final String email = mEmailField.getText().toString();
final String password = mPasswordField.getText().toString();
final String username = mUsernameField.getText().toString();

final FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();

firebaseAuth.createUserWithEmailAndPassword(email, password)
    .addOnSuccessListener(new OnSuccessListener<AuthResult>() {
         @Override
         public void onSuccess(AuthResult authResult) {
             final FirebaseUser firebaseUser = authResult.getUser();
             UserProfileChangeRequest changeNameRequest = new UserProfileChangeRequest.Builder()
                 .setDisplayName(username).build();

             firebaseUser.updateProfile(changeNameRequest)
                 .addOnCompleteListener(new OnCompleteListener<Void>() {
                     @Override
                     public void onComplete(@NonNull Task<Void> task) {
                         onAuthSuccess(firebaseUser);
                     }
                 });
             }
        });

private void onAuthSuccess(FirebaseUser user){
    System.out.println("displayname : " + user.getDisplayName());
}