I downloaded go-ethereum .aar file from here :
For my Android phone for invoking functions of my smart contract.
But I don't know how to use this file?
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you should have an IDE. Recommended is official - AndroidStudio. Once you have this you should: 1) Open up the project structure by right-clicking on your project and choosing “Open Module Settings” or choosing “File” > “Project Structure…” 2) Click the “+” button in the top left to add a new module. 3) Choose “Import .JAR or .AAR Package” and click the “Next” button.
from this point you should use the module/package services/API