package dbcontract.db;

public interface DbContract {

    public static final String HOST = "jdbc:postgresql://localhost:5432/";
    public static final String DB_NAME = "db_notespesa";
    public static final String USERNAME = "postgres";
    public static final String PASSWORD = "";
}

package dbcontract;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

import dbcontract.db.DbContract;

public class TestConnection {

    public static void main(String[] args) {
        try {
            Class.forName("org.postgresql.Driver");
            Connection conn = DriverManager.getConnection(
                    DbContract.HOST+DbContract.DB_NAME,
                    DbContract.USERNAME,
                    DbContract.PASSWORD);
            System.out.println("DB connected");

        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
    }
}

java.lang.ClassNotFoundException: org.postgresql.Driver
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:583)
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
    at java.base/java.lang.Class.forName0(Native Method)
    at java.base/java.lang.Class.forName(Class.java:315)
    at TestConnection.main(TestConnection.java:13)

<`<!-- https://mvnrepository.com/artifact/org.postgresql/postgresql -->
<dependency>
    <groupId>org.postgresql</groupId>
    <artifactId>postgresql</artifactId>
    <version>42.2.5</version>
</dependency>