Develop Reference

Does every Android device contains all previous SDK versions?

I'm just wondering, if the latest Android SDK installed on a device contains code of all the previous versions as well?
So if I target API level 10 in my app and install it on a device with Lollipop, will it just take and use Gingerbread SDK exactly as it was 3 years ago?
Or is there just one codebase for all versions with a lot of checks and switches which is then run by some kind of compatibility mode picking the correct code and enabling methods of the version of SDK I target?
I read the article about android:targetSdkVersion specified in Manifest but still would like to know how this works internally.

Ok, I just surfed a bit around on the source code (which you can find here: https://github.com/android/platform_frameworks_base). I'm not an engineer of the Android framework, I was just curious about your question and here is what I found.
It does not contain all the different versions of source code. You can imagine that this would result in a nightmare if more and more versions become available. Foremost, you would have different (buggy) versions of the same method without fixing them just to keep them the same.
In the source code, you can find places like these: (see https://github.com/android/platform_frameworks_base/blob/59701b9ba5c453e327bc0e6873a9f6ff87a10391/core/java/com/android/internal/view/ActionBarPolicy.java#L55)
public boolean hasEmbeddedTabs() {
final int targetSdk = mContext.getApplicationInfo().targetSdkVersion;
if (targetSdk >= Build.VERSION_CODES.JELLY_BEAN) {
return mContext.getResources().getBoolean(R.bool.action_bar_embed_tabs);
}
// ...
return mContext.getResources().getBoolean(R.bool.action_bar_embed_tabs_pre_jb);
}
So the Android developers do the version check in the code where necessary. But these checks are not as necessary as you think (I guess). What's the reason for changing a method?
method is buggy: All they need to do is fix the bug. Tests will make sure that the general behavior of the method keeps the same
method is deprecated: The guys can not remove the method, all they can do is mark it as deprecated and hope for the best. Compilers will do the rest.
method behavior has to change: Well, I guess that is something they can not do easily. They can work around with version codes (which is pretty ugly and becomes a maintenance nightmare), or they just introduce a new API. That's the reason why you'll find a lot of APIs just doing the same

If you have write down a code with latest android sdk and install it in your device. It means actually you are using latest android.jar(you can see android.jar in your project) file while compiling/Executing code.
Now If you install your application in ginger bread device then android.jar(latest) has a backward compatibility(if required) to run code in Gingerbread device.and if you define target sdk version 10 and running app on Higher API level ,then it will run smooth except your compatibility behavior disable in respective device other than targeted devices.

Almost finished with my first Android application made with Ice Cream Sandwich and was wondering about backwards comptibility

Hello I have developed my first application for Android using the Ice Cream Sandwich SDK and I was wondering if there was a way to make this compatible for devices also running Gingerbread without having to re-do the entire program. I have tried to find the answer to this from other sources but haven't found anything yet. Thank you for your time.

Congratulations for the development of your first application ;)
It's difficult to answer to your questions without knowing what your application is using. If your application use new features of the Android ICS API, you have to implements some compatibility code (see Support package). If not, you just need to add this in your AndroidManifest.xml :
<uses-sdk
android:targetSdkVersion="14"
android:minSdkVersion="7"
/>
Note that 7 is for Android 2.1
The best way to know if you use specific API of Android 4 (ICS) is to try to launch your project on an emulator under a lower version. If this give you error(s), it's because you have compatibility problems.

Just set your build target in Eclipse to Android 2.3. If you get any compilation errors in Eclipse, than this means that you are using APIs that are not available in 2.3 and your application will crash when it is going to reach at those lines when running on 2.3> .

Just make sure youre using APIs that are there on older versions of Android. Try to read the wahts new for ICS and avoid those :) which pretty much beats the purpose of using a newer SDK that changes alot and breaks lots of things. Beware of the new layouts....

API8 to 7 compatibility

What is the best way to make my app originally designed for API8 to be compatible with 7? I've noticed that AlertDialog features seem to cause trouble, for instance showDialog() doesn't work.

In your manifest file you can target 8 and require 7. This will allow the app to run on 7 and show up in the market. But obviously you need to test the dickens out of the app on 7 to make sure it runs well. A good blog post on it is here
http://devtcg.blogspot.com/2009/12/gracefully-supporting-multiple-android.html
Another option that I don't think is good because you will have to support two or more code bases is to the use the multipal apk file support of the market.
http://developer.android.com/guide/market/publishing/multiple-apks.html

Android compatibilty settings in AndroidManifest.xml

I have an app to release which works on all android screen-sizes (except smaller) and densities above SDK version 2.0.
It will also run on extra large screens.
Currently I have added this:
<supports-screens
android:largeScreens="true"
android:normalScreens="true"
android:smallScreens="false"
android:anyDensity="true"
/>
But I also need to add android:xlargeScreens="true" , to allow it visible in android market on extra large screen devices, since by default it is false.
But to add android:xlargeScreens I need to change my eclipse targetsettings to 2.3 as this attribute was added from API level 9.
So what should I do with my target compilation settings for this scenario ? Should it be 2.3 while compiling ? If yes, then will the app not give any problems while running on devices with 2.0 version ?

Yes you need to change the uses sdk to 2.3 but make sure that you are not using any newer apis which are not in 2.0 or whatever your minimum supported sdk version is. Or in case you want to use them you have to use reflection.
But more about how to use the sdk versions is here and more about uses-sdk is here.
I do the same in my application and make sure you test your application in both[all] the versions before you release.
Best,
Achie.

I'm moving this from the comments to make it more clear for others looking at this question in the future.
When supporting both old and new versions of Android it can be confusing how applications manage to run despite many things change with in the frameworks during each new release, I'm going to try and clarify this here.
An application written for the 1.5 sdk can only call functions that exist for that API level, so for instance the multi touch api's didn't exist in 1.5 and never will. Now you say "Ok but I don't need to call any newer APIs, I just want my app to work in 2.3 and have a2sd support" And I say "Ok, just change your targetApi in the manifest, set the minSDK and compile against 2.3 and you're good to go."
Now why does that work? What if the onMeasure() method for ListView was changed in 2.2 and now calls betterCalculateFunction() within onMeasure()? Why does my app still work?
This is the advantage of late binding in Java. You see, Java is never compiled until it reaches a device and is running, what you are doing in Eclipse is converting it to byte code which contains a bunch of byte code instructions that are later interpreted by the device. The byte code will NEVER contain a reference to betterCalculateFunction() though (unless you directly call it. Calling onMeasure() is indirect). This can happen because when your code is running on the device it gets linked against the Android framework on the device and your code calls onMeasure() directly because it is a public outward facing API. The path of execution will then enter the framework and call whatever it needs to, then once its done return to your code.
So on 1.5 you might see
doStuff (your code) -> onMeasure
(public API) -> done
and 2.2
doStuff (your code) -> onMeasure
(public API) ->
betterCalculateFunction (private
function) ->done
Now if you need to call functions that may or may not exist depending on API level then I suggest you look at a related answer of mine here stackoverflow: gracefully downgrade your app
Hope that clears some things up.

I haven't tried 2.3, but that's what I do with 2.2.
I compile for 2.2 and test on 1.6 to make sure everything works how I'm expecting. I haven't run in to any issues with it.
To double check, set your target for 2.3 and then setup an emulator for a lower rev version to make sure it all works.

The default value for android:xlargeScreens is true, so you don't have to change anything - it's on by default, as long as your minSdkVersion or targetSdkVersion is higher than 4.
http://developer.android.com/guide/topics/manifest/supports-screens-element.html

Here is an official Android developer blog explanation of how this works:
http://android-developers.blogspot.com/2010/07/how-to-have-your-cupcake-and-eat-it-too.html
In summary: you can use the newest XML whilst still supporting the older OS versions in a back compatible way.

While reading this blog post I guess I have an answer on my old question. An extract below (which is for another manifest attribute "requiresSmallestWidthDp" introduced from 3.2):
"The catch is that you must compile your application against Android 3.2 or higher in order to use the requiresSmallestWidthDp attribute. Older versions don’t understand this attribute and will raise a compile-time error. The safest thing to do is develop your app against the platform that matches the API level you’ve set for minSdkVersion. When you’re making final preparations to build your release candidate, change the build target to Android 3.2 and add the requiresSmallestWidthDp attribute. Android versions older than 3.2 simply ignore that XML attribute, so there’s no risk of a runtime failure."

For different screens you have to create multiple apk then it reduces size of your application.In each application's manifest you have to define according to following link.
http://developer.android.com/guide/practices/screens-distribution.html

Incorporating both Contacts and ContactsContract into an Android application

Greetings all,
I realize this issue has been posed before in other forms, and believe me, I have been searching the net for days trying to find the answer. However, I'm fairly new to Android and Java, and I need a little guidance please.
I presently have two version of my app, one for Android 1.5 and 1.6, and another for 2.0 and beyond. As you probably guessed, it's because the Contacts API is different.
I recently became aware that it was possible to combine both methods into a single app, by using dynamically loadable classes. Very cool! After days of attempts, I still haven't been able to do it successfully... or at least, it won't run.
I have come across 3 examples of how to do this - one by Google, called "Business Card", another that had something to do with Spinners, and a 3rd was something someone here created. My problem is that each one seems to me to have a showstopper.
I'm using Eclipse on Windows 7. My app was first created for A1.5, so that's the one I'm upgrading. What's happening is that my ContactsAccessorNewApi class needs to import the ContactContracts, and according to Eclipse, that won't work because my project was originally built without support for it. Hence, it won't run. I've tried adding the android.jar for SDK level 5 into the project, but that creates a whole mess of other problems.
My code for this pat of my app is exactly like the Google example "Business Card" - so if someone could help me cross this hurdle, I'd be very grateful. I'll be happy to post any code that you need to answer my question.
Thank you!

another that had something to do with Spinners
If you mean this sample project, that's mine.
I'm using Eclipse on Windows 7. My app was first created for A1.5, so that's the one I'm upgrading. What's happening is that my ContactsAccessorNewApi class needs to import the ContactContracts, and according to Eclipse, that won't work because my project was originally built without support for it.
You need to set your build target (Project Properties > Android) to be high enough that ContactsContract exists (android-5 or higher).
So long as your android:minSdkVersion in your manifest is set to support earlier versions of Android (e.g., 3 for Android 1.5), your app will still install on older emulator AVDs and devices.
I've tried adding the android.jar for SDK level 5 into the project, but that creates a whole mess of other problems.
Yeah, don't do that.