Develop Reference

Android Studio does not show layoutpreview

I am trying to create a layout in Android Studio but the preview of the layout is not showing at all. There is no errour though.
How can i get it to work properly? I will add a screenshot.
Preview not loading the layout
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: "Error parsing XML: must not undeclare prefix"

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">

Image view in layout android

I am trying to make a imageview in android but during the execution I find this e error in Resource id cannot be an empty string (at 'id' with value '#+id/').

Yo have to declare an ID
<RelativeLayout
xmlns:android="schemas.android.com/apk/res/android";
xmlns:tools="schemas.android.com/tools";
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
android:paddingBottom="#dimen/activity_vertical_margin"
tools:context=".MainActivity"
android:id="#+id/ExampleID"> // Give your Id here..

do it like this:
<?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" >
<ImageButton
android:id="#+id/btnSpeak"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:layout_marginRight="10dp"
android:layout_marginTop="10dp"
android:contentDescription="#string/speak"
android:src="#android:drawable/ic_btn_speak_now" />
</LinearLayout>

Gradle: Error parsing XML: not well-formed (invalid token)

I'm getting this compilation error in my XML file but I cant seem to spot the problem.. The error actually randomly came and I don't recall even touching this file but maybe I just cant see the problem.
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:orientation="vertical"
android:layout_height="match_parent"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
android:paddingBottom="#dimen/activity_vertical_margin"
tools:context=".MyBetsWindow">
<Button
android:id="#+id/AddBetButton"
android:text="Add Bet"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
<LinearLayout
android:id="#+id/BetListLayout"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical">
</LinearLayout>
</LinearLayout>

By me, your code works ; the error doesn't come from your layout.
Here is what it's doing on a old program that I made called ABar:
So check something else !

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.