Develop Reference

Cannot Understand this code functionality

<LinearLayout 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:gravity="center"
android:orientation="vertical"
tools:context="com.bignerdranch.android.geoquiz.CheatActivity">
<TextView
android:id="#+id/answer_text_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="24dp"
tools:text="Answer"/>
I am a novice in programming. I started with Big Nerd Ranch programming book. I came across this code. In the book it is stated as:
"This namespace allows you to override any attribute on a widget for the
purpose of displaying it differently in the Android Studio preview. Since TextView has a text attribute,
you can provide a literal dummy value for it to help you know what it will look like at runtime. The
value “Answer” will never show up in the real app. Handy!"
What does that actually mean? I am completely new. I know this is foolish question, please help me in this.

Thanks to that line
xmlns:tools="http://schemas.android.com/tools"
you can use in all your XML something like that
tools:text="Answer"
Thanks to that line
xmlns:android="http://schemas.android.com/apk/res/android"
you can use in all your XML element the android attribute, for example
android:id="#+id/answer_text_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="24dp"
The book you're reading is a good book, keep continue and don't give up!

Tools is basically a collection of extra properties you can add to your TextView that assist you when designing layouts in Android Studio. In this particular example, tools:text allows you to put a fake value into your TextView which will only show up in the the layout preview in Android Studio.
This will allow you to see what a TextView looks like when designing your layout in Android Studio, but you don't have to worry about removing that dummy text from your layout when you build a "real" version of your app for a phone.
See also: Tools Attribute Reference

Android, background drawable appear black in genymotion but looks fine in intellij

I'm trying to set background image for my activity like this:
<LinearLayout
android:id="#+id/container"
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:gravity="center"
android:background="#drawable/login_background"
android:orientation="vertical">
which looks great in the preview in intellij
but in genymotion it looks black:
after trying to change the name of the file, the size of the file and trying various export methods from photoshop, I'm clueless.
Any ideas?

Have you tried another genymotion config yet?
Or have you tried it on a real Device?
I would suggest to use some other Configuration first, according to this post it could help.
https://github.com/facebook/fresco/issues/298
PS: I know not a real answer, got not enough reputation yet, thought i could help anyway.

After playing with the issue more I truly don't know what was the problem. After trying the same image in different activity it started to show even without modified the genymotion settings.

ActionBarSherlock + Google Maps API, Failed to find style 'mapViewStyle' in current theme

First of all, I know there are thousens of post asking the same, but after few days looking for a valid answer and try everything I'm still having the same problem...
So let's expose my situation:
I'm using ABS + google maps, so my map view xml is the following:
<?xml version="1.0" encoding="utf-8"?>
<com.actionbarsherlock.internal.view.menu.ActionMenuItemView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/ViewContainer"
android:layout_width="fill_parent"
android:layout_height="fill_parent" >
<com.google.android.maps.MapView
android:id="#+id/mapview"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:apiKey="0PGmnrCR70lothfjc_sFGTdFtESQPOksYKQWYpw"
android:clickable="true" />
</com.actionbarsherlock.internal.view.menu.ActionMenuItemView>
Few days ago everything was good, I was able to do my stuff in the app and everything. But suddenly I just made a little change, insert a framelayout to display more options in the screen. I tried to go back but was impossible...
I'm really deseparated, what can I do? maybe I can create a new project and paste all my code, but not really sure if it's going to work...
Any suggestions?
BTW my error code is this one
Missing styles. Is the correct theme chosen for this layout?
Use the Theme combo box above the layout to choose a different layout, or fix the theme style references.
Failed to find style 'mapViewStyle' in current theme
java.lang.NullPointerException
Exception details are logged in Window > Show View > Error Log
Thank you for your time!

I finally solve the problem, the error in the screen is still there, but just like a warining and when I execute the app same to work fine.
The only think I made, is change the specific layout of ABS and put a frame layout.

Android Layout Auto Complete fail when using custom component

In my layout I have a subclass of a TextView.
<com.class.path.views.CustomTextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="HELLO WORLD!"
/>
This works fine. My problem is that the "auto complete" doesn't work. If I remove the class path and just put TextView instead the "auto complete" works like a charm again.
This isnt a big issue but it is a bit annoying when you cant remember the android:properties that you need.
So my question is should this work? is it a documented bug?

This is a known issue and this is to be fixed in ADT 12 (current latest version is ADT 11).
We've fixed a number of smaller issues in ADT 12 that don't deserve blog posts on their own, but here's a quick roundup:
...
* Code completion inside a custom view layout has been improved
...
Source: http://tools.android.com/recent/adt12bugfixroundup

My Android "Hello World" app doesn't say "Hello"

I created a Hello World application, and the system generated most of the Android language below. When running the app without the System.out statement, there is no "Hello" displayed in the emulator. Then, using the Eclipse tutorial, I read that I can add the System.out.println statement to main. Again the app runs, but there is no output.
What am I not understanding here?
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<TextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="#string/hello"
System.out.println =" Hello world!"
/>
</LinearLayout>

I had this same problem, turns out all I had to do was be more patient.
The initial load time of the AVD seems like FOREVER... and in Eclipse before it actually looks like the device is booting up it just says "ANDROID_" against a fully black screen for quite a while. I thought it wasn't loading my text "Hello, Android" correctly, but it was actually just a load screen and I didn't wait long enough. (Yes, I now feel like a moron and a noob.)
If you're following along with the Android Developers Hello World Tutorial, and it doesn't seem like it's working, if you've never loaded the AVD before... maybe just wait longer?

Beginners' questions are fine, but boy, this really is a beginner question :) As for your problem, it can be a few things. What's probably happened is that you've adapted the basic "Hello, Android" tutorial which defines the TextViews in code, to make it display using an XML file. However, when you did that, you didn't change the code to use that XML file, and instead it's trying to display your old TextView. Also, "System.out.println="hello world!" won't do anything when in your XML file - you need to put statements like that in the code itself. In fact, offhand I can't remember if System.out.... even does anything in Android - debugging lines should be issued using Log.d("some title", "your message"), as that outputs to the Android specific logging device.
Anyway, it'd be easier to help solve your problem if you showed a bit more of your code. Try to make sure it's formatted properly, e.g. indenting code lines by four spaces. You can preview your post before you submit your edited version in the lower window to make sure it looks right.

Try this.
Hardcoded in [your_layout].xml
...
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text=" Hello world" />
or this
Use reference in [your_layout].xml
...
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="#strings/hello" />
In res/values/strings.xml
<resources>
<string name="hello">Hello world</string>
</resources>
That should help you get started a bit. A few thing to remember, System.out.println doesn't do a thing in Android, especially in XML file.
When addressing text with "#string/hello", it means that the application will look for string name "hello" inside strings.xml.
Anyway, you should try Android tutorial to get start.

One thing is for sure, putting println in your layout resource file won't do anything. This code(if it may be called so) is NOT executed. This is just definition of view. If at all, this statement should be called from your Activity class implementation(the java file) to be executed.