APK files are analogous to other software packages such as APPX in Microsoft Windows or a Debian package in Debian-based operating systems. To make an APK file, a program for Android is first compiled, and then all of its parts are packaged into one container file. An APK file contains all of a program's code (such as .dex files), resources, assets, certificates, and manifest file. As is the case with many file formats, APK files can have any name needed, provided that the file name ends in the file extension ".apk".[2][3][4][5] APK files can be used to downgrade apps to an older file version (i.e. downgrade app version 78.9 to 55.1) by uninstalling the upgraded app and installing the older version via the "Unknown Sources" setting in settings (see below for more information).[6] This can be done for various reasons, such as an upgrade causing the app to repeatedly freeze and/or crash, the upgrade changing the app drastically (e.g. background too dark or too bright or preferred features not present, changed, or hidden), or restoring/removing something that the upgrade removed or installed.
9Apps 4.9
