各位，我有个小问题，不知道为什么；新人

public class USBConsole
{
    public static void main(String[] args)
    {
        IUSBable m = new mouse();
        USBConsoleTable.insertInUSB(m);
        IUSBable k = new keyboard();
        USBConsoleTable.insertInUSB(k);
        USBConsoleTable.insertInUSB(new keyboard());
    }
}
interface IUSBable
{
    void dosomth();
}

class mouse implements IUSBable
{   
    public void dosomth()
    {
         System.out.println("tic...tic...mouse working.");
    }
}

class keyboard implements IUSBable
{
    public void dosomth()
    {
        System.out.println("da....da...da..keyboard working.");
    }
}

class USBConsoleTable
{   
    private static IUSBable[] USBss = new IUSBable[6];
    private static int indx = 0;
    static void insertInUSB(IUSBable USBs)
    {   
        if (indx > USBss.length)
         {   
            System.out.println("USB插口不足");
             return;
         }
        else if (USBss[indx] != null)
        {
            USBss[indx]=USBs;
            USBs.dosomth();
            indx++;
        }
    }
}