Here's an example of how to connect to a PostgreSQL database server in Java using JDBC:

1. Import the necessary packages

You need to import the necessary JDBC packages for connecting to a database. Here's an example:

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

2. Register the JDBC driver

You need to register the JDBC driver before connecting to the database. Here's an example:

try {
    Class.forName("org.postgresql.Driver");
} catch (ClassNotFoundException e) {
    e.printStackTrace();
}

In this example, we are registering the PostgreSQL JDBC driver.

3. Establish a connection to the database

You need to establish a connection to the database using the getConnection() method of the DriverManager class. Here's an example:

String url = "jdbc:postgresql://hostname:port/mydb";
String username = "myuser";
String password = "mypassword";

try {
    Connection connection = DriverManager.getConnection(url, username, password);
} catch (SQLException e) {
    e.printStackTrace();
}

In this example, we are connecting to a PostgreSQL database with the URL "jdbc:postgresql://hostname:port/mydb", and the username "myuser" and password "mypassword". Note that the URL includes the protocol jdbc:postgresql, the hostname and port hostname:port, and the name of the database mydb.

4. Closing the connection

Once you are finished using the database, you should close the connection to free up resources. Here's an example:

try {
    connection.close();
} catch (SQLException e) {
    e.printStackTrace();
}

In this example, we are closing the connection object that was created earlier.

This is a basic example of how to connect to a PostgreSQL database server in Java using JDBC. There are many other options and configurations that can be used to fine-tune the connection, but this should get you started.