C++ reference http://www.cplusplus.com/
C# reference https://docs.microsoft.com/en-us/dotnet/csharp/
JAVA https://www.oracle.com/java/
http://openbook.rheinwerk-verlag.de/javainsel/
Android https://developer.android.com/
https://www.xda-developers.com/
Agile Dev https://www.agilealliance.org/agile101/
Testing https://www.istqb.org/