triangle(b: double, h: double) |
| + getArea(): double | + getArea(): double | + getArea(): double |
+-----------+----------+-----------+

+---------------------------+
|        ShapeUtil          |  <-- 工具类
+---------------------------+
| + printArea(s: Shape): void|
+---------------------------+