+---------------------------+
|        <<abstract>>       |
|          Shape           |
+---------------------------+
|                          |
+---------------------------+
| + getArea(): double      |  <-- 抽象方法
+---------------------------+
           ^
           |
+----------+----------+----------+
|                     |                     |
+----------------+  +----------------+  +----------------+
|     Circle     |  |   Rectangle    |  |    Triangle    |
+----------------+  +----------------+  +----------------+
| - radius: double|  | - width: double |  | - base: double |
|                 |  | - height: double|  | - height: double|
+----------------+  +----------------+  +----------------+
| + getArea(): double|  | + getArea(): double|  | + getArea(): double|
+----------------+  +----------------+  +----------------+


+---------------------------+
|        ShapeUtil          |
+---------------------------+
|                           |
+---------------------------+
| + printArea(Shape): void  |  <-- 静态方法
+---------------------------+