# Database administrative login by Unix domain socket
local   all             postgres                                peer

# TYPE  DATABASE        USER            ADDRESS                 METHOD
host    email           app_postfix     127.0.0.1/32            md5
host    email           app_courier     127.0.0.1/32            md5
host    email           app_sasl        127.0.0.1/32            md5

# "local" is for Unix domain socket connections only
local   all             all                                     peer
# IPv4 local connections:
host    all             all             127.0.0.1/32            md5
# IPv6 local connections:
host    all             all             ::1/128                 md5
# Allow replication connections from localhost, by a user with the
# replication privilege.
#local   replication     postgres                                peer
#host    replication     postgres        127.0.0.1/32            md5
#host    replication     postgres        ::1/128                 md5

virtual_mailbox_maps = pgsql:/etc/postfix/db/users.cf
virtual_maps = pgsql:/etc/postfix/db/aliases.cf

#hosts = 127.0.0.1
user = app_postfix
password = 'abcd'
dbname = email
query = SELECT 'email_local' FROM users_aliases WHERE 'email_alias' = '%s'

#hosts = 127.0.0.1
user = app_postfix
password = 'abcd'
dbname = email
query = SELECT 'path_maildir' FROM users WHERE 'email_local' = '%s' AND 'user_active' = '1'

 warning: connect to pgsql server localhost: FATAL:  password authentication failed for user "app_postfix"?FATAL:  password authentication failed for user "app_postfix"?

public class DerbInterfaz {
        public final String DRIVER = "org.apache.derby.jdbc.EmbeddedDriver";
        public final String JDBC_URL = "jdbc:derby:zadb;create=true";
        public final String SQL_STATEMENT = "select * from citas";
        public Statement statement;
        public void conexion() throws ClassNotFoundException, SQLException {
        Class.forName(DRIVER);
        Connection connection = DriverManager.getConnection(JDBC_URL);
        Statement statement = connection.createStatement();
        try{
            ResultSet resultSet = statement.executeQuery(SQL_STATEMENT);
        }catch(SQLException e){
            connection.createStatement().execute("CREATE TABLE citas(nombre varchar(20),apellidos varchar(20), dia varchar(20), hora varchar(20), motivo varchar(255))");
            ResultSet resultSet = statement.executeQuery(SQL_STATEMENT);
        }
    }
} 

public class saveData {
    derbinterfaz.DerbInterfaz bd = new derbinterfaz.DerbInterfaz();
    public void registrar_datos(){
        try{
            bd.conexion();
            bd.statement.execute("INSERT INTO citas(nombre, apellidos, dia, hora, motivo)VALUES('ad','ad','ad','ad','ad')");
        }catch(ClassNotFoundException | SQLException e){
            JOptionPane.showMessageDialog(null, "No se han podido guardar los datos","Error",JOptionPane.PLAIN_MESSAGE);
        }
    }
}

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at derbinterfaz.saveData.registrar_datos(saveData.java:11)
	at derbinterfaz.interfaz.jButton1ActionPerformed(interfaz.java:130)
	at derbinterfaz.interfaz.access$100(interfaz.java:5)
	at derbinterfaz.interfaz$2.actionPerformed(interfaz.java:54)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
	at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
	at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
	at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
	at java.awt.Component.processMouseEvent(Component.java:6505)......