当我尝试重新登录时，Android应用程序崩溃

Question

I have an android app that is connected to an API through retrofit, ive succesfully logged in, if i press back button to return back to the login activity again, if i try re-logging in again, the app crashes and give me a NullPointerException.

here's connection code

    private void loginUser(String email, String password) {

    UnifyAuthenticationApiInterface service = this.client.create(UnifyAuthenticationApiInterface.class);
    Call<UnifyAuthenticationApiResponse> call = service.staffLogin(email, password);

    call.enqueue(new Callback<UnifyAuthenticationApiResponse>() {
        @Override
        public void onResponse(Call<UnifyAuthenticationApiResponse> call,
                               Response<UnifyAuthenticationApiResponse> response) {

            UnifyAuthenticationApiResponse result = response.body();
            School school = new School();
            com.peterstev.unify.login.Data data = result.getData();
            mySchoolsList = new ArrayList<School>();
            mySchoolsList = data.getSchools();
            staff = data.getStaff();

            gotoHomeActivity();

        }

        @Override
        public void onFailure(Call<UnifyAuthenticationApiResponse> call, Throwable t) {
            progressDialog.dismiss();
            Toast.makeText(MainActivity.this, "Login Failed @ onFailure", Toast.LENGTH_SHORT).show();
        }
    });
}

and the goToHomeActivity() is

 private void gotoHomeActivity() {
    progressDialog.dismiss();
    if (mySchoolsList.size() > 1) {
        schoolsListView = new ListView(MainActivity.this);
        schoolsArrayAdapter = new SchoolListAdapter(MainActivity.this, android.R.layout.simple_list_item_1, mySchoolsList);
        schoolsListView.setAdapter(schoolsArrayAdapter);

        dialog = new Dialog(MainActivity.this);
        dialog.setContentView(schoolsListView);
        dialog.setTitle("Welcome " + staff.getFullName());
        dialog.show();

    } else {
        Intent intent = new Intent(MainActivity.this, NavMainActivity.class);
        startActivity(intent);
    }
}

the NullPointerException gets thrown at

com.peterstev.unify.login.Data data = result.getData();

at first, it gets the data n succesfully logs in, but when i use the back button n try to log in again it crashes.

Answer 1

Debugger is your answer - check if you aren't loosing any data when going back - maybe you're storing login params somewhere in activity class but you're not saving instance state properly and second request is triggered without necessary data. Check state of variables just before calling your request first and second time.

In situation like that always best bet to place breakpoint and trigger your work step by step. You cannot be good developer without debugger skills.

Answer 2

I think for some reason, the data object wasn't receiving the result when i used the back button to navigate to the parent activity. so i used and if condition to make it get the required data.

 private void loginUser(String email, String password) {

    UnifyAuthenticationApiInterface service = this.client.create(UnifyAuthenticationApiInterface.class);
    Call<UnifyAuthenticationApiResponse> call = service.staffLogin(email, password);

    call.enqueue(new Callback<UnifyAuthenticationApiResponse>() {
        @Override
        public void onResponse(Call<UnifyAuthenticationApiResponse> call,
                               Response<UnifyAuthenticationApiResponse> response) {

            if(response.isSuccessful()) {

                UnifyAuthenticationApiResponse result = response.body();
                School school = new School();
                data = result.getData();
                if(data == null) {
                    try{
                        this.onResponse(call, response);
                    }catch(NullPointerException NPE){
                        Log.d("NPE", NPE.getMessage());
                    }
                }
                mySchoolsList = new ArrayList<School>();
                mySchoolsList = data.getSchools();
                staff = data.getStaff();

                gotoHomeActivity();
            }
        }

        @Override
        public void onFailure(Call<UnifyAuthenticationApiResponse> call, Throwable t) {
            progressDialog.dismiss();
            Toast.makeText(MainActivity.this, "Login Failed @ onFailure", Toast.LENGTH_SHORT).show();
        }
    });
}