facebookAuth.java

  /**
     * a public method to handel the facebook authentication
     *
     * @param token holds the user Facebook accessToken
     */
    public void handleFacebookAccessToken(AccessToken token) {

        AuthCredential credential = FacebookAuthProvider.getCredential(token.getToken());
        FirebaseUtils.auth.signInWithCredential(credential)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (task.isSuccessful()) {
                            Log.d(TAG, "signInWithCredential: success");
                            startChatActivity();
                        } else {
                            Toast.makeText(LauncherActivity.this, task.getException().getMessage(),
                                    Toast.LENGTH_SHORT).show();
                        }

                    }
                });

    }