Small knowledge of common classes

StringBuffer

 StringBuffer sb=new StringBuffer();
        //1,append Additional 
        sb.append("kankan");
        //2,insert() Add or insert 
        sb.insert(0,12);
        //3,replace() Replace  
        sb.replace(0,2,"world");
        //4,delete  Delete 
        sb.delete(0,2);
        System.out.println(sb.toString());

Calendar The use of the class

Calendar calendar=Calendar.getInstance();
        System.out.println(calendar.getTime().toLocaleString());// Get the current time 
        System.out.println(calendar.getTimeInMillis());// from 1970 The number of milliseconds from January 1 to now 

        // Get date 
        int year=calendar.get(calendar.YEAR);
        int month=calendar.get(calendar.MONTH); // from 0-11 Start 
        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+" year "+month+" month "+day+" Japan "+hour+" when "+minute+" branch "+second);

        // Modification time 
        Calendar calendar2=Calendar.getInstance();
        calendar2.set(calendar.DAY_OF_MONTH,19) ;// Set days to 19
        System.out.println(calendar2.getTime().toLocaleString());

        //add Method modification time 
        calendar2.add(calendar.HOUR,1); // Add one to the hours , Subtraction can input negative numbers directly 
        System.out.println(calendar2.getTime().toLocaleString());

simpleDateFormate class

 // use SimpleDateFormat Class will Date Convert to string 
        SimpleDateFormat sdf=new SimpleDateFormat("yyy year MM month dd Japan  HH:mm:ss");
        //2, Create a Date
        Date date=new Date();
        // Convert date to string 
        String str =sdf.format(date);
        System.out.println(str);