How whatsapp in Android checks which contacts in your address book have installed the application? I have to create an application that requires this type of contact synchronization.
I have a simple approach for this but i'm not pretty sure. Possibly, when a user registers his/her account on Whatsapp, the app would insert their number in online database and after when you register an account on whatsapp, it gets your local contact numbers and search them in the online database.
As per my knowledge when you install the app and register your number then this number goes to the Whatsapp server and stored in their database. Now the main logic comes is that whatapp has its own contact list which shows the number that are present your phone book which has whatapp installed on their phone. It takes all of your phone book entries and send this enteries to their server, now on their server it checks all the number which has whatapp installed from their database and mark a flag on that. After that the list of contact which has this flag enabled will be shown on the user's phone.
来源：https://stackoverflow.com/questions/17339872/whatsapp-contacts-synchronization