Accessing Google Account Id /username via Android

I've ran into the same issue and these two links solved for me:

The first one is this one: How do I retrieve the logged in Google account on android phones?

Which presents the code for retrieving the accounts associated with the phone. Basically you will need something like this:

AccountManager manager = (AccountManager) getSystemService(ACCOUNT_SERVICE);
Account[] list = manager.getAccounts();

And to add the permissions in the AndroidManifest.xml

<uses-permission android:name="android.permission.GET_ACCOUNTS"></uses-permission>
<uses-permission android:name="android.permission.AUTHENTICATE_ACCOUNTS"></uses-permission>

Additionally, if you are using the Emulator the following link will help you to set it up with an account : Android Emulator - Trouble creating user accounts

Basically, it says that you must create an android device based on a API Level and not the SDK Version (like is usually done).

This Method to get Google Username:

 public String getUsername() {
    AccountManager manager = AccountManager.get(this);
    Account[] accounts = manager.getAccountsByType("com.google");
    List<String> possibleEmails = new LinkedList<String>();

    for (Account account : accounts) {
        // TODO: Check possibleEmail against an email regex or treat
        // account.name as an email address only for certain account.type
        // values.
        possibleEmails.add(account.name);
    }

    if (!possibleEmails.isEmpty() && possibleEmails.get(0) != null) {
        String email = possibleEmails.get(0);
        String[] parts = email.split("@");
        if (parts.length > 0 && parts[0] != null)
            return parts[0];
        else
            return null;
    } else
        return null;
}

simple this method call ....

And Get Google User in Gmail id::

 accounts = AccountManager.get(this).getAccounts();
    Log.e("", "Size: " + accounts.length);
    for (Account account : accounts) {

        String possibleEmail = account.name;
        String type = account.type;

        if (type.equals("com.google")) {
            strGmail = possibleEmail;

            Log.e("", "Emails: " + strGmail);
            break;
        }
    }

After add permission in manifest;

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

<uses-permission android:name="android.permission.GET_ACCOUNTS" />

Retrieve profile information for a signed-in user Use the GoogleSignInResult.getSignInAccount method to request profile information for the currently signed in user. You can call the getSignInAccount method after the sign-in intent succeeds.

GoogleSignInResult result = 
Auth.GoogleSignInApi.getSignInResultFromIntent(data);
GoogleSignInAccount acct = result.getSignInAccount();
String personName = acct.getDisplayName();
String personGivenName = acct.getGivenName();
String personFamilyName = acct.getFamilyName();
String personEmail = acct.getEmail();
String personId = acct.getId();
Uri personPhoto = acct.getPhotoUrl();

if (Plus.PeopleApi.getCurrentPerson(mGoogleApiClient) != null) {
  Person currentPerson = Plus.PeopleApi.getCurrentPerson(mGoogleApiClient);
  String userid=currentPerson.getId(); //BY THIS CODE YOU CAN GET CURRENT LOGIN USER ID
}

Used these lines:

AccountManager manager = AccountManager.get(this);
Account[] accounts = manager.getAccountsByType("com.google");

the length of array accounts is always 0.