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R doesn't generate button

I've just finished to do the layout I want, and even although I saved everything, when I try to find by id the button, the button I'm looking for is missing.. Both the button's I have.. I've checked in the R class, under the ID, and it seems it doesn't sees' the buttons. Doesn't generate them? I dunno.. Any way, it happens every time I use relative layout!! Is there any connection? It works with TableLayout and with linear layout.. Only with relative layout it make's problems.
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="#drawable/Background" >
<TextView
android:id="#+id/corpTag"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:text="#string/corpTag"
android:textColor="#color/White" />
<Button
android:id="#+id/addBusinessButton"
style="#android:style/ButtonBar"
android:layout_width="200dp"
android:layout_height="25dp"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="208dp"
android:text="#string/addYourBusiness" />
<Button
android:id="#+id/searchBusinessButton"
style="#android:style/ButtonBar"
android:layout_width="200dp"
android:layout_height="25dp"
android:layout_alignBottom="#+id/addBusinessButton"
android:layout_alignLeft="#+id/addBusinessButton"
android:layout_marginBottom="36dp"
android:text="#string/searchBusiness" />

I found this happening to me with a broken layout. No need to be worry. I am trying my best to giving you the solution
Solution
Make sure that anything the R. links to is not broken. Fix all errors in your XML files. If anything in the ADKs are broken, R will not regenerate.
If you somehow hit something and created import android.R in your activity, remove it.
Run Project -> Clean. This will delete and regenerate R and BuildConfig.
Make sure Project -> Build Automatically is ticked. If not, build it manually via Menu -> Project -> Build Project .
Wait a few seconds for the errors to disappear.

This problem isn't related to relative layout, but is actually set on the background because Android drwable is case-sensitive and now allows you any capital letters for image or layout. Rename your image name or drwable name to "background" instead of Background."
Here is the problem with set background name:
android:background="#drawable/Background"

It might be the Android Style ButtonBar. It isn't available for every API level. So if you have a low minSDK, it's not going to build and Eclipse/IntelliJ should give you an error message in its error pane.
The rule given by Google, is that you should always define your own resources. If you want to use a resource that is in the Android name space, that's fine, but you should copy it first and package it with your apk, because that's the only way you can be assured that it will always be bundled with your application.
And yes, I realize many people and many tutorials on the web don't do it that way, but that's because most people are lazy.

Element type "Button" must be followed by either attribute specifications, ">" or "/>"

I am creating a button in my XML and here is the creation parameters
<Button android:id="#+id/btn2"
---> android:layout_width="wrap_content" <----
android:layout_height="wrap_content"
android:text="#string/PNR"
/>
I am getting and error in the line indicated saying :
" Element type "Button" must be followed by either attribute specifications, ">" or "/>" "
Not only in button id I try to create TextView or so then also same error comes and at same place.
I have checked earlier posts but they said that the tags were not closed and did not worked for me.
Please suggest me, what to do? Here is the full 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="vertical" >
<Button android:id="#+id/btn2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/PNR"
/>
</LinearLayout>

Try cleaning your project
Project --> Clean... then choose your project
Sometimes Eclipse doesn't pick up changes to your xml. When you get goofy errors like this always try cleaning first. Sometimes you will get ClassCastException in Java code when running right after changing something in xml like
cannot cast Button to EditText
or something similar that won't make sense. This is also a good time to clean your project.
I would also recommend getting rid of whitespace within elements because I have had trouble with that as well (especially on older versions of Eclipse) plus I think it looks cleaner. So I would change your <Button... to
<Button android:id="#+id/btn2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/PNR"/> <!-- just moved your end tag to this line -->

Why the ImageView class is not found?

i'm having troubles with creating the first android app (Silent Toggle Mode from the pdf "Android.Application.Development.for.For.Dummies" ) with inserting an Imageview in the main.xml file, when I go to the graphical layout, the image isn't showing itself and this error appears:"The following classes could not be found:
- ImageView (Change to android.widget.ImageView, Fix Build Path, Edit XML)"
this is my xml code
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:contentDescription="#string/hello_world"
android:orientation="vertical"
tools:context=".MainActivity" >
<EditText android:id="#+id/edit_message"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:hint="#string/edit_message" />
<ImageView
android:id="#+id/phone_icon"
android:contentDescription="#string/description"
android:layout_width="wrap_content"
android:scaleType="fitXY"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:src="#drawable/phone_on" />
<Button
android:id="#+id/toggleButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/ToggleSilentMode" />
</LinearLayout>
thanks

This is one issue that I have also run across recently. It was sort of a misleading error to me and kept me stumped for around 30 minutes this morning. It isn't that Android cannot find the class. It is that it is having trouble importing the image source file.
Now the reason that it is having trouble finding the image is because I did not copy the source image into the directory correctly. I have chosen to drag the source file from another folder directory into the appropriate locations. In the process, it did not copy the file over properly. Or it copied it over as just a link to the other resource.
As a result Android does not know what to do with the resource and throws that odd error. In order to resolve this problem, you need to delete the old image sources and then replace them with direct full copies of their equivalents. Then you should see this error go away and the application working as expected from the xml layout code.

I faced the same problem once. To get it out, I replaced all the existing images, now it worked.

I had the same issue, when added a new ImageView in my xml with an image src.
I tried to open this image with Photoshop and get an "module format error". So the image was corrupted. The problem was solved creating a new image similar to the first one.
Hope it works :)

TextView in all projects moved to the right

This is easily the weirdest problem I've ever faced.
So today, I created a new Android Project. The first layout xml had just a textview that has the following properties (its inside a relative layout):
<TextView
android:id="#+id/textView1"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="124dp"
android:background="#80ffffff"
android:gravity="center_vertical"
android:text="Welcome"
android:textSize="48dp"
android:textStyle="bold" />
What I wanted was to have a textview that has text in its center. But the text STARTS from the center and goes out of screen towers the right side.
So I thought I must be doing something wrong. I went into one of my older projects (it was also open in eclipse). I opened an XML there, and shockingly, all the text there has also moved to the left (It starts from the center, rather than actually centering the text). It was fine yesterday. Here's the button:
<Button
android:id="#+id/gettingstarted"
android:layout_width="210dp"
android:layout_height="wrap_content"
android:layout_alignLeft="#+id/login"
android:layout_below="#+id/login"
android:layout_marginTop="10dp"
android:text="Getting Started"
android:textColor="#ffffff" />
I have no idea what happened. Why is the text no longer centered in buttons in all my projects? I'm in a really tough spot right now. Kindly help.

For your first problem try
android:gravity="center_vertical|center"
but for your text views in your all projects , it's a general question , maybe it's occured because of changing in your apps theme or projects styles

LinearLayout in App Widget

I am working with modified version of sample WeatherListWidget to get a better understanding of App Widgets. Things are fine - except when I try to replace the dark_widget_item and light_widget_item layout files with slightly more complex layout files. Here is original layout:
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/widget_item"
android:layout_width="match_parent"
android:layout_height="46dp"
android:paddingLeft="25dp"
android:gravity="center_vertical"
android:background="#drawable/item_bg_light"
android:textColor="#e5e5e1"
android:textSize="24sp" />
I would like to be able to have multiple text lines. But:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/widget_item"
android:background="#drawable/item_bg"
android:layout_width="match_parent"
android:layout_height="46dp"
android:paddingLeft="25dp">
<TextView android:id="#+id/type_string"
android:textColor="#666666"
android:textSize="20sp" />
<TextView android:id="#+id/title_string"
android:textColor="#666666"
android:textSize="18sp" />
</LinearLayout>
fails.
In fact, it results in "Sorry! The application Launcher (process com.android.launcher) has stopped unexpectedly. Please try again. Force close".
Reinstating TextView widget_item.xml fixes this. I suspect that part of the problem is how I reference RemoteViews in WeatherWidgetService.getViewAt() - but I am getting very little help from DDMS or LogCat or anything else.

Thanks guys, I got the notification. (SO requires a username with length > 2, hence the dot)
Answer as per comment:
I don't see any layout_width and layout_height attributes for both of the TextViews in your LinearLayout - they are mandatory. Also, if you want the two TextViews to be above eachother, add android:orientation="vertical" to the LinearLayout. And just to the record, you can break a CharSequence to multiple lines in a single TextView by adding "\n" inbetween the different elements.
If you're going to include an image as well, then you're probably better off with a LinearLayout than a single TextView indeed, although you could potentially use the intrinsic drawable option of the latter. That could get a little messy though, especially if you're planning on using different styles for the different lines of text... Not impossible, but I'd stick with the LinearLayout. ;)