It's not possible to do without API. and API for that doesn't exist. I you want to create chat application you can use GCM.
You can write extentions for hangouts : Hangouts API.
But android application integration is not possible at this moment. (And i think it never will be, because they have their protocol)
You have only one way to do this. Download whireshark
or fidler
and see what http
requests are sent/received during the chat on your android or web hangouts chat. :)
But still i think they have some authentication and hashing that will make this impossible.
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