利用JavaCC分析并计算算术表达式。支持变量、常见函数。分析过程与计算过程分开，计算速度可与Java内置表达式相比。调用方法见主程序的main方法。-JavaCC analyzed and calculated use of arithmetic expressions. Support variables, common functions. Analysis and calculation process separately, the calculation speed can be compared with Java built expressions. See the main program main method calls the method.