If you're like me, or tons of other developers, you'll have started out with a limited number of build flavors in your Gradle build script — probably two, in fact: production and development. However, as you add different variables to expose in your BuildConfig, and extended the number of flavors, your code might start to look like a copy and paste mess. 

Here's an example: 

production {
   buildConfigField 'String', 'REST_API', '"https://api.myserver.com/"'
   buildConfigField 'Boolean', 'DEBUG_MODE', 'false'
   buildConfigField 'Boolean', 'DEMO_MODE', 'false'
   }
development {
   buildConfigField 'String', 'REST_API', '"https://test-api.myserver.com/"'
   buildConfigField 'Boolean', 'DEBUG_MODE', 'true'
   buildConfigField 'Boolean', 'DEMO_MODE', 'false'
   }
demo {
   buildConfigField 'String', 'REST_API', '"https://demo-api.myserver.com/"'
   buildConfigField 'Boolean', 'DEBUG_MODE', 'false'
   buildConfigField 'Boolean', 'DEMO_MODE', 'true'
   }

By using a combination of environment variables and functions, we can simplify these flavors. Most continuous integration systems, like TeamCity and Jenkins, will allow you to set parameters during the build, which in turn can be set as environment variables.

In the example above, the REST API variable is an obvious candidate for this. Here's how it would look, assuming the API_SERVER environment variable is used in the build script: 

productFlavours{
  def getAPIServer = {
	def server = System.getenv("API_SERVER")
	if(server)
        return '"'+server+'"'
   	else
        return '"https://api.myserver.com"'
  }
  //other functions and the rest of the flavors 
  //....
}

As you can see the function reads from the environment variables set, but will also return a default if the variable isn't set. Typically it makes sense to have the production variables as the defaults, as you really don't want to send out a production build pointing at a development setup. 

To use this in the flavor definition itself: 

production {
   buildConfigField 'String', 'REST_API', getAPIServer()
   buildConfigField 'Boolean', 'DEBUG_MODE', 'false'
   buildConfigField 'Boolean', 'DEMO_MODE', 'false'
   }

While it might not reduce the lines of code in your script, it certainly a clearer way of expressing the variations.