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hello guys welcome to so much' solutions my name is somnath Mukhopadhyay and today i am going to discuss about one of the very important design patterns called observer pattern so as I promised in my last video which discussed about the callback mechanism using Java I told you that there are there are other design patterns similar to this callback mechanism one is an out of them one the more the one is and that design pattern is called observer pattern okay and so today we I am going to discuss about the observer pattern okay now let me give you the practical scenario why we need an observer pattern suppose there is the data in the tabular from in a central repository now few now two three people are viewing this data from three different machine one one guy is viewing the data in establish form another guy viewing the data in this pie chart form and another guy is probably seeing the data in a bar graph form okay now suppose the data gets change or data data is being updated in the central repository so unless the views that only these three views update themselves with the change state of the central repository data their views will be in the inconsistent state which is not at all desirable right so so there there like the significance of observer pattern if we use the observer pattern then our central repository will be able to notify all of its viewers that my data has been changed or my data has been updated please update your views accordingly okay and then all the viewers will update the state with the change state of the sub in a central repository traitor this central repository data in observer pattern is called subject and all the viewers that were that were being updated with the change state of the subject is are called observers so what is actually so what is what is the main point there in the subject will be in a particular state so when able it state changes then it will notify all of its observers who were observing to the subject state change and these observers will then update themselves in accordance with that change state of them a subject okay this is the main point of the observer pattern so now let's implement the observer pattern in we in Java okay so to implement observer pattern in Java we should have first of all we should have a two interfaces we should have one interface is called the observer interface and the other interface is called subjecting to face as I told you as I told you earlier that interface is like contract so so this that means whoever wants to be observed or whoever wants to notify about exchange should implement the subject interface and whoever wants to observe or update themselves with the change state of the subject should implement them observer interfaces okay so let's let's - let's create the tool interface first so I'll create two interfaces one is one the subject interface subs interface and finish it off and then the other is the observable interface new interface and the others is the observe interphase okay these are two interfaces I have created now let's add methods to the subject interface so in the subject interface it should have a method called attach observer observer and which would say take a data and observer and then it should have another method called detach observer observer and again it will take additional parameter as observer the pattern and then it should have a function called notified observers okay and it should have a function of a lot of observers that's it for subject interface now let's go to the observer intervals observer interface should have something like update themes and have a function limited which should enable them to update the state with subjects avoidable and this function this method we can have maybe maybe to shade the subject of a specific option okay this method these two methods will be sufficient for the observer interface now let's let's define Phi let's do the fire say ball let's now define the concrete subject class which will basically so which will be basically having some data and whose data will be observed which data will be watched by the concrete of Java class okay so let's create two classes one is the concrete subject concrete subject and it should implement subject interface implements an interface finish it off and then we should have a concrete observer class we should have a concrete observer class or concrete observer class and then it should again interface observer observer and then okay and fill it up okay so these are the two classes two concrete classes which will implement one will implement subject interface and the other will implement the observable interface okay now let's go to the concrete subject class what it will do concrete subject last naturally will have a list of observers okay so let's do it I release a list up sorry list let's have it list off observer observers and lesson give name observers you list all right you are less now what we'll do actually we're simply up clear this input list input list and then we'll improve that of these artists as well okay all right and then as I told you we should have a we should have a private seeing data which will give them which will keep the state of the observer subject and then then we should have a getter setter for this nature and the our seat okay now what will the attached observer will do attach observer will simply of river the ordered and an observer okay that's it and then each will simply move that don't remove that observer move that observer from yourself notify observer which will simply traverse to the list of the observers and it will take all I know it will it will ask them to update themselves so what it will do actually it will simply so I can do it using the iterator but I am NOT doing it because there are many people who won't be able to understand that Italy iterator you know III even though I'm not assume it but let us do it in the most simple fashion so what I will do in lanes is the cultural observer thought sighs all right and then I will do a for loop the entire is equal to zero I less than Lane and then I plus plus what it will do observer observer surveys you can to observers dot gate I and then what it will do actually it is simply ask each observer to update itself so updated same okay they are safe - Sadie - ask everybody to update yourself okay to travel through the you know it will traverse through the list of observers and it allows them to update themselves then that's that's all for this concrete subject class now let's do the observer class in the observer class observer concrete observer class what we will do say to simply have a reference to the concrete subject me to suit it will have a reference to the concrete subject concrete subject subject and then it will have these have the subject or the other these are subject subject easy-care to subject and it any problem so it should typecasted I think it should be typed a state subject okay this is one thing done and then in the update function what it will do so first of all let's do the same old same as ever all is done what we will do actually in the update function it will simply get the data right so again each it should also have the string dr8 string data which should have a we should get the local copy of the subject data so get a satyr all right so what did we do they simply have data or rather these dot data you can to you easy care to him it was simply have subject subject dot here to data and not only that it should also simply system dot out dot println three until then what it will print print the these dot data okay so this is basically painting that these are data to tell us what the current state of each energy observer okay and then so this is everything file save all is done so this is everything now what we'll do we'll go to the main will create a concrete subject Congress subject subject is equal to nu concrete congress subject is called a new country subject with kiddush observers observer one which was you observer the observer okay you know observer one is done the concrete of some national issue has a new on concrete observer then server ruby is - it was new concrete observer all right then one this is done subject should attach an observer that's an observer ruby is one and then it should attach subjects should attach this observers an observer movies - all right so this is this it will create and then what you will do at the same time will be is one dot said subject set subject subject and it will also have movies to door set subject subject so everything is done so observer will have subject would have a list of observers similarly observer will have a reference to the subject okay now what we will do we will have the created state of the subject subject dot say data what he will do let's have a concrete subject class suppose the data was initially initially suppose hello okay suppose it will initially hello whatever we do we will change the subject it will change this subject a strong seeing subject current current data equals subject taught and get data okay and then suppose we are simply changing the data and then what we will do actually this is current data subject current data was in front of a large today I have been updated subject teacher have been updated sadly data has been updated okay and then it will simply update USA itself to say subjected and updated now once the subject it has been updated then it should notify the it should notify in the observer right so what he will do subject subject thought notify of rama's okay simply to notify us and then it will once it is notifying the observer then it should basically say issued the observer will get their state updated okay so previously previously so it would observer will basically get them updates updated state of the in a subject okay so let's see let's see how it works so let's observe a pattern so what we will see actually in because in the note in this as notified observers will call object function of the officer all the observers attached so each and every observer in the object function of the concrete observer what we are doing we are getting the data from the subject and we are basically you know painting their data so what you will do and great actually in the output will get to print out having the new data of them having the new data of the subject okay so let's see let's see how it works known as application so fellow subject data has been hello subject it has been updated so probably having let's hear of it like this again down it and hello subject data has been updated hello submitted an updated okay so probably there is some spinning message has been updated this would be the right thing so again submitted has been updated okay so so you are getting the point so it when able the subject state changes its subject data changes its state subjects would notify all the observers that my data has been updated so please you know greater yourself updated okay so this is all our observer pattern and hopefully you have enjoyed the session thank you for attending the class bye