Observer pattern steps:

$1.$ Create an interface called AlarmListener. This is the observer interface. In AlarmListener, there is a void alarm$()$ method defined.

$2.$ Create a class called SensorSystem. This is the publisher class. Define one instance variable

ArrayList$$ AlarmListener $>$ listeners $=$ new ArrayList$()$; This ArrayList saves all the observers of this publisher.

$3.$ In SensorSystem, define a method void register$($AlarmListener alarmListener$).$ What it does is to add alarmListener to the ArrayList listeners.

$4.$ In SensorSystem, define another method void soundTheAlarm$().$ What it does is to use a for loop to loop through all the listeners$/$observers in the ArrayList listeners, and call their alarm$()$ method.

$5.$ Different three concrete observer classes: Lighting, Gates, and Surveillance. Make them implement the interface AlarmListener. Implement the alarm$()$ method in all three classes. In Lighting, alarm$()$ prints out "lights up$".$ In Gates, alarm$()$ prints out "gates close". In Surveillance, alarm$()$ prints out "Surveillance $-$ by the numbers:".

$6.$ Use the following client code to try it$.$

$```$

public class ObservserDemo $\{$

public static void main$($String$[]$ args $)\{$

SensorSystem sensorSystem $=$ new SensorSystem$()$;

sensorSystem$\_$register$($new Gates$[$l$)$;

sensorSystem$\_$register$($new Lighting$($l$)$;

sensorSystem$\_$register$($new Surveillance$())$;

sensorSystem$\_$soundTheAlarm$()$;

$\}$

$\}$

$```$