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So library
A dynamic library file under a Linux system, just like a DLL file under a win system. Change the APK to RAR, in the Lib directory.
Dex (Classes.dex)
Dex is a file format that can be run directly on a Dalvik virtual machine in an Android system. The Java source code is transformed into a Dex file after the complex compilation of ADT, which is a gradual optimization process. The instruction code for the Dex file is a set of instruction sets that are proprietary to the Dalvik virtual machine, optimized for embedded systems, which are small and efficient compared to standard Java. class files.

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