用java的Swing中的2d绘图做的3d魔方，实践了一下3d数学计算。这个魔方是可以用方向键旋转的。-A 3d magic cube written in java using the Swing 2d graphic functions which is aimed at practicing the 3d math theory.The cub can be rotate by direction keys.

已知二维空间点Point2D类的定义如下，在此基础上派生出三维空间点Point3D类。根据数学，增加成员变量z，定义三个参数的构造方法和无参数的构造方法(对应原点的对象)，重写toString()和getDistance()方法，重写equals()方法。TestPoint类的main()方法定义3个以上点做测试，结果格式如下所示。(The definition of Point2D class of two dimensional space points is as follows. On