Hi friends,
in this post I wrote about a tutorial to build Extensions:

Hi friends, this is a tutorial to create extension. Build extension.
Letâ€™s create an extension on the Pythagorean theorem: pitagoras.aix
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Create in your PC-Windows this folder:
C:/AppInventorExtensiones
Download in that folder these packages [or other recent version]
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Continue in this tutorialâ€¦
http://kio4.com/appinventor/125B_extensiones_crear_i.htm
Install java JDK, GIT and ANT.
Change User variables:
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Download Source code Pitagoras.java for Piâ€¦
Then in this post a Template of extension:

In other post I sent information about how build extensions:
Now I show a template file (source code) to build a simple extension, is only a didactic code.
User set:
textInput = Hola
numberInput = 234
and extension return double number: 468 and this sentence by Event: Ya terminado Hola
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Copy, paste and try.
Get template file in this link: http://kio4.com/appinventor/126_extensiones_plantilla.htm
AquĂ presento un archivo llamado Plantilla.java, es una archivo para coâ€¦
And now other Template to build Extension : (Copy, Paste and Study: Plantilla2.java file)

http://kio4.com/appinventor/126A_extensiones_plantilla.htm

If you want to start building extensions, I recommend you start with mathematical functions, for example multiply two numbers, calculate the body mass index (BMI), obtain a random number, calculate the area of a triangle, calculate the volume of a cylinder, â€¦

When you know how to perform these mathematical functions, you can launch yourself to learn a bit of Java and write more complicated extensions.

Regards.