Android: "Error parsing XML: must not undeclare prefix" - android

I am very new to android development and was in the process of trying to create my first process when I came across the build error with which the question is titled.
Here is my XML code:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app= ""
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
tools:context="com.example.austinc.clicker.MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="0"
android:id="#+id/textViewCount"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:textSize="24dp" />
<Button
android:text="Reset"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="#+id/ButtonClick"
android:layout_alignLeft="#+id/ButtonClick"
android:layout_alignStart="#+id/ButtonClick"
android:layout_marginTop="61dp"
android:id="#+id/buttonReset" />
<Button
android:text="Click"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="55dp"
android:id="#+id/ButtonClick"
android:layout_below="#+id/textViewCount"
android:layout_centerHorizontal="true" />
</RelativeLayout>
From what I can infer by double clicking on the error, the problem has to do with the second line of code (the meaning of which I have no idea). Is this a quick fix, or have I made a mistake somewhere in editing some piece of code?

I also faced the same issue, searched for 15 minutes and found the solution, i modified first three lines in content_main.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="#+id/content_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
app:layout_behavior="#string/appbar_scrolling_view_behavior"
tools:context="com.example.user.demoapp.MainActivity"
tools:showIn="#layout/activity_main">

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Android Studio does not show layoutpreview

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Thanks in advance.
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/content_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
app:layout_behavior="#string/appbar_scrolling_view_behavior"
tools:context="com.example.diceout.MainActivity"
tools:showIn="#layout/activity_main">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Let's Play!" />
<Button
android:id="#+id/rollButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_alignParentTop="true"
android:layout_marginLeft="155dp"
android:layout_marginStart="155dp"
android:layout_marginTop="25dp"
android:text="Roll" />
</RelativeLayout>
TextView
#ivan franken Could you post your #layout/activity_main Xml code. I saw this line tools:showIn:#layout/activity_main
in code on your content_main.xml. I tested it in my project and got similiar result to yours. But when I deleted it, it worked fine.

Android Studio - ViewFlipper - ImageView

I had my viewflipper working, flipping between two images. I messed around with sizes and long story short the app crashes now. I undid everything and tried putting it in again, and I can't get it to not crash anymore. Without the viewflipper it runs fine.
here is my content_main.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/content_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
android:background="#drawable/background"
app:layout_behavior="#string/appbar_scrolling_view_behavior"
tools:context="com.example.melanie.cskiosk.MainActivity"
tools:showIn="#layout/activity_main">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Welcome!"
android:visibility="visible"
android:textColor="#android:color/background_dark"
android:textStyle="normal|bold" />
<ViewFlipper
android:id="#+id/viewflipper"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:autoStart="true"
android:flipInterval="2000">
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="#drawable/woit_161013_0749" />
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="#drawable/woit_161013_0761" />
</ViewFlipper>
</RelativeLayout>
Not sure what I'm doing wrong here. Like I said I had this working just fine a few hours ago

Error parsing XML: unbound prefix

I'm new to android and trying to learn some basic stuff, My problem is if i paste this line android:layout_gravity="center" in the default "activity_main.xml" file I'm getting the error in the title but if I delete the line the error disappears!
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
tools:context="com.example.helloworld.MainActivity" >
<TextView
android:id="#+id/textView1"
android:layout_width="wrap_content"
android:textSize="70dp"
android:layout_height="wrap_content"
andriod:layout_gravity="center"
android:text="#string/hello_world"
/>
</RelativeLayout>
I appreciate any help
Your error is simple
Instead of
andriod:layout_gravity
write
android:layout_gravity

XML documents must start and end within the same entity help me

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="${packageName}.${activityClass}" >
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#drawable/bg"
tools:context=".MainActivity" >
<AnalogClock
android:id="#+id/analogClock1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="290dp" />
<TextClock
android:id="#+id/textClock1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_marginBottom="146dp"
android:text="#+id/textclock"
android:textColor="#FFFFFF"
android:textSize="100sp" />
</RelativeLayout>
Can I have some help on this I'm really confused would really appreciate some advice. I was just cleaning up my code when this error appeared. I have no idea on how to fix
The error comes because you didn't close RelativeLayout tag in your xml file,so add </RelativeLayout>at the end of xml file.
You don't need to add two relative layout where one parent layout is useless
So simply replace in your code
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="${packageName}.${activityClass}" >
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#drawable/bg"
tools:context=".MainActivity" >
with
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#drawable/bg"
tools:context=".MainActivity" >
Second Option is
Remove your xml code and saved the xml file. After that,replace it with the above code and save. The error will be gone. It is probably a remnant from the time when "automatically build project" enabled.

Error : The markup in the document following the root element must be well-formed Please help I am new at android

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
tools:context=".HelloActivity" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/hello_world"
</textview>
Not a concrete question, but Is there a difference between 'valid xml' and 'well formed xml'? and http://www.w3.org/TR/REC-xml/#sec-well-formed may be of help.
You forgot to end first TextView tag.
The must be between the tags like this:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
tools:context=".HelloActivity" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/hello_world" />
</RelativeLayout>
Just replace the below code : -
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/hello_world"
with : -
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/hello_world" />
If this works then please mark this answer as accepted.

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