LogIn

public class LogIn extends AppCompatActivity {


    private CallbackManager mFacebookCallbackManager;
    private TextView info;
    private LoginButton loginButton;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        FacebookSdk.sdkInitialize(getApplicationContext());
        mFacebookCallbackManager = CallbackManager.Factory.create();

        setContentView(R.layout.login_layout);
        info = (TextView) findViewById(R.id.info);

        loginButton = (LoginButton) findViewById(R.id.login_button);
        loginButton.setReadPermissions(Arrays.asList("public_profile", "email"));

        loginButton.registerCallback(mFacebookCallbackManager, new FacebookCallback<LoginResult>() {
            @Override
            public void onSuccess(LoginResult loginResult) {
                info.setText("User ID:  " +
                        loginResult.getAccessToken().getUserId() + "\n" +
                        "Auth Token: " + loginResult.getAccessToken().getToken());

                String accessToken = loginResult.getAccessToken().getToken();

                // save accessToken to SharedPreference
                PrefUtils.saveAccessToken(accessToken);

                GraphRequest request = GraphRequest.newMeRequest(
                        loginResult.getAccessToken(),
                        new GraphRequest.GraphJSONObjectCallback() {
                            @Override
                            public void onCompleted(JSONObject jsonObject, GraphResponse response) {

                                // Getting FB User Data
                                Bundle facebookData = getFacebookData(jsonObject);
                            }
                        });

                Bundle parameters = new Bundle();
                parameters.putString("fields", "id,first_name,last_name,email,gender");
                request.setParameters(parameters);
                request.executeAsync();
//                Enable Main activity after login. Take username and store on DB!!
            }

            @Override
            public void onCancel() {
                info.setText("Login attempt cancelled.");
            }

            @Override
            public void onError(FacebookException e) {
                e.printStackTrace();
                info.setText("Login attempt failed.");
                deleteAccessToken();
            }
        });
    }

    private Bundle getFacebookData(JSONObject object) {
        Bundle bundle = new Bundle();

        try {
            String id = object.getString("id");
            URL profile_pic;
            try {
                profile_pic = new URL("https://graph.facebook.com/" + id + "/picture?type=large");
                Log.i("profile_pic", profile_pic + "");
                bundle.putString("profile_pic", profile_pic.toString());
            } catch (MalformedURLException e) {
                e.printStackTrace();
                return null;
            }

            bundle.putString("idFacebook", id);
            if (object.has("first_name"))
                bundle.putString("first_name", object.getString("first_name"));
            if (object.has("last_name"))
                bundle.putString("last_name", object.getString("last_name"));
            if (object.has("email"))
                bundle.putString("email", object.getString("email"));

//            PrefUtils.saveFacebookUserInfo(object.getString("first_name"),
//                    object.getString("last_name"), object.getString("email"),
//                    object.getString("gender"), profile_pic.toString());

        } catch (Exception e) {
            Log.d("APOEL", "BUNDLE Exception : " + e.toString());
        }

        return bundle;
    }


    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        mFacebookCallbackManager.onActivityResult(requestCode, resultCode, data);
    }

    private void deleteAccessToken() {
        AccessTokenTracker accessTokenTracker = new AccessTokenTracker() {
            @Override
            protected void onCurrentAccessTokenChanged(
                    AccessToken oldAccessToken,
                    AccessToken currentAccessToken) {

                if (currentAccessToken == null) {
                    //User logged out
//                    PrefUtils.clearToken();
                    LoginManager.getInstance().logOut();
                }
            }
        };
    }

}

prefUtil

public class PrefUtils {

    private Activity activity;

    // Constructor
    public PrefUtils(Activity activity) {
        this.activity = activity;
    }

    public void saveAccessToken(String token) {
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(activity);
        SharedPreferences.Editor editor = prefs.edit();
        editor.putString("fb_access_token", token);
        editor.apply(); // This line is IMPORTANT !!!
    }


    public String getToken() {
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(activity);
        return prefs.getString("fb_access_token", null);
    }

    public void clearToken() {
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(activity);
        SharedPreferences.Editor editor = prefs.edit();
        editor.clear();
        editor.apply(); // This line is IMPORTANT !!!
    }

    public void saveFacebookUserInfo(String first_name, String last_name, String email, String gender, String profileURL){
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(activity);
        SharedPreferences.Editor editor = prefs.edit();
        editor.putString("fb_first_name", first_name);
        editor.putString("fb_last_name", last_name);
        editor.putString("fb_email", email);
        editor.putString("fb_gender", gender);
        editor.putString("fb_profileURL", profileURL);
        editor.apply(); // This line is IMPORTANT !!!
        Log.d("MyApp", "Shared Name : "+first_name+"\nLast Name : "+last_name+"\nEmail : "+email+"\nGender : "+gender+"\nProfile Pic : "+profileURL);
    }

    public void getFacebookUserInfo(){
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(activity);
        Log.d("MyApp", "Name : "+prefs.getString("fb_name",null)+"\nEmail : "+prefs.getString("fb_email",null));
    }
}