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hello subscribers and YouTube Watchers from slide nerd this is Viv's over here in this video I'm going to show you guys how to perform validation of the data that has been entered inside an edit text using something called text watcher before we discuss anything let me show you a small problem you have an app like this where you ask the user to enter a percentage value and he enters 200 do you think this is acceptable well of course not let me show you the piece of code here if you guys notice I have nothing great but just a text view and an edit text and I'm using this Android input type equals two number to ensure that the user can only enter numbers inside and I'm using this Android max length equals to three to ensure that the user cannot enter more than three characters inside the edit text I have discussed about both these attributes in my previous video you guys can definitely go ahead and refer to see how these attributes work but for now think about this despite all these attempts by you to ensure that the user enters the proper value well the user is entering 200 what can you say so here we are going to use a different method to validate and ensure that this kind of error doesn't happen so once again I have already discussed about events previously in my vid about button click here I'm going to again give you a small overview there is a person at your door and he rings your doorbell what happens the door circuit gets triggered and the doorbell rings now remember everybody hears the doorbell but only one person comes outside and opens the door the same way events work in Android think about this you have an edit text the text changes inside your edit text and event is triggered because of this change and there is some guy called text watcher who is actually interested in listening to this changes they have this text watcher has certain methods that you can implement and you can write code to decide what should be done when your edit text changes its content right so what are the different steps to end sure that the user will never ever enter an incorrect value inside your edit text here is something you should do first bring the edit text that you want to track or monitor inside your java code then implement this text watcher either inside your activity or as an inner class or as a separate class or any other means that you feel comfortable with then implement all the methods inside the text watcher now remember these are the methods that you're going to override to ensure that you take certain action when the edit text changes its content right now remember the most important step is the forth step where you have to actually link the edit text and the text watcher because the text water should know who is the guy who is going to throw events at me and that is this edit text for that you have to use this step where you add the text change listener and last but not the least use this method after text change to perform your validation now if you guys are not so clear about this let's actually jump into Android studio and figure out how this works all right so here I'm using Android studio I have this project which is called edit text test and I'm my main activity to Java file which contains the Java code for the app and I am an activity underscore main tour XML which is my appearance defined inside XML right so here you guys can see there is a text view which says enter the percentage and there's an edit text now this is the edit text that we want to validate and make sure that the user never enters an incorrect value now remember there is already some level of validation over here by saying this input type equals to number I have made sure that the user cannot enter anything other than a number over here and I also have this max length equals to 3 to ensure that the user cannot enter a value greater than 999 but we still further need to customize this validation and ensure that the user will never enter a value greater than 100 so let's actually see how to do that the first step to doing that is to bring this edit text inside Java code so I'm going to go here to my main activity to Java I'm going to remove this menu that you guys see we're here and I'm going to say edit text by text so I've created a reference next step I need to link this reference to my XML by say fine view by ID Arnaud ID edit text at this point your Android studio is going to say blah blah blah incompatible types very simple present press alt enter on your keyboard and you can use this cast and there you see Android studio is automatically typecasted it for us right now the next step we need to implement this interface called text watcher so let's actually do that in our activity by saying implements so there you see text watcher if I click implement again Android studio is going to give an error saying blah blah blah because you need to implement certain methods for this interface very simple I'm going to go ahead press enter over here implement methods and there you can see gives me an option which methods I should implement I want to select all and click OK and there you go these are the three methods that you have now remember at this point everything is set but we have still not specified that this is the edit text that we want to monitor using this text watcher for that I'm gonna say my text dot add exchange listener and there you guys can see this is needed inside the bracket it says text watcher watcher over here and since our activity implements this text watcher this is the reference I'm going to pass inside so before I do anything more complicated let me actually show you guys what happens with these methods alright so at this point I have done nothing great I just have a toast in three different places with three different messages showing what has happened right so let's actually go ahead and run this and see what happens as I try to change the text inside the edit text and see which method gets triggered right alright so here my emulator is up let's actually try entering something over here I'll just enter two over here as you guys can see just before change on text change and after change you can see that the three methods are trigger defiant or something like three over here again you can see those three calls methods being sequential cold again if I try and changing something you are removing something over here again as you guys can see those three methods get cold right and we are going to use the after-tax change which is after I've changed this text so let's actually go ahead and override that method to make sure that the user never enters a value greater than 200 100 right all right so here in my after text change method there is this parameter which is called editable and you guys are probably wondering what is this it is nothing great it is just the text that we have inside our edit text so I'm going to remove this toast over here and let's actually get the value the first thing I'm going to do is kind of convert this to a string now the 200 which the user has entered is actually this editable object that you guys see over here so I'm going to convert this to a string first then I need to check if this number is greater than 100 right so I'm going to convert that to integer by saying integer so at this point whatever value the user is entered inside the edit text is right here inside our integer dot parseint blah blah blah right so it is the form of this number that you guys can see over here and I'm going to simply check if this number is greater than 100 well I can simply say something like this if then what do we do in that case very simple we simply replace the editable by saying editable dot replace so if the user enters 200 we want to wipe that 200 out and replace it with 100 for that I'm going to say the starting position is 0 the ending position s and the value that we want to replace is simply 100 so in other words what I have done very simple the user into 200 that was represented by this editable that you guys see over here I converted that to a number by saying integer dot parseint blah blah blah and if I check if the number is greater than 100 then simply replace everything by hundred right that's what I did so let me actually go ahead and run this alright so here my emulator is running now let's try to enter a value like 200 now remember for each character you enter the editable is being continuously monitored because of the text watcher so if I see 2 0 0 there you go there you go guys you saw that it happened dynamically it entered converted into 100 automatically but there is one small problem with this code let me show you what the problem is if you're a beginner this is where you're gonna make a mistake or take a look if I remove everything over here our app is going to crash saying blah blah blah test has shop stopped and you guys are like what the hell very simple think about this when there is nothing inside the edit text here you're trying to convert empty string into an integer and that is going to through a number format exception in Java so for that the best part is that you put a try-catch over here and you do nothing when the exception is thrown over here so now what happens whenever there is an empty text this thing is going to give an error for numbered format exception but nothing will happen since you have not done anything inside but the code will run and the app will not crash let me again demonstrate that by saying ctrl s over here and clicking run at the top all right so this time we are back to the same problem now again I'm going to enter 200 over here and you guys will see automatically it get converted to 100 this time I'm actually going to remove the text by using this backspace button over here and take a look nothing happens the app does not crash because we have this exception handled by doing nothing inside the number format exception so if you guys have understood something out of this video please subscribe to my channel let me know your thoughts I would love to hear from you guys thanks for watching I'll catch you guys the next bit have a nice day