The document discusses the APK file format used on Android devices. It explains that an APK is an archive file based on the JAR format, containing compiled code in DEX format which is compiled from Java CLASS files. The key components of an APK are the AndroidManifest file containing permissions and entry point, the Classes.dex file containing the compiled code, and the Resources file containing images and other app resources. APK files can be opened and decompiled using tools to view the contained Java code.

1. Mobile
APK
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APK – Android Application Package
APK
Android Package (APK) is the package file format used by the Android operating
system for distribution and installation of mobile apps.
APK files are a type of archive file, based on the JAR file format.

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APK
DEX (Dalvik Executable)
DEX is binary file format, so it is compiled. We could say, that .dex file is for DVM (Dalvik
Virtual Machine) something like .class files for JVM.
DEX file format is really generated from java CLASS files by dex compiler from Android
SDK.
This compiler translates JVM bytecode to DVM bytecode and put all class files to one
dex file.
JAR (Java Archive)
JAR is a package file format designed for distribution of Java application on its platform.
It contains compiled Java class files + some more files like MANIFEST.
Basically it is just an ZIP archive with some restrictions.

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APK
APK
Each APK contain a basic file.
•Folder named META-INF – Contain the certificates for the application.
•AndroidManifest.xml – Contain the permission and the landing page.
•Classes.dex – dex (Dalvik Executable) files, Contain the JAVA code of the
application.
•Resources.arsc - Contains compiled resources in a binary format (may include
images, strings, or other data used by the program).

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APK
APK
There are few options to open apk and review the code.
•Change the extension to rar.
•Use Software like android studio, dex2jar and jd-gui.
The Classes dex contain a JAVA code with dex2jar converting the file to jar(java
file), with jd-gui can review the JAVA code the application contains.