int max = 5;
int min = 2;
形成过程0:int saleNumber = (int)(Math.random()*(max)); // 0~4 , Math.random()返回 [0,max)
形成过程1:int saleNumber = (int)(Math.random()*(max+1)); // 0~5
形成过程2:int saleNumber = (int)(Math.random()*(max+1))+ min; // 2~7
形成过程3:int saleNumber = (int)(Math.random()*(max+1-min))+ min; // 2~5