To connect to a PostgreSQL database from a Java application, you will need to do the following:

1. Add the PostgreSQL JDBC driver to your project’s classpath. You can do this by adding the following dependency to your project’s pom.xml file if you are using Maven:

<dependency>
  <groupId>org.postgresql</groupId>
  <artifactId>postgresql</artifactId>
  <version>42.2.16</version>
</dependency>

Alternatively, if you are using a different build system or if you want to add the JDBC driver manually, you can download the postgresql JAR file from the Maven Central repository and add it to your classpath.

2. Create a Connection object to establish a connection to the database. To create a Connection object, you will need to use the DriverManager class and call the getConnection() method, passing in the JDBC URL of the database, the username, and the password as arguments. Here is an example of how to create a Connection object:

String jdbcUrl = "jdbc:postgresql://localhost:5432/mydatabase";
String username = "postgres";
String password = "password";

Connection connection = DriverManager.getConnection(jdbcUrl, username, password);

In this example, the jdbcUrl variable contains the JDBC URL of the PostgreSQL database, the username variable contains the username to use to connect to the database, and the password variable contains the password. The getConnection() method returns a Connection object that represents the connection to the database.

3. Use the Connection object to create Statement and ResultSet objects to execute SQL statements and retrieve the results. Once you have a Connection object, you can use it to create Statement and ResultSet objects to execute SQL statements and retrieve the results. Here is an example of how to execute a SELECT statement and retrieve the results:

String sql = "SELECT * FROM employees";

Statement statement = connection.createStatement();
ResultSet resultSet = statement.executeQuery(sql);

while (resultSet.next()) {
  int id = resultSet.getInt("id");
  String firstName = resultSet.getString("first_name");
  String lastName = resultSet.getString("last_name");

  // Process the retrieved data
}

In this example, the sql variable contains the SELECT statement to execute, and the createStatement() method of the Connection object is used to create a Statement object. The executeQuery() method of the Statement object is then used to execute the SELECT statement and return the results as a ResultSet object. The next() method of the ResultSet is used to iterate through the rows of the result set, and the getXxx() methods are used to retrieve the values of the columns.