常用类的小知识

StringBuffer

 StringBuffer sb=new StringBuffer();
        //1,append追加
        sb.append("kankan");
        //2，insert()添加或插入
        sb.insert(0,12);
        //3,replace()替换 
        sb.replace(0,2,"world");
        //4,delete 删除
        sb.delete(0,2);
        System.out.println(sb.toString());

Calendar类的使用

Calendar calendar=Calendar.getInstance();
        System.out.println(calendar.getTime().toLocaleString());//获得当前时间
        System.out.println(calendar.getTimeInMillis());//从1970年一月一到现在的毫秒数

        //获取年月日
        int year=calendar.get(calendar.YEAR);
        int month=calendar.get(calendar.MONTH); //从0-11开始
        int day=calendar.get(calendar.DAY_OF_MONTH);
        int hour=calendar.get(calendar.HOUR_OF_DAY);
        int minute=calendar.get(calendar.MINUTE);
        int second=calendar.get(calendar.SECOND);
        System.out.println(year+"年"+month+"月"+day+"日"+hour+"时"+minute+"分"+second);

        //修改时间
        Calendar calendar2=Calendar.getInstance();
        calendar2.set(calendar.DAY_OF_MONTH,19) ;//将天数设为19
        System.out.println(calendar2.getTime().toLocaleString());

        //add方法修改时间
        calendar2.add(calendar.HOUR,1); //将小时数加一，减法可以直接输入负数
        System.out.println(calendar2.getTime().toLocaleString());

simpleDateFormate类

 //用SimpleDateFormat类将Date转化为字符串
        SimpleDateFormat sdf=new SimpleDateFormat("yyy年MM月dd日 HH:mm:ss");
        //2,创建一个Date
        Date date=new Date();
        //将日期转化为字符串
        String str =sdf.format(date);
        System.out.println(str);