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When I install an application, what happens in the background, where is the application installed, where are its preferences stored, where is information such as usernames and passwords stored, etc?

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where is the application installed

The .apk file is put into /data/app (see: Where in the file system are applications installed?)

where are its preferences stored

This depends on the application. If it uses the SharedPreferences classes then they go in /data/data/<app's package name>/shared_prefs, but they could feasibly be stored anywhere the app can write data to (i.e. all of /data/data/<app's package name>, the SD card, etc).

where is information such as usernames and passwords stored

This also depends on the app. Most often, probably somewhere in /data/data/<app's package name>. They could also use AccountManager APIs (the app must request this permission) which I believe will use the /data/system/accounts.db database.

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Thanks. Do most applications document where they store user data? Is this available somewhere on the Android Market? – PeanutsMonkey Apr 22 '12 at 20:58
@PeanutsMonkey: I don't think that I've ever seen an app that provided any kind of publicly available documentation for that, so I'd guess (but it's just a guess) that it's uncommon. – eldarerathis Apr 22 '12 at 21:05
What is the best way to find out? – PeanutsMonkey Apr 22 '12 at 21:07
@PeanutsMonkey: Email the developer. – eldarerathis Apr 22 '12 at 21:10
@PeanutsMonkey: Google has been relatively vocal about their hands-off approach to the Market. They don't vet or test any of the apps up front aside from running a background process on the server that checks for known malware. I've also never heard of any app store requiring developers to enumerate where and how they store every piece of data in their app and then release it publicly through the store. That seems like it would be a fairly obnoxious requirement since 99% of users probably won't ever care or ask for that kind of information. They may not even understand what to ask. – eldarerathis Apr 22 '12 at 22:31

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