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