backward compatible android

I downloaded the androind adk and the only version of android I checked was android 4.0

now is android backward compatible? If I want to develop for android 2.1 or 3.0 etc do I also need to download this from the android sdk? that would involve running the android adk again and checking only those versions?

can I also set the screen size as android phone come in different screen sizes or doesnt that matter?
jagguyAsked:
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for_yanCommented:
My guess is that if you want to develop for earlier versions - you need to do it on earlier versions to be sure. Let's assume, that this 4.0.3 as a newer version has morer options - so it may take advantage of them and when you then move to the earlier device it may complain.
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for_yanCommented:
The analogy which comes to my mind is suppose you'll compile your java code with JDK 7 - it will use something specific to Java 7 in the class file. Then youi'll start to execute this class with java 6; well JVM will check version in the very beginning and will complain.
Maybe Android will not be as strict, but it may encounter something which is not suppoterted in the earlier versions.

In the opposite way - if you dveleop for earlier versions - if Android is backward compatible, then the newer versions should run that older android programs; jut like Java 7 will run the classes compiled with javac 6.
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for_yanCommented:
also read bottom answers here and about compatibilty package link in the last answer:

http://stackoverflow.com/questions/7841473/can-android-4-0-apps-work-with-android-2-0-and-android-3-0-devices
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SanthanaTechnical LeadCommented:
Hi jagguy,

Check this below link.....
http://developer.android.com/resources/articles/backward-compatibility.html

What i am thinking is....If you develop any app with recent version (Android 4.1(IceCream)), It will have newer concepts like fragment, loader...etc...It will not be available in the older version 2.3(GingerBread). So while running  your code in older version it will break....
If you add a <android:minSdkVersion> to the application's manifest, to ensure your app won't be installed on older devices.

can I also set the screen size as android phone come in different screen sizes or doesnt that matter?

Go through the below link.....
http://developer.android.com/training/multiscreen/index.html
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