计算器

代码说明：

// calc-ui-model/Calc.java -- Fred Swartz//     Level     : Intermediate.//     Structure : Three files: main, GUI (subclass of JFrame), logic.//     Components: JButton, JTextField (right justified).//     Containers: JFrame, several JPanels.//     Layouts   : BorderLayout to put the other panels together.//                 Two GridLayout panels for the buttons.//     Listeners : One ActionListener which is shared by all//                 numeric key buttons.  Similarly share//                 an ActionListener for all operator buttons.//