I would like to be able to test my Android app on different Android versions on an actual device, rather than on the emulator. I have a Nexus S which currently has Android 2.3.6 installed.
My understanding is that I can boot into "recovery" mode on my phone and load an Android package from the SD card, but where can I find the official Android releases? Are they distributed along with the SDK somewhere?
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The Nexus S was the first android 2.3 phone as far as I'm aware. There won't be any official android releases earlier than that for it.
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I do not believe 2.2 and earlier are listed anywhere, however, you can check the factory images for nexus devices and you might find an explanation why they do not list earlier versions
I suspect you will have to root your device and then 'flash' another version of android onto it.
Here is a tutorial to get you started: http://nexusshacks.com/nexus-s-hacks/how-to-root-nexus-s-or-nexus-s-4g-new/