I'll try my best to make this as clear as possible since I'm new to Java. If there are parts which are unclear to you, please let me know. I apologize beforehand if the structure of my coding is messy.
I'm making a combobox event wherein if an item is selected (or changed), it will display items of that database and if the Update button is clicked, the program will connect to the database based on the item selected in the combobox.
The problem is that the variable cn after the if-else statement, has an error of "variable may have not been initialized".
Please note that I would like to focus on the condition since it is where I'm having difficulties and that the Update part is not a concern at this time.
private void btn_UpdateActionPerformed(java.awt.event.ActionEvent evt)
{
String value2 = (String) combobox_location.getSelectedItem();
String value4 = (String) combobox_category.getSelectedItem();
try
{
if(value2.equals(value4))
{
Connection cn = db.itemconnector.getConnection();
}
String sql = "UPDATE items set location = '"+value2+"' where id = '"+value4+"' " ; //Please ignore this line for the now
PreparedStatement ps1 = cn.prepareStatement(sql);
ps1.execute();
JOptionPane.showMessageDialog(null, "Update Successful");
PreparedStatement ps2 = cn.prepareStatement(ref);
ResultSet rs = ps2.executeQuery();
DefaultTableModel tm = (DefaultTableModel)itemTable.getModel();
tm.setRowCount(0);
while(rs.next())
{
Object o[] = {rs.getInt("id"), rs.getString("location"), rs.getString("product_name"),rs.getString("product_category"),rs.getString("product_description"),rs.getInt("product_stock"), rs.getFloat("product_price"), rs.getString("product_status")};
tm.addRow(o);
}
}
catch (Exception e)
{
System.out.println("Update Error!");
}
}
I have a package db that has itemconnector class for database connection and here is my code I've written in it. I hope I have provided all necessary details for your assistance. If you need more info, please let me know. Thank you.
package db;
import java.sql.*;
public class itemconnector {
/**
*
* #return
* #throws Exception
*/
public static Connection getConnection() throws Exception
{
Class.forName("com.mysql.jdbc.Driver");
Connection cn = (Connection)
DriverManager.getConnection("jdbc:mysql://192.168.1.50:3306/sales","root","");
return cn;
}
public static Connection getConnection1() throws Exception
{
Class.forName("com.mysql.jdbc.Driver");
Connection cn = (Connection)
DriverManager.getConnection("jdbc:mysql://192.168.1.50:3306/sales1","root","");
return cn;
}
public static Connection getConnection2() throws Exception
{
Class.forName("com.mysql.jdbc.Driver");
Connection cn = (Connection)
DriverManager.getConnection("jdbc:mysql://192.168.1.50:3306/sales2","root","");
return cn;
}
}
Related
So I'm currently working on a project that will be using a database but its my first time trying fiddling with it on java.
But I'm already seeing my first problem is how would i make one single file that handles connection while other files handles GET/ADD/UPDATE/DELETE (one for each table) what would properly be the best way on doing this ?
To not having to place connection values in each file and do the connection
I though about extending the connection class with the other classes but idk if its a great idea.
import java.sql.*;
public class DatabaseConnection {
public static void main(String[] args) {
final String url = "jdbc:postgresql://localhost:5432/Database";
final String user = "dbuser";
final String password = "dbpass";
try(Connection conn = DriverManager.getConnection(url, user, password)) {
System.out.println("Connection successful!");
} catch (SQLException e) {
System.out.println("Connection failure.");
e.printStackTrace();
}
}
}
What would be the best approach?
Maybe i'm wrong, but i think you need connection pool.
Try to find instruction here https://www.baeldung.com/java-connection-pooling
You could move the database connection related code to a utility class, and use the PreparedStatement class to precompile the SQL Query
public class doSomething {
Connection conn = null;
PreparedStatement pst = null;
public static void main(String [] args){
conn = DatabaseConnection.connect()
String qry = "Select * from table_name";
pst = (PreparedStatement) conn.prepareStatement(qry);
}
}
Well, I'm trying to use SQLite in my Libgdx game, but don't know how.
public class Main {
public static void main(String[] args){
LwjglApplicationConfiguration config = new LwjglApplicationConfiguration();
config.title = Game.TITLE;
config.width = Game.V_WIDTH * Game.SCALE;
config.height = Game.V_HEIGHT * Game.SCALE;
new LwjglApplication(new Game(), config);
}}
What I need to do in my main? lol
I've been looking for this but, all I can find is related to Android application.
I already have the driver in my ref libraries, and connection class..
What I usually do when using a database with an application, is make a ConnectionFactory, that returns a new connection to the database.
public class ConnectionFactory {
public static Connection getConnection() {
Connection con = null;
Class.forName("org.sqlite.JDBC");
con = DriverManager.getConnection("jdbc:sqlite:test.db"); //change to whatever db you want
return con;
}
}
now we have a ConnectionFactory that can pump out connections to our database. Now when we want to interact with the database, you can get the connection appropriately. inside your main, it might look something like this:
public static void main(String[] args) {
Connection con = null;
String firstName = null, lastName = null;
try {
con = ConnectionFactory.getConnection();
PreparedStatement pstmt = con.prepareStatement("SELECT * FROM myTable where myId = ?");
pstmt.setInt(1, /*some id here, ill put this as example:*/ 1234567);
//execute the query and put into result set so we can get the values.
ResultSet rs = pstmt.executeQuery();
//the resultset iterates through rows, by calling next
if( rs.next() ) //could be while(rs.next()) if expecting multiple rows
{
firstName = rs.getString("firstName"); //column name you want to grab here
lastName = rs.getString("lastName");
}
} catch(SQLException sqle) {
sqle.printStackTrace();
}
finally {
try {
con.close(); //dont forget to close your connection to database!
} catch(SQLException sqle) {
sqle.printStackTrace();
}
}
}
You will need to create tables within the SQLite database and insert records before you can do any interactions though, so keep that in mind.
first time posting so sorry if my question is slightly strange.
So I have a project in school that requires us to create java classes using netbeans that open up a window with three options, check stock, purchase item and update stock.
We had a class called stockdata that held the details of 5 different items for us to use in our three classes to check, purchase and update items. The latest stage of our coursework requires us to create a derby database and enter the items into a table.
I have done this with no issues but I am having a problem getting the items from the table back into my classes to use. We were given the following code but I can't get it to work, even using the commented hints.
package stock;
// Skeleton version of StockData.java that links to a database.
// NOTE: You should not have to make any changes to the other
// Java GUI classes for this to work, if you complete it correctly.
// Indeed these classes shouldn't even need to be recompiled
import java.sql.*; // DB handling package
import java.io.*;
import org.apache.derby.drda.NetworkServerControl;
public class StockData {
private static Connection connection;
private static Statement stmt;
static {
// standard code to open a connection and statement to an Access database
try {
NetworkServerControl server = new NetworkServerControl();
server.start(null);
// Load JDBC driver
Class.forName("org.apache.derby.jdbc.EmbeddedDriver");
//Establish a connection
String sourceURL = "jdbc:derby://localhost:1527/"
+ new File("UserDB").getAbsolutePath() + ";";
connection = DriverManager.getConnection(sourceURL, "use", "use");
stmt = connection.createStatement();
} // The following exceptions must be caught
catch (ClassNotFoundException cnfe) {
System.out.println(cnfe);
} catch (SQLException sqle) {
System.out.println(sqle);
} catch (Exception e) {
System.out.println(e);
}
}
// You could make methods getName, getPrice and getQuantity simpler by using an auxiliary
// private String method getField(String key, int fieldNo) to return the appropriate field as a String
public static String getName(String key) {
try {
// Need single quote marks ' around the key field in SQL. This is easy to get wrong!
// For instance if key was "11" the SELECT statement would be:
// SELECT * FROM Stock WHERE stockKey = '11'
ResultSet res = stmt.executeQuery("SELECT * FROM Stock WHERE stockKey = '" + key + "'");
if (res.next()) { // there is a result
// the name field is the second one in the ResultSet
// Note that with ResultSet we count the fields starting from 1
return res.getString(2);
} else {
return null;
}
} catch (SQLException e) {
System.out.println(e);
return null;
}
}
public static double getPrice(String key) {
// Similar to getName. If no result, return -1.0
return 0;
}
public static int getQuantity(String key) {
// Similar to getName. If no result, return -1
return 0;
}
// update stock levels
// extra is +ve if adding stock
// extra is -ve if selling stock
public static void update(String key, int extra) {
// SQL UPDATE statement required. For instance if extra is 5 and stockKey is "11" then updateStr is
// UPDATE Stock SET stockQuantity = stockQuantity + 5 WHERE stockKey = '11'
String updateStr = "UPDATE Stock SET stockQuantity = stockQuantity + " + extra + " WHERE stockKey = '" + key + "'";
System.out.println(updateStr);
try {
stmt.executeUpdate(updateStr);
} catch (SQLException e) {
System.out.println(e);
}
}
// close the database
public static void close() {
try {
connection.close();
} catch (SQLException e) {
// this shouldn't happen
System.out.println(e);
}
}
}
Sorry if this seems a stupid question but I am fairly new to Java and was making good progress until this roadblock.
Thanks in advance!
Alex
Searching for "java sql" on Google delivers this link: https://docs.oracle.com/javase/tutorial/jdbc/basics/processingsqlstatements.html
From a connection you can create a statement (you can find this in the link and in your code) , then fetch a result set and loop over that with rs.next(). That should get your started.
Of course you have to make sure that the driver and database are there/running, just saying...
Here netbeans has nothing to do with database. This is a Java-based integrated development environment(IDE) that will help you to reduce syntactic error.
public void dataAccess(){
try {
String connectionUrl = "suitable connection url as per your database";
Connection con = null;
Statement stmt = null;
ResultSet rs = null;
Class.forName("JDBC driver name as per your database");
con = DriverManager.getConnection(connectionUrl, userName, password);
String SQL = "SQL query as per your criteria";
stmt = con.createStatement();
rs = stmt.executeQuery(query);
while (rs.next()) {
// look into ResultSet api and use method as per your requirement
}
rs.close();
}
catch (Exception e) {
//log error message ;
}
}
What is the problem in this code? where it is update the all column values with the same last one .
public class dbconnection {
java.sql.Connection con;
java.sql.Statement st;
ResultSet rs;
public EncBean getConnection()throws SQLException{
EncBean encBean1 = new EncBean();
String v_url= "jdbc:oracle:thin:#192.168.2.138:1522:orcl2";
String v_username= "scott";
String v_password = "tiger";
try
{
DriverManager.registerDriver(new oracle.jdbc.OracleDriver());
con = DriverManager.getConnection(v_url,v_username,v_password);
System.out.println ("Connection to Oracle database was Established");
}
catch ( SQLException e)
{
e.printStackTrace();
}
return encBean1;
}
public EncBean selectRows()
{
EncBean encBean2 = new EncBean();
try
{
String SQLselect="select JOB_NAME from job";
st=con.createStatement();
rs=st.executeQuery(SQLselect);
while (rs.next()) {
encBean2.setName(rs.getString("JOB_NAME"));
}
}
catch ( Exception ex )
{
ex.printStackTrace();
}
return encBean2;
}
public void updateRows(String updatedname){
try
{
Statement stmt = con.createStatement(ResultSet.TYPE_FORWARD_ONLY,ResultSet.CONCUR_UPDATABLE);
ResultSet srs = stmt.executeQuery("select job_name from job " );
while (srs.next()) {
srs.updateString("job_name", updatedname);
srs.updateRow();
con.commit();}
System.out.println("An existing user was updated successfully!");}
catch(SQLException err){
System.out.println(err.getMessage());
}}}
This is the main
public class mainenc {
public static void main(String[] args) throws Exception{
dbconnection dbcon = new dbconnection();
EncBean encbeancon= dbcon.getConnection();
EncBean encBean5 = dbcon.selectRows();
enc concatinputs = new enc();
EncBean encBeanconcat = concatinputs.funconcat(encBean5.getName());
EncBean encBean4 = concatinputs.inputencryption(encBeanconcat.getConcatenatedData());
String vReserverbin= encBean4.getReversedBinary();
String ascistring= concatinputs.convertBinaryStringToString(vReserverbin);
dbcon.updateRows(ascistring);
}}
What is the problem in this code? where it is update the all column values with the same last one .
After updated method you should write list method again.
Try to take this example:
UPDATE tableB
SET tableB.value , tableA.value, tableB.value)
WHERE tableA.name = 'Joe'
It is kind of obvious: dbcon.updateRows(...) calls for the update method and that method does the job.
But as Erhan said, you don't get to see the result because you don't actually make use of updated records, e.g. show them etc. At least, you can check it out at the DB level if op is completed.
But I really disliked your comment:
plz can you do it for me?
You should do your own task and ask help when you need a hand. But never expect someone else to do your job mate.
I am trying to add ComboxItem from database. I have write the code given below. When I run the program it give a error message.
Message is
java.sql.SQLException:Driver does not support this function
someone please help me.
here is my code
public class DepositFirstForm extends javax.swing.JFrame {
Connection conn=null;
ResultSet rst=null;
PreparedStatement pst=null;
private void ItemComb(){
private void ItemComb(){
String sql="SELECT * FROM account_type";
try
{
pst=conn.prepareStatement(sql);
rst=pst.executeQuery(sql);
while(rst.next()){
String actype=rst.getString("account_type");
dfcmb1.addItem(actype);
}
}
catch(Exception e)
{
JOptionPane.showMessageDialog(null, e);
}
}
private void formWindowOpened(java.awt.event.WindowEventevt) {
// TODO add your handling code here:
conn=Connect.connectDB();
ItemComb();
}
}
my table name is account_type
serial_no account_type
1 regular
2 premium
3 golden
Try removing the (sql) parameter from you call to pst.executeQuery:
rst = pst.executeQuery();
Since your PreparedStatement is already created using the SQL.
I trust you are declaring conn, pst and rst somewhere.