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How to instance 1 connection class for multiple requests?

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);
}
}

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;
}
}

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.

why can't java see my mysql driver

I'm having trouble working out why java can't see my mysql driver:
I've downloaded the driver .jar from the mysql website
I've added the jar to my runtime classpath
I can confirm the jar is on the classpath, by printing out the relevant system property
But I'm still getting ClassNotFound Exceptions. Is there anything else I need to be doing?
class example:
package org.rcz.dbtest;
import java.sql.*;
public class DB {
private Connection connect = null;
private Statement stmt = null;
private PreparedStatement prepared = null;
private ResultSet rset = null;
private String driverClassName = "com.myqsl.jdbc.Driver";
private String jdbcUrl = "jdbc:mysql://localhost/ctic_local?user=root&password=server";
private String queryString;
public DB(String query)
{
System.out.println(System.getProperty("java.class.path"));
queryString = query;
}
public void readFromDatabase()
{
try
{
Class.forName(driverClassName);
connect = DriverManager.getConnection(jdbcUrl);
stmt = connect.createStatement();
rset = stmt.executeQuery(queryString);
writeResultSet(rset);
}
catch (ClassNotFoundException cex)
{
System.out.println("Could not find mysql class");
}
catch(SQLException sqex)
{
}
}
private void writeResultSet(ResultSet resultSet) throws SQLException {
// ResultSet is initially before the first data set
while (resultSet.next()) {
// It is possible to get the columns via name
// also possible to get the columns via the column number
// which starts at 1
// e.g. resultSet.getSTring(2);
String user = resultSet.getString("name");
String comment = resultSet.getString("comment");
System.out.println("User: " + user);
System.out.println("Comment: " + comment);
}
}
}
My main class simply passes the query into the DB class:
package org.rcz.dbtest;
import java.io.IOException;
public class Main {
public static void main(String[] args) throws IOException
{
String qstring = "SELECT * FROM comments";
new DB(qstring).readFromDatabase();
System.in.read();
}
}

You've a typo in the driver class name.
private String driverClassName = "com.myqsl.jdbc.Driver";
it should be
private String driverClassName = "com.mysql.jdbc.Driver";
// -------------------------------------^
Unrelated to the concrete problem, holding DB resources like Connection, Statement and ResultSet as an instance variable of the class is a bad idea. You need to create, use and close them in the shortest possible scope in a try-finally block to prevent resource leaking. See also among others this question/answer: When my app loses connection, how should I recover it?