public void getTime()
        {
          Calendar  cal = new GregorianCalendar();   
           int year=cal.get(Calendar.YEAR);   
           int month=cal.get(Calendar.MONTH);   
           int day=cal.get(Calendar.DATE);
           int hour=cal.get(Calendar.HOUR_OF_DAY);
           int minute=cal.get(Calendar.MINUTE);
           int second=cal.get(Calendar.SECOND);
         
          String s = "当前时间: "+String.valueOf(year)+"年"+
                   String.valueOf(month)+"月"+String.valueOf(day)+"日"+
                   String.valueOf(hour)+"时"+String.valueOf(minute)+"分"+
                   String.valueOf(second)+"秒";
           showtime.setText(s);
           showtime.setColumns(s.length());

大家运行它时会发现showtime这个JTextField显示的长度要比那些字符长度长