Accessing Views in Code: Assigning Variables

Awesome now it's time to assign values to these new variables. 0:00 Let's start by adding four new lines to the bottom of onCreate. 0:04 Then on the second new line let's add a comment, // Assign 0:10 the Views from the layout file to the corresponding variables. 0:16 It's important that we don't assign the views until 0:26 after the call to setContentView. 0:28 If we try to access a view from our layout before calling setContentView, 0:31 it won't exist and we'll get an error. 0:36 On the next line, let's start typing factTextView and then an = sign. 0:39 And now what we need is a method that will take in the ID that we set on the layout 0:45 and return our TextView. 0:50 Fortunately, every activity has a method for this, and it's called findViewById. 0:52 Let's start typing findViewById after the equals sign. 0:57 And notice that as you're typing the auto complete feature is filling it in below. 1:02 Android Studio analyzes what we're typing offers suggestions that 1:07 match what we've typed so far. 1:11 Automatic code completion is awesome, it's super convenient, and it makes it so 1:13 that we only need to remember the first few letters of something and 1:17 Android Studio will take care of the rest. 1:20 Now we can either finish typing this method, or 1:23 we can just hit Enter to select it from code completion. 1:25 And now we see a hint about the parameter we need to use here. 1:29 The findViewById method requires an ID as its parameter. 1:33 But rather than just type in the ID from the layout, 1:37 we must refer to it through a generated resource class. 1:40 When we built our project, Android automatically builds a class for 1:43 us called simply R, which stands for resources. 1:47 This class contains all the IDs of our files and 1:51 the res directory, as well as a ton of other default resource IDs. 1:54 If we open the project pane and 1:59 switch to the project perspective and then drill into app, 2:02 build, generated, source, r, debug, and 2:09 then our package name, we can even open the R class. 2:14 Though notice at the top there's a warning saying, 2:21 files under the build folder are generated and should not be edited. 2:24 We're not here to edit anything, it's just nice to know where the R class comes from. 2:29 Let's close this and switch back to the Android perspective. 2:34 And I'll hide the project pane again. 2:41 Back in funfactsactivity.java, let's type a capital R followed by a dot. 2:43 Then we need to pick which type of resource we're looking for. 2:49 Since we're looking for an ID, let's type id followed by another dot. 2:53 And look at that, it's showing us the IDs that we set in our layout. 2:59 Let's pick factTextView, and hit Enter to select it, and semicolon. 3:04 Now depending on which version of Android you're using, 3:07 you might see an error, like I do, or 3:11 if you're targeting a newer version of Android, everything will be just fine. 3:14 So what happened? 3:20 Let's take a look at some code to see what changed in the new version of Android. 3:21 Here we've got a Java project where we've mocked out some classes from 3:26 the Android API. 3:29 We start by creating a fake view class, and 3:32 then we create a button and text view classes that extend from our view class. 3:34 Then we create an activity class and give it a findViewById method that takes in 3:39 an integer and returns a view, just like you'd find in the actual activity class. 3:43 Inside this method, we create a local view variable. 3:49 And depending on whether the ID is a one or a two, we set it equal to 3:53 either a new button or a new text view before returning a view variable. 3:58 Finally we've got a main activity class which extends from activity, and inside 4:03 that class' constructor we try to set up two variables, a button and a text view. 4:08 This is the same error I'm getting back in Android Studio, incompatible types. 4:14 It's expecting a button but we're setting it equal to a view to fix this. 4:20 Since we know that findViewById with the parameter 4:25 of 1 will indeed return a button, we can just cache the returned 4:30 value to a button by adding the word Button and parentheses. 4:34 And we can do the same thing for 4:41 a text view by using Alt Enter to apply a quick fix. 4:43 However, in Android Oreo they made a change to the way we 4:48 use findViewById that means we no longer need the cast. 4:52 So let's undo the casts. 4:56 And now let's take a look at how they change the findViewById function. 4:59 Up in the findViewById method, instead of returning a view, 5:03 we want to use something called generics to let us return multiple types. 5:08 Don't worry too much if this part doesn't make sense. 5:13 What's important is how it affects our view variables, not how it's implemented. 5:16 Let's start by declaring our generic and 5:22 what types it's allowed to have inside angle brackets. 5:24 Here we're declaring a generic, named T, and 5:31 saying it can only be a class with the view class as a parent. 5:35 Then instead of returning a view, let's return our new generic type, T. 5:40 Finally since we've changed the return type from View to our generic type 5:48 that extends View, we need to add a cast to our view variable before we return it. 5:53 Perfect, now we no longer need to add a cast to our findViewById calls. 6:00 Though since this change is fairly recent, you're likely to encounter 6:07 a lot of code that still uses casts with findViewById. 6:10 So if you see any code that's still using casts and 6:15 it's not giving you issues, feel free to ignore it. 6:18 You're just on a newer version of Android than we are. 6:21