Develop Reference

Why does my for loop in android studio stop after the first iteration?

I created a system in which I can run all my postgre sql queries with only one single Async Task Class in Android Studio. This was really(!!) challenging due to the big amount of limitations that I had to face. But this works actually really great!
//Used for connecting to database and executing queries.
//Index 0 of input string must be the query, Index 1 must be the tablename we demand
//We can only gather data from 1 table for each query, so if you need data from several tablecolumns, use multiple queries like:
//[0] = query, [1] = tablename, [2] = 2nd query, [3] = 2nd tablename, [4] = 3rd query, [5] = 3rd table name ... and so on (each query must come with a tablename)
public class DBHandler extends AsyncTask<String, Void, List<String>>
{
public AsyncResponse delegate;
#Override
protected List<String> doInBackground(String...query)
{
List<String> result = new ArrayList<String>();
String sql;
String tableresult = null;
Connection conn = null;
Statement st = null;
ResultSet rs = null;
try {
Class.forName("org.postgresql.Driver");
conn = DriverManager.getConnection("jdbc:postgresql://192.168.200.300:5439/dbname?user=anonymous&password=secretpw");
st = conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE); //necessary if you want to use rs.first() after rs.next(), it makes the resultset scrollable
for (int i = 0; i <= query.length-1; i = i+2) //queries are always stored in i=0 and/or in i+2, because i+1 contain the demanded tablenames for resultset handling
{
System.out.println("I is: " +i);
if (!query[i].isEmpty())
{
System.out.println(query[i]);
sql = query[i];
rs = st.executeQuery(sql);
while (rs.next())
if (!query[i + 1].isEmpty() || !rs.getString(query[i + 1]).isEmpty()) //if i+1 is empty, there is no demanded tablename. Used when we dont need any return values (ie. INSERT, UPDATE)
result.add(rs.getString(query[i + 1])); //demanded tablename is always stored in i+1
//We add an empty entry if we demand multiple tablenames so we can keep them seperate
//Might be replaced with any other char, but you will have to backtrack all usages of DBHandler and fix the filters there
if(i+2 < query.length)
result.add(" ");
}
rs.first(); //reset pointer for rs.next()
}
rs.close();
st.close();
conn.close();
System.out.println("End of AsyncTask");
}
catch (SQLException ex)
{
ex.printStackTrace();
}
catch (Exception e)
{
e.printStackTrace();
}
return result;
}
//onPostExecute returns query result in a List.
//We need to use interaces delegate feature to send the result to other classes, like "Auslieferung", which is implementing the interface
#Override
protected void onPostExecute(List<String> result)
{
super.onPostExecute(result);
System.out.println("Result: " +result.toString());
if (!result.isEmpty())
delegate.processFinish(result);
}
}
There is a for-loop in this Async Task.
for (int i = 0; i <= query.length-1; i = i+2)
And now finally I can explain my issue:
I usually use SELECT queries, sometimes I use an INSERT query (which can be done by a single query), but when I parse an Update Query, my for-loop stops iterating after the first pass, so i+2 never happens. The update queries look like this:
String updatequeries[] = {UPDATE delivery SET contactperson = 'Jon Doe' WHERE officeid = 5, " ", UPDATE delivery SET contactemail = 'abd#def.gh' WHERE officeid = 5, " "};
Why does this for loop stop running right after the first run? The debugger does not show anything unusual, everything was parsed right and there are no queries missing. Updating a table does not return any results, but nothing depends on result values here. I tried to run 20 update queries in a single string var, but the for loop stops after the first iteration anyway. No issues are displayed in the debugger or in the logs. Have I overseen something or is there anything I don't know? Might this be a bug? Please help me! This issue drives me crazy.

Select multiple column values with Java ResultSet

I am trying to select values from a row in my MySQL table.
SELECT fortnite,psn,steam,twitch,xbox,youtube
FROM `social_media`
WHERE id = '16483378715464928'
When I try to convert the result into a string, the ResultSet only receives the first "fortnite" row. My question is, how do I retrieve the following columns and put them all into one string to return.
Here is my example code:
public static String getSocialMedia(String id) {
String ret = "";
int i = 1;
try {
Statement stmt = null;
ResultSet resultSet = null;
getConnection();
stmt = con.createStatement();
resultSet = stmt.executeQuery("SELECT fortnite,psn,steam,twitch,xbox,youtube FROM `social_media` WHERE id ='" + id + "'");
while(resultSet.next()) {
ret += resultSet.getString(i) + " ";
i++;
}
if(resultSet != null) {
resultSet.close();
}
if(stmt != null) {
stmt.close();
}
} catch (Exception e) {
e.printStackTrace();
}
return ret;
}

This is happening due to this.
while(resultSet.next()) {
ret += resultSet.getString(i) + " ";
i++;
}
In the above code inside while you need to fetch all the values either by name or index. next() function gives you the complete row not a single column.
You should change it to:
while(resultSet.next()) {
for(i = 1, i <=6, i++){
ret += resultSet.getString(i) + " ";
}
}

When i try to convert the result into a string, the ResultSet only
receives the first "fortnite" row. My question is, how do i retrieve
the following columns and put them all into one string to return.
Terminology is important here, because misunderstanding terminology may lead you to misinterpret documentation. In this case, the important terminology distinction is "row" vs. "column".
Your query SELECTs fortnite,psn,steam,twitch,xbox,youtube. Those six identifiers define six columns that each row in your result set will have. It looks like your particular query is selecting by the table's primary key, so you'll only ever have zero or one row, but other queries can return multiple rows.
You want to extract the values of all six columns of one row, but you iterate while(resultSet.next()), and ResultSet.next() moves the result set to the next row, if any, not the next column. Since you have only one row, the loop terminates after only one iteration.
It looks like you want something more like this:
if (resultSet.next()) {
for (i = 1; i <= 6; i++) {
ret += resultSet.getString(i) + " ";
}
}
The if (resultSet.next()) is necessary to move the result set to the first row, and to detect when there isn't any. Then you iterate over the columns of the result, whose number you know statically, based on the query.

Navigation button is not working

I have read a similar post but I still cannot get what is the problem.
I created a table in ms access, named DOCTOR, there are columns: DoctorID(number), Name(text), PhoneNumber(number), Department(text) and Specialization(text)
I connect the database to java through UCanAccess, below is the code to get connection
import java.sql.*;
public class Doctor
{
public static Connection connection; //sharing the memory
public static Connection connect() throws ClassNotFoundException, SQLException
{
String db = "net.ucanaccess.jdbc.UcanaccessDriver";
Class.forName(db);
String url = "jdbc:ucanaccess://C:/Users/user.oemuser/Documents/Doctor.accdb";
connection = DriverManager.getConnection(url);
return connection;
}
}
In my GUI class, i have a method called getConnect to show the data from database to textfield
public void getConnect()
{
try
{
connection = Doctor.connect();
statement=connection.createStatement();
String sql = "SELECT * FROM DOCTOR";
results = statement.executeQuery(sql);
results.next();
id = results.getInt("DoctorID");
name = results.getString("DoctorName");
phone = results.getInt("PhoneNumber");
dept = results.getString("Department");
spec = results.getString("Specialization");
textField1.setText("" +id);
textField2.setText(name);
textField3.setText("" +nf3.format(phone));
textField4.setText(dept);
textField5.setText(spec);
}
catch(Exception ex)
{
ex.printStackTrace();
}
}
and below is the code for the button1 which is the next button.
if(evt.getSource() == button1)
{
try
{
connection = Doctor.connect();
connection.setAutoCommit(false);
statement=connection.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);
String sql1 = "SELECT * FROM DOCTOR";
results = statement.executeQuery(sql1);
if(results.next())
{
textField1.setText("" +results.getInt("DoctorID"));
textField2.setText(results.getString("DoctorName"));
textField3.setText("" +nf3.format(results.getInt("PhoneNumber")));
textField4.setText(results.getString("Department"));
textField5.setText(results.getString("Specialization"));
}
else
{
results.previous();
JOptionPane.showMessageDialog(null, "No more records");
}
connection.commit();
}
catch(Exception ex){
ex.printStackTrace();
}
}

Obviously the best component to use here is a JTable if you want to query all records within a particular database table or at the very least place the result set into an ArrayList mind you database tables can hold millions+ of records so memory consumption may be a concern. Now, I'm not saying that your specific table holds that much data (that's a lot of Doctors) but other tables might.
You can of course do what you're doing and display one record at a time but then you should really be querying your database for the same, one specific record at a time. You do this by modifying your SQL SELECT statement with the addition of the WHERE clause statement and playing off the ID for each database table record, something like this:
String sql = "SELECT * FROM DOCTOR WHERE DoctorID = " + number + ";";
But then again we need to keep in mind that, if the schema for your DoctorID field is set as Auto Indexed which of course allows the database to automatically place a incrementing numerical ID value into this field, the Index may not necessarily be in a uniform sequential order such as:
1, 2, 3, 4, 5, 6, 7, 8, 9, 10,.......
instead it could possibly be in this order:
1, 3, 4, 5, 6, 9, 10, 11, 16, 17,....
This sort of thing happens in MS Access Tables where a table record has been deleted. You would think that the ID slot that is deleted would be available to the next record added to the table and would therefore hold that removed ID value but that is not the case. The Auto Index Increment (autonumber) simply continues to supply increasing incremental values. There are of course ways to fix this sequencing mismatch but they are never a good idea and should truly be avoided since doing so can really mess up table relationships and other things within the database. Bottom line, before experimenting with your database always make a Backup of that database first.
So, to utilize a WHERE clause to play against valid record ID's we need to do something like this with our forward and reverse navigation buttons:
Your Forward (Next) Navigation Button:
if(evt.getSource() == nextButton) {
try {
connection = Doctor.connect();
connection.setAutoCommit(false);
number++;
long max = 0, min = 0;
ResultSet results;
Statement statement=connection.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);
// Get the minimum DoctorID value within the DOCTOR table.
String sql0 = "SELECT MIN(DoctorID) AS LowestID from DOCTOR";
results = statement.executeQuery(sql0);
while (results.next()){ min = results.getLong("LowestID"); }
// Get the maximum DoctorID value within the DOCTOR table.
sql0 = "SELECT MAX(DoctorID) AS HighestID from DOCTOR";
results = statement.executeQuery(sql0);
while (results.next()){ max = results.getLong("HighestID"); }
if (max <= 0) {
JOptionPane.showMessageDialog(null, "No records found in Doctor Table.");
return;
}
if (number > min) { previousButton.setEnabled(true); }
if (number > max) {
nextButton.setEnabled(false);
JOptionPane.showMessageDialog(null, "No more records");
number--;
}
results = null;
while (results == null) {
String sql1 = "SELECT * FROM DOCTOR WHERE DoctorID = " + number + ";";
results = statement.executeQuery(sql1);
long id = 0;
// Fill The GUI Form Fields....
while (results.next()){
//id = results.getLong("DoctorID");
textField1.setText("" +results.getInt("DoctorID"));
textField2.setText(results.getString("DoctorName"));
textField3.setText("" + results.getString("PhoneNumber"));
textField4.setText(results.getString("Department"));
textField5.setText(results.getString("Specialization"));
connection.commit();
return;
}
// ----------------------------------------------------------
if (id != number) { results = null; number++; }
}
}
catch(Exception ex){ ex.printStackTrace(); }
}
Your Reverse (Previous) Navigation Button:
if(evt.getSource() == previousButton) {
try {
connection = Doctor.connect();
connection.setAutoCommit(false);
number--;
long max = 0, min = 0;
ResultSet results;
Statement statement=connection.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);
// Get the minimum DoctorID value within the DOCTOR table.
String sql0 = "SELECT MIN(DoctorID) AS LowestID from DOCTOR";
results = statement.executeQuery(sql0);
while (results.next()){ min = results.getLong("LowestID"); }
// --------------------------------------------------------------------------
// Get the maximum DoctorID value within the DOCTOR table.
sql0 = "SELECT MAX(DoctorID) AS HighestID from DOCTOR";
results = statement.executeQuery(sql0);
while (results.next()){ max = results.getLong("HighestID"); }
// --------------------------------------------------------------------------
if (max <= 0) {
JOptionPane.showMessageDialog(null, "No records found in Doctor Table.");
return;
}
if (number < min) {
previousButton.setEnabled(false);
JOptionPane.showMessageDialog(null, "No more records");
number++;
}
if (number < max) { nextButton.setEnabled(true); }
results = null;
while (results == null) {
String sql1 = "SELECT * FROM DOCTOR WHERE DoctorID = " + number + ";";
results = statement.executeQuery(sql1);
long id = 0;
// Fill The GUI Form Fields....
while (results.next()){
textField1.setText("" +results.getInt("DoctorID"));
textField2.setText(results.getString("DoctorName"));
textField3.setText("" + results.getString("PhoneNumber"));
textField4.setText(results.getString("Department"));
textField5.setText(results.getString("Specialization"));
connection.commit();
return;
}
// ----------------------------------------------------------
if (id != number) { results = null; number--; }
}
}
catch(Exception ex){ ex.printStackTrace(); }
}
Things To DO...
So as to remove duplicate code, create a method named
getMinID() that returns a Long Integer data type. Allow this method to accept two String Arguments (fieldName and
tableName). Work the above code section used to gather the minimum DoctorID value within the DOCTOR table into the new
**getMinID() method. Use this new method to replace the formentioned code for both the Forward (Next) and Revese (Previous)
buttons.
So as to remove duplicate code, create a method named
getMaxID() that returns a Long Integer data type. Allow this method to accept two String Arguments (fieldName and
tableName). Work the above code section used to gather the maximum DoctorID value within the DOCTOR table into the new
getMaxID() method. Use this new method to replace the formentioned code for both the Forward (Next) and Revese (Previous)
buttons.
So as to remove duplicate code, create a void method named
fillFormFields(). Allow this method to accept two arguments, one as Connection (*connection) and another as ResultSet
(results) . Work the above code section used to Fill The GUI
Form Fields into the new fillFormFields() method. Use this new
method to replace the formentioned code for both the Forward (Next)
and Revese (Previous) buttons.
Things To Read That Might Be Helpful:
The SQL WHERE clause statement and the SQL ORDER BY statement for sorting your result set.
Searching For Records

How to get the first 2 records from Access's Table and Add them to Jtable (Data filter)

String[] Titulo = {
"ID",
"CDEP",
"NOMDEP",
"POB"
}; //This is the head of the Jtable
Mnc2 = new DefaultTableModel(null, Titulo);
String[] Filas = new String[4];
try { //HERE IS THE CONECTION TO THE BDD(ACCESS)
Conexion_DDB Conn = new Conexion_DDB();
//Prepared Query
PreparedStatement Consulta = Conn.getConnection().prepareStatement("SELECT * FROM [DEPARTAMENTOS]");
ResultSet Resultadobm = Consulta.executeQuery();
while (Resultadobm.next()) {
//This is the part thath I don't know how to get the Strings or values for each row
Filas[0] = Resultadobm.getString("CDEPID");
Filas[1] = Resultadobm.getString("CDEP");
Filas[2] = Resultadobm.getString("POBDEP");
Filas[3] = Resultadobm.getString("NOMDEP");
//There are more records but just for example
while (Resultadobm.next()) {
//This is the part thath I don't know how to get the Strings or values for the first two rows
Mnc2.addColumn(Filas);//Add the columns
}
/*At this part, I'm trying to get the values equals or higher than 50,000 to show then in the same table at the same time with the two first records found*/
//If you can help me here too I would appreciate it a lot
int Menora = 50000;
if (Mnc2.getValueAt(1, 1).equals(Menora)) {
Tabla_Busquedas.setValueAt(Mnc2, 1, 1);
}
Conn.Desconexion(); //Close the connection
} catch (SQLException ex) {
Logger.getLogger(Form_Tabla.class.getName()).log(Level.SEVERE, null, ex);
}
}

while (Resultadobm.next()) { //This is the part thath I don't know how
to get the Strings or values for the first two rows
I am not sure I understand right.I think you can terminate the loop after you fetched first two rows:
while (Resultadobm.next() && Resultadobm.getRow() <= 2) {
// ...your code
}

You can go to the source:
"SELECT TOP 2 * FROM [DEPARTAMENTOS] ORDER BY [SomeUniqueField]");

MySQL - Iterating over columns in a result set

I'm working on a project where a user can assemble the components of a yoga class. It's spread across several files and thus too large to put it all here. The trouble I'm having is in one method where I need to iterate over the horizontal columns of a result set that is returning only ONE ROW from a MySQL database.
I understand that I have to position the cursor on the first row of the result set (which I believe I am doing). Since I have only one row in the result set (my variable is rset), I should be using rset.next() only one time, correct? And then I should be able to use a simple loop to iterate over each column and append the value to my String Builder. I want to skip the first column and append each subsequent value until the loop reaches columns with null values. I cannot find why my code keeps returning a "Before start of result set" exception.
Can anyone spot anything wrong?
I'll post the method as well as the method called by this method. (I posted this in a different question, but I believe the nature of my question has changed, so I'm re-posting this with a different title.)
// Query that returns the poses within a specific section
public String listPosesInSection(String tableName, String sectionName) {
String strList;
StringBuilder strBuilderList = new StringBuilder("");
// Run the query
try {
statement = connection.createStatement();
// Query will collect all columns from one specific row
rset = statement.executeQuery("SELECT * FROM " + tableName + " WHERE " + tableName + "_name = '" + sectionName + "'");
rset.next();
System.out.println("Column count is " + countColumnsInTable(tableName));
for (int i = 2; i <= countColumnsInTable(tableName); i++) {// First value (0) is always null, skip first column (1)
System.out.println("test");
strBuilderList.append(rset.getString(i) + "\n"); // This is line 126 as indicated in the exception message
}
} catch (SQLException e) {
e.printStackTrace();
}
strList = strBuilderList.toString();
return strList.replaceAll(", $",""); // Strips off the trailing comma
}
// Method for getting the number of columns in a table using metadata
public int countColumnsInTable(String sectionType) {
int count = 16;
try {
statement = connection.createStatement();
rset = statement.executeQuery("SELECT * FROM " + sectionType);
rsMetaData = rset.getMetaData();
count = rsMetaData.getColumnCount();
} catch (SQLException e) {
e.printStackTrace();
}
return count;
}
And here is the first part of the exception message:
Column count is 26
java.sql.SQLException: Before start of result set
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1078)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:989)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:975)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:920)
at com.mysql.jdbc.ResultSetImpl.checkRowPos(ResultSetImpl.java:855)
at com.mysql.jdbc.ResultSetImpl.getStringInternal(ResultSetImpl.java:5773)
at com.mysql.jdbc.ResultSetImpl.getString(ResultSetImpl.java:5693)
at YogaDatabaseAccess.listPosesInSection(YogaDatabaseAccess.java:126)
at YogaSectionDesigner$5.actionPerformed(YogaSectionDesigner.java:231)

Looks to me like you're re-using the rset between your two methods. So when countColumnsInTable has completed, the rset variable is pointing to a different result set than it was before, in listPosesInSection. And that result set has not been advanced with next, hence the error message you're getting. You probably want to assign it to a local ResultSet within that method instead.
public int countColumnsInTable(String sectionType) {
int count = 16;
try {
Statement statement = connection.createStatement();
ResultSet rset = statement.executeQuery("SELECT * FROM " + sectionType);
ResultSetMetaData rsMetaData = rset.getMetaData();
count = rsMetaData.getColumnCount();
} catch (SQLException e) {
e.printStackTrace();
}
// Remember to clean up
return count;
}

try this
public String listPosesInSection(String tableName, String sectionName) {
String strList;
StringBuilder strBuilderList = new StringBuilder("");
// Run the query
try {
statement = connection.createStatement();
// Query will collect all columns from one specific row
rset = statement.executeQuery("SELECT * FROM " + tableName + " WHERE " + tableName + "_name = '" + sectionName + "'");
while (rset.next()){
System.out.println("Column count is " + countColumnsInTable(tableName));
for (int i = 2; i <= countColumnsInTable(tableName); i++) {// First value (0) is always null, skip first column (1)
System.out.println("test");
strBuilderList.append(rset.getString(i) + "\n");
}
}
} catch (SQLException e) {
e.printStackTrace();
}
strList = strBuilderList.toString();
return strList.replaceAll(", $",""); // Strips off the trailing comma
}

rset is a ResultSet object
i think you are using the same ResultSet object in listPosesInSection and also in countColumnsInTable
so what is happening here is in listPosesInSection rset holds a result and you have also move the cursor but again in countColumnsInTable you are using the same rset so it is overwritten ie it holds a new result now, you are getting the number of columns but since it holds a new result now the cursor will be before the 1st record so use different Resultset object in countColumnsInTable