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Cannot find added elements in Tabbed Activity

I created a Tabbed Activity template project using Android Studio.
Then I created a LinearLayout with a nested TextView on it:
<LinearLayout
android:id="#+id/updateView"
android:layout_width="match_parent"
android:layout_height="20dp"
android:layout_marginTop="707dp"
android:orientation="vertical">
<TextView
android:id="#+id/updateTextView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="right"
android:text="TextView" />
</LinearLayout>
This view is shown on preview, but there's none of it in the built app.
When I try to get a reference to it from my code:
LinearLayout lastUpdate = findViewById(R.id.updateView)
I get null in runtime.
So, how can I modify the code?
Preview screenshot

I solved my problem by moving xml code of LinearLayout upper, than code of AppBarLayout, and modification of dynamic pixels (dp) of ViewPager and my textView

Can't set TextView to VISIBLE

I set the following to visible in my onCreate method.
findViewById(R.id.noDesignsCallToAction).setVisibility(View.VISIBLE);
In the XML, the view is set to visibility gone.
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:weightSum="1"
android:orientation="vertical"
android:layout_below="#id/toolbar"
android:id="#+id/linearLayout">
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="#string/go_ahead_create_something_n_it_s_really_easy_n_just_click_here_to_get_started"
android:textAppearance="?android:attr/textAppearanceLarge"
android:id="#+id/noDesignsCallToAction"
android:padding="10dp"
android:textSize="14sp"
android:typeface="sans"
android:visibility="gone"
android:onClick="LaunchCustomizer"
android:gravity="center_vertical|center_horizontal" />
</LinearLayout>
Why won't the textview appear as visible in my app?

Change textColor of your textview in layout file or check if any view is not overlapping your textview.

Inflate it and then set its visibility.
setContentView(R.layout.something);
TextView tv = (TextView) findView...;
tv.setVisibility(View.VISIBLE);
and maybe string res is empty ?

Make sure you set the content view first. Otherwise you'll get an NPE as a result of the ID not being found in the active layout.
Use an instance (don't call findViewById(id).setVisibility() as it uses more memory compared to caching the TextView as a field) and then set the visibility.
setContentView(R.layout.layout);
TextView temp = (TextView) findViewById(R.id.noDesignsCallToAction);
temp.setVisibility(View.VISIBLE);

Running TextView failure/Bug (Singleline marquee)

Im trying to let my textview run from left to right in an infinite way.But somehow it looks really weird I even cant explain it in words so I decided to record and upload it.
Here is the video : https://www.youtube.com/watch?v=gj3FF7fEutk&feature=youtu.be
So thats how I wrote my Textview :
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:singleLine="true"
android:ellipsize="marquee"
android:marqueeRepeatLimit ="marquee_forever"
android:focusable="true"
android:focusableInTouchMode="true"
android:scrollHorizontally="true"
android:textAppearance="?android:attr/textAppearanceMedium"
android:text="Title"
android:layout_marginTop="20dp"
android:textColor="#color/white"
android:id="#+id/textView_title_full"
android:layout_centerHorizontal="true" />
And this TextView is in a RelativeLayout declared like this :
<RelativeLayout
android:layout_width="wrap_content"
android:layout_height="170dp"
android:id="#+id/BELOW_FULL"
android:layout_gravity="bottom"
android:background="#drawable/color_below_player_full"
android:layout_below="#+id/viewpager_albumart_full"
android:layout_alignParentBottom="true">
where the RelativeLayout is also in a Relativelayout like this :
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:weightSum="1"
android:background="#0E0E0E">
I of course used the txt_song_title.setSelected(true); method.
How it looks like what I want to achieve :https://www.youtube.com/watch?v=o4uAA4pik68
Thank you all very much !
EDIT:
All parts where I do sth with my TextView :
public void init_textviews()
{
...
txt_song_title = (TextView)findViewById(R.id.textView_title_full);
txt_song_title.setSelected(true);
}
//init_textviews() is called in my onCreate method
txt_song_title.setText("Some Long text"); // called in onCreate after init_textviews()
And lastly:
public static void update_GUI_full (Context context) //Called when it is a button is clicked
{
...
txt_song_title.setText("longTexthere");
...
}

Working side by side with #Ahmet Kazaman we've found that there seemed to be a problem when combining a marquee TextView with another TextView periodically updated using a Handler.
After some tests we've found that this strange behaviour only happens when the TextViews are iside a RelativeLayout and everything woeks as expected when the enclosing layout is a LinearLayout. Thus, the solution (more a workaround) is to change the enclosing layout to be a Linearlayout.

how to define correctly textview width

Hi everyone i got a little problem about the width of my textview
it looks like this
as you can see, my textview is bigger than my text :/
here is my code :
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal"
android:weightSum="2">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="destinataire"
android:id="#+id/tv_destinataire"
android:layout_gravity="left|top"
android:layout_weight="1"
android:layout_marginLeft="25dp"
android:layout_marginTop="10dp"
android:padding="10dp"
android:layout_marginBottom="10dp"
android:textColor="#000000" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="envoyeur"
android:id="#+id/tv_envoyeur"
android:layout_gravity="right|top"
android:layout_weight="1"
android:layout_marginRight="25dp"
android:layout_marginTop="10dp"
android:padding="10dp"
android:textColor="#ffffff"
android:layout_marginBottom="10dp" />
</LinearLayout>
i would like something like this, if the message contain some word well "wrap_content" whereas is long long message something like maximum 70% of width :
thank you :)

You can reach what you said adding some logic directly on your activity and not in the XML file. You can set the width by Java code with something like:
TextView t = (TextView)findViewById(R.id.myTextView);
where myTextView is the id that you declared in your XML.
Now go ahead and write some logic...
If is necessary:
t.setWidth(200);
Note that 200 is only an example, you can calculate the width you need before.

If I were you, I might use the relative layout and add maxWidth limit to TextView containing text, and as the dialog going, just place the TextView below the last TextView and use alignStart/alightParentStart and alignEnd/alignParentEnd to indicate who's speaking.
when a new message arrives
prepare your relative layout parameter, add layout rules
set text, maxwidth limit, layout parameter and other style you want for your text
add the view to a scrollable relative layout container
Sorry for my bad English in case of you have any reading problem. :)

Custom button programmatically android

I'm creating a list of buttons in Android with the same icon for each of them and then set the text programmatically. So my list has:
ImageView (always the same) + Text (label for the icon which I set programmatically).
How can I create a something like the icon below but where I can change text dinamically?
Thank you!

You can use android:drawableLeft attribute in the TextView. Here's the sample:
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceMedium"
android:text="Medium Text"
android:id="#+id/textView"
android:gravity="center_vertical"
android:layout_margin="16dp"
android:drawableLeft="#drawable/ic_launcher"
/>
And the result

I would suggest you using the following
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/button_text"
android:drawableLeft="#drawable/button_icon"
... />
If you are looking for more customized button, then probably you should learn more about Custom Views:
https://developer.android.com/training/custom-views/index.html

What I understand from your question is that you want to change/set the text of the button programmatically.
for this use the following code in onCreate event:
Button mybutton = (Button) findViewById(R.id.your_button_name);
mybutton.setText("Your Text Here");