How to execute stored procedure with JDBC

executePLSqlFunctions.java

import java.sql.*;

public class Main {

    public static void main(String[] args) {
        String sql = "{? = call fn_test()}";
        try (Connection conn = getConnection(); CallableStatement callableStatement = conn.prepareCall(sql);
            PreparedStatement preparedStatement = conn.prepareStatement("select fn_test()")) {
            // Execute stored procedured CallableStatement (recommend way)
            callableStatement.registerOutParameter(1, Types.VARCHAR);
            callableStatement.execute();
            System.out.println("Response: " + callableStatement.getString(1));
            // Execute stored procedured using PreparedStatement
            ResultSet rs = preparedStatement.executeQuery();
//            ResultSet rs = preparedStatement.getResultSet();
            rs.next();
            System.out.println("Response: " + rs.getString(1));
            rs.close();
        } catch (SQLException e) {
            e.printStackTrace();
            System.err.println("Error al conectarse: " + e.getSQLState() + "-" + e.getMessage());
        } catch (NullPointerException e) {
            e.printStackTrace();
            System.err.println("Error al conectarse: " + e.getMessage());
        }
    }

    static Connection getConnection() throws SQLException {
        return DriverManager.getConnection("jdbc:postgresql://localhost/dbtest", "juan", "12345");
    }
}