Problems encountered in Java real-world development (differs from net)

1 Looping syntax

for (Rteachersubject teacher:teachersubjectlist)

{System. out. println (Teacher.getteacherclasssubjectid ());

}

2 MySQL Syntax

SELECT DISTINCT TSC. ClassID, CONCAT (Tsc.grade, ' Grade ', tsc.name) as ClassName from

Cloud_archivesmanage.t_teacherinfo TTi

JOIN

Cloud_archivesmanage.r_teacher_subject RTS on TTi. Userid=rts. Teacherid

JOIN Cloud_archivesmanage.t_schoolclass TSC on TSC. Classid=rts. ClassID

WHERE TTi. userid=13969823

In SQL Server

SELECT DISTINCT TSC. ClassID, CAST (Tsc.grade as

VARCHAR) + ' grade ' +tsc.name as ClassName from

Cloud_ArchivesManage.dbo.T_TeacherInfo TTi

JOIN

Cloud_ArchivesManage.dbo.R_Teacher_Subject RTS on TTi. Userid=rts. Teacherid

JOIN Cloud_ArchivesManage.dbo.T_SchoolClass TSC on TSC. Classid=rts. ClassID

WHERE TTi. userid=13969823

3 Condition Judgment

<if test="SSID! = null and ssid!=" and ssid!= '-1 ' ">

and TSS. ssid=#{SSID, Jdbctype=varchar}

</if>

4 Database IsNull and ifnull issues

In sql: ISNULL (s.num,0) as Studentnumber

In MySQL: Ifnull (s.num,0) as Studentnumber

5 Problems in Mybatits

Pay attention to the return type when seeking the total quantity

6 instantiation problems

list<rclassversion> classversionlist = new arraylist<rclassversion> ();

7 Self-growth ID is null

8 Database type for datetime

TIMESTAMP

9 Reasons for reading less data

LIMIT 0,3 default starting from 0

Ten like query

and SCC. Name like CONCAT ('% ', ' ${classname} ', '% ')

11 Get the day of the month

Calendar cal = Calendar. getinstance ();

Int day=cal.get (Calendar. DATE));

12 Regular parsing

String regex = "\\{[^\\{*].*\\}";

Pattern pattern = pattern. Compile (regex);

Matcher Matcher = Pattern.matcher (res);

Jsonobject jsonobject = jsonobject.fromobject (matcher.group (0));

JSON parsing

Jsonobject jsonobject = Jsonobject. Fromobject (Matcher.group (0));

Nested data inside data

Jsonarray ja1 = Jsonobject.getjsonarray ("Data");

list<productinfo> Product = new arraylist<productinfo> ();

Try {

for (int i = 0; i < ja1.size (); i++) {

Jsonobject Jsonitem = Ja1.getjsonobject (i);

ProductInfo item = new productinfo ();

Item.setproductid (Jsonitem.getint ("ProductID"));

Item.setproductname (jsonitem.getstring ("ProductName"));

Item.setstatus (Jsonitem.getint ("Status"));

Item.setlogoimg (jsonitem.getstring ("logoimg"));

Item.seturl (jsonitem.getstring ("Url"));

Product.add (item);

}

} catch (Jsonexception e) {

E.printstacktrace ();

}

A layer of data

String regex = "\\{[^\\{*].*\\}";

Pattern pattern = pattern. Compile (regex);

Matcher Matcher = Pattern.matcher (res);

if (Matcher.find ()) {

Jsonobject jsonobject = Jsonobject. Fromobject (Matcher.group (0));

Try {

if (Jsonobject.getint ("Code") = = && Jsonobject.getboolean ("Data") {

Json.setmessage (jsonobject.getstring ("Message"));

Json.seturl (URL);

Json.setstate (true);

} Else {

Json.setmessage (jsonobject.getstring ("Message"));

Json.seturl (URL);

Json.setstate (false);

}

} catch (Jsonexception e) {

E.printstacktrace ();

}

}

Substring

String str = url.substring (Url.length ()-1, Url.length ());

from which to the end, to who?

The question of Jdbctype not to write

D_type = #{awbtype,jdbctype=varchar} The Jdbctype does not write any impact

Answer: Do not write as empty will error, write it will not

16 Chinese characters converted to GB2312 method

Public String Changecharset (String str, string newcharset) throws unsupportedencodingexception {

if (str = null) {

Decodes a string with the default character encoding.

byte [] bs = Str.getbytes ();

Generate a string with a new character encoding

return New String (BS, Newcharset);

}

return null;

}

17 string Concatenation method

C#

string res = string. Format ("key={0}&userid={1}&productid={2}&datatype={3}&name={4}&schoolid={5}", Key, UserID, ProductId, "JSON", Usercentername, Schoolareaid)

Java implementation
String param1 = string. format ("Key=%1$s&userid=%2$s&productid=%3$s&datatype=%4$s&name=%5$s&schoolid=%6$s", Key, UserID,

ProductId, "JSON", Usercentername, Schoolareaid);

%s String type such as "Mingrisoft"

%c character type ' m '

%b Boolean type True

%d integer type (decimal) 99

%x integer type (hexadecimal) FF

%o Integer type (octal) 77

%f floating-point type 99.99

%a hexadecimal floating-point type Ff.35ae

%e Index Type 9.38e+5

%g General floating-point types (shorter in type F and e)

Percent% type%

%n line break

%tx Date and Time type (x represents a different date and time conversion character)

Why Tomcat failed to start

Mapper.xml problems, case-sensitive

Problems encountered in Java real-world development (differs from net)