实验3继承和多态定义下述5个类，类属性如下：Employee:firstName,lastName,socialSecurityNumberSalaridEmployee:weeklySalary(周薪）HourlyEmployee:wage(每小时的工钱),hours（月工作小时数）CommisionEmployee:grossSales(销售额),commissionRate(提成比率)BasePlusCommisionEmployee:baseSalary（月基本工资）Employee类中定义了抽象方法earning，用于计算员工的月工资。SalaridEmployee月工资计算为：weeklySalary*4