Google's Android app ecosystem, borrowing from Apple's App Store, tries to keep bare technical information out of the hands of everyday users. This includes keeping file type information relatively obscure; while most Windows users know what a .exe file is, the same cannot be said about the analogous files on Android devices, which have an ".apk" extension, standing for "Android Package." Android Package files are the vehicle that developers (and software stores like Google Play) use to deliver applications to Android devices.
Android packages are used to deliver self-contained Android applications and are distributed through the Google Play store. Android packages may include executable binary code or a mixture of scripts that call on standard Android services, such as Web browsing and location services. In short, if it's an app, it has an associated APK file. In addition, independent developers distribute APK files as hobby projects rather than sending them through Google Play or Amazon.
Loading APK Files
The usual place to download an APK file is from the Google Play store or from the Amazon App Store. Both will install an app by downloading the APK file and running it. It is also possible to sideload an APK file -- first, download it to your desktop PC, then connect your Android device via USB and transfer it over. You will need to change a security setting in Android to use APK files from untrusted sources. Android's update process sometimes results in version incompatibilities between apps and the version of Android used on a particular device; loading an APK file directly can let you try older apps with newer versions of Android or vice versa to see if they still run.
File Format Specification
The APK file is Android's version of the Java JAR file, which is a ZIP file containing a program and its installation instructions. You can examine the contents of an APK file with Windows Explorer or any other program capable opening a ZIP file, though you may need to temporarily change the file extension to .zip to get it to open.
Mandatory File Contents
An Android Package file will contain the following sub-files in the following locations: "AndroidManifest.xml" in its root directory, which specifies what's in the package; classes.dex containing any binary code already compiled into Dalvik bytecode; a resources ARSC file and a subfolder called META-INF, which contains digital signature files. Other components used by the app will be in subfolders usually named "assets," "res," and "raw." There may be additional subfolders, depending on the app.
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