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How to Connect on a different database in Try-Catch in Java?

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;
}
}

SQLite connection w/ Libgdx .Desktop

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.

connecting Java to MySQL

I'm doing my dissertation on software engineering and im building a small application that makes use of a SQL DB, in this case MySQL. I'm also using the application controller pattern. So the code I have working for retrieving data from the db is;
public static void main(String[] args)
{
try
{
String url = "jdbc:mysql://localhost:3306/tm470_returns_stock_management_system";
Class.forName("com.mysql.jdbc.Driver").newInstance();
Connection con = DriverManager.getConnection(url,"root","root");
Statement st = con.createStatement();
ResultSet res = st.executeQuery("SELECT * FROM test_table");
while (res.next())
{
int id = res.getInt("test_id");
String msg = res.getString("test_info");
System.out.println(id + "\t" + msg);
}
con.close();
}
catch(Exception e)
{
System.out.println("DB connection unsuccesful");
}
}
I now want to transfer this out of my Main class/string and into my Application Controller Class (which is called Facility).
Now my question is, for every method in my Facility Class that needs to access the DB, do i have to do the full code each time? Or can i create a method within the Facility class that each application method can just call whenever it needs to access the DB. If i can condense all this into a method, can you advise me how to go about it please?
Be gentle with me guys, I am a learner :)

How about adding a utility class like ConnectionUtil and using the static method to access the connection.
import java.sql.Connection;
import java.sql.DriverManager;
public class ConnectionUtil{
static final String url = "jdbc:mysql://localhost:3306/";
static final String dbName = "test";
static final String driver = "com.mysql.jdbc.Driver";
static final String userName = "userparatest";
static final String password = "userparatest";
Connection con = null;
static Connection getConnection() throws Exception {
if(con == null)
{ Class.forName(driver).newInstance();
con = DriverManager.getConnection(url + dbName, userName,password);
}
return con;
}
}
this can be further improved but just providing a start..
just call below whenever you want a statement..
Statement st = ConnectionUtil.getConnection().createStatement();

I would map it as a own class, which is used by your application other classes. When you define it as a singleton you will only need one instance in your complete application

Yes , you can write a method for accessing db and you can reuse it across all the applications.
Keep the following in a method and reuse it.
String url = "jdbc:mysql://localhost:3306/tm470_returns_stock_management_system";
Class.forName("com.mysql.jdbc.Driver").newInstance();
Connection con = DriverManager.getConnection(url,"root","root");
Statement st = con.createStatement();
int productID = 6;
String skuCode = "ABC123";
ResultSet res = st.executeQuery("SELECT * FROM test_table");
while (res.next())
{
int id = res.getInt("test_id");
String msg = res.getString("test_info");
System.out.println(id + "\t" + msg);
}

Why i can not connect to mysql db using java

I have used jdbc driver before.But for this piece of program i can't connect to the db.This doesn't throw any exception or anything. Just won't connect. I couldn't find a solution online either.Below is the code i tried to run :( Please help in solving this. Thank you in advance :)
public class HeapMySql<T extends Comparable<T>> implements HeapInterface {
static final String DRIVER = "com.mysql.jdbc.Driver";
static final String DB_URL = "jdbc:mysql://localhost/Heap";
static final String USERNAME = "root";
static final String PASSWORD = "";
private int size = 0 ;
String sql;
static Statement stmt = null;
static Connection conn = null;
static ResultSet rs = null;
public void HeapMySql(){
try
{
sql = "CREATE TABLE testHeap (index integer, value integer);";
stmt.executeUpdate(sql);
System.out.println("Done");
}catch(Exception e){
}
}
public static void main(String [] arg){
try{
Class.forName("com.mysql.jdbc.Driver");
System.out.println("Connecting to a selected database...");
conn = DriverManager.getConnection(DB_URL, USERNAME, PASSWORD);
System.out.println("Connected database successfully...");
System.out.println("Creating table in given database..."); //lets create a table in our database
stmt = conn.createStatement();
HeapMySql test1 = new HeapMySql<>();
}catch(ClassNotFoundException | SQLException ex){
}finally{
}

A constructor does not have a return type: docs
Remove void from public void HeapMySql() and it will do the work.
Also as said in comments, you should print the stacktrace in your catch blocks. This makes it easy to understand the exception and resolve the problem.

How to use the mysql connection on multiple classes?

I have some issues with the conection between java application and mysql.
This is my file(this file work very well):
import java.sql.*;
import javax.swing.JFrame;
public class MysqlConnect{
public static void main(String[] args) throws SQLException {
Connection conn = null;
String url = "jdbc:mysql://localhost:3306/";
String dbName = "jdbctutorial";
String driver = "com.mysql.jdbc.Driver";
String userName = "birthday";
String password = "123456";
try {
Class.forName(driver).newInstance();
conn = DriverManager.getConnection(url+dbName,userName,password);
System.out.println("Connected to the database");
} catch (Exception e) {
e.printStackTrace();
}
}
}
Is possible "to separate" the main and mysql connection ??
My idea is something like that :I have the MysqlConnection file and another GUI file.
In the GUi file I have a button (ADD) and whenever I click this button some datas will be stored to database .My problem is that I don't know how run the query ,because I need the Statement variable ,Connection variable,etc..What I suppose to do ?To do the mysqlConnection and GUI in the same file ?Another idea of mine is to do an object of type MysqlConnection and work with that object.And here is the problem :If I remove the (public void main .....) i have an error at try and catch.
Sorry if my english is bad but I hope i make myself clear .
Thanks in advance .

What I understand from your question is that you want to make an application that shows data from a database in a GUI. Maybe you should look into an architecture like MVC (Model-View-Controller) where you have the model as an representation of the data in the database and having the view as a graphical representation of the model.
Since it didn't came to mind to apply a certain architecture, I would recommend you to look into that first, do a little bit of research and then implement your system. When looking into the MVC-architecture, I recommend you to start here. This is really the most easy example you could think of.
About your database connection: your setup looks good, though first of all, put it in a separate class and add query functionality to it. While implementing that part, this would come in handy. After that, you can let the Controller call the database to manipulate the Model on a button press, which will update the View (GUI) in your MVC-architecture.
So, do NOT put your database connection and your Main or GUI in the same class! This is a bad code style, violates the Single Responsibility Principle and will give you more trouble in future developing! Instead, use a proper architecture
If you want further help, always feel free to ask! I have recently studied this kind of stuff and made an application like this.

Hi RvanHeest thank you very much for your time.I try to do like that :
MysqlConnect.java
public class MysqlConnect{
public Connection conn = null;
public String url = "jdbc:mysql://localhost:3306/";
public String dbName = "jdbctutorial";
public String driver = "com.mysql.jdbc.Driver";
public String userName = "birthday";
public String password = "123456";
public String query="Select * From Person";
public Statement stmt;
public ResultSet rs;
public void crearedatabase(){
try {
Class.forName(driver).newInstance();
conn = DriverManager.getConnection(url+dbName,userName,password);
System.out.println("Connected to the database");
Statement stmt = conn.createStatement();
} catch (Exception e) {
System.out.println("Error!!!");
}
}
}
and in mine Gui class like that :
GUi file:
.................
................
MysqlConnect mysqlitem = new MysqlConnect();
mysqlitem.crearedatabase();
String query = "INSERT INTO persons("
+ "id"
+ "name"
+ "lastname"
+ "date) "
+ "VALUES(null,Maxim,Alexandru-Vasile,1990-12-28)";
try{
mysqlitem.rs=mysqlitem.stmt.executeQuery(query);
}
catch(Exception e1){
System.out.println("Eroare");
}
On the " mysqlitem.rs=mysqlitem.stmt.executeQuery(query);" I have an Exeption error and I don't know how to resolve..
Thank you very much again !!!

I ran in to the same issue.
I found the root cause to be that you are declaring the stmt variable twice.
Your code should look this like:
public class MysqlConnect{
public Connection conn = null;
public String url = "jdbc:mysql://localhost:3306/";
public String dbName = "jdbctutorial";
public String driver = "com.mysql.jdbc.Driver";
public String userName = "birthday";
public String password = "123456";
public String query="Select * From Person";
public Statement stmt;
public ResultSet rs;
public void crearedatabase(){
try {
Class.forName(driver).newInstance();
conn = DriverManager.getConnection(url+dbName,userName,password);
System.out.println("Connected to the database");
stmt = conn.createStatement();
} catch (Exception e) {
System.out.println("Error!!!");
}
}
}
Note the change to the line 18 "stmt = conn.createStatement();"

I wrote this code for create a separate dbconnection class on a separate java file and its working fine for me.
public class dbConnection{
public Connection getConnection()
{
String url = "jdbc:mysql://localhost:88/shop";
String username = "root";
String password = "";
Connection con = null;
try
{
Class.forName("com.mysql.jdbc.Driver");
}
catch (ClassNotFoundException e1)
{
// TODO Auto-generated catch block
e1.printStackTrace();
}
try
{
con = DriverManager.getConnection(url, username, password);
}
catch (Exception e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
return con;
}
}
// USING THE ABOVE CONNECTION ON DIFF CLASS
-----------
Connection con=new dbConnection().getConnection();
------------
Credits to StackOverFlow...

public class LoadDriver {
public static void sqlDriver(String[] args) throws InstantiationException,
IllegalAccessException, ClassNotFoundException {
// TODO Auto-generated method stub
try {
Class.forName("com.mysql.jdbc.Driver").newInstance();
and in your main class
try {
LoadDriver.sqlDriver(null);