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Posting messages for intended location Android application

I am new to android programming and I am trying to create an application that helps me to send a message/post (based on intended location) to those users using the same app in that particular location.
Can i get some resources on this?
Also currently i am trying to implement the feature wherein i post a message about a particular location on my wall and that same message has to seen by the users in that location. How can i implement this.
please post suggestions/open-srouce codes, examples etc.
thank you

You're probably looking for solutions related to the Geo-Fences concept.
There are plenty of applications that can use your mobile coordinates to interact and provide feedback. Notice that the accuracy of the mobile phone GPS is not THAT good, but people can play :-)
Take a look on this application -- https://www.onx.ms/#!landingPage
Official Android docs about geofencing here -- http://developer.android.com/intl/pt-br/training/location/geofencing.html

Best path googleMaps

I need to create an application that identifies the best path for a person. But it would need to identify the importance of uphill and downhill on the way.
How getting this information of which path would be straight?
This app is for android.
Tanks!

Check out Elevation API
and Distance Matrix API
The distance API may be the only thing you need.
Route information would be obtained by the Directions API
A combination between the 3 may be somewhat necessary, but once you setup an HttpClient to the google server you can just create back-end web service calls to grab the information you need on your android phone.

can we change the gps values of a mobile device?

I am trying to test a Location aware application on Android and iPhone in which I need to go to different locations.
Is it possible to mask/change the GPS location of the smart phone in which the GPS app is running using some code in.Net /Java? This is to make testing easier...

Pretty sure something like that would require hacking the onboard GPS chip. The easiest solution here would be to actually have your app mock the GPS coordinates instead.

I use a free 'Fake GPS' application from the Google Play Store for that purpose. I'm sure there are several available on both your target platforms

Getting location coordinates from device like iPhone in Java (not based on IP more like GPS or cell signal)

How can one get more accurate location coordinates from mobile device (smarthphone, iPhone, BlackBerry ...) other than from IP address. For example by using GPS query from smartphone API. And how would one go about integrating with Java?

This depends on what you're asking.
If you mean "How can one retrieve GPS coordinates using Java in a native application on a smartphone?", then that answer will differ for every smartphone platform, and there is no answer for iPhones because iPhones do not run Java.
If you mean "How can one retrieve GPS coordinates in a web application using Java on the backend?", then you can use the Geolocation API in HTML5, assuming the browser supports it (as far as I know, Mobile Safari and the browser in Android support this. I don't know about Blackberry, and would assume that it does not). See here.

How to define GPS module in Android?

I'm porting android to Devkit8000 which is a BeagleBoard clone. I have a GPS module connected on /dev/ttyS0. I could successfully get NMEA output when writing "cat /dev/ttyS0" in the terminal emulator.
I want to know how to let android know that there is a GPS module on /dev/ttyS0 and it is outputting NMEA standard? To be able to use the android.location class with it! or test it with any GPS app in the store!
Is there something to edit in the android's source? adding driver for example, writing a code with android-ndk or what?

The interface you must implement is described in the Platform Development Kit and you may want to use the source for the emulated GPS as a reference.