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Android Java - Race Condition in OnCreate with two Observers and making lists

sorry if this is a convoluted question. Working on creating an app for a college course and I'm running into (what appears to be) a race condition in my OnCreate method.
TL;DR - sometimes my spinner populates and I can get an index from it. Sometimes it's not populated yet when trying to get a specific index. Details and code below.
The app is a "course scheduler" for a college student.
I'm creating an Activity that displays existing course information and allows you to edit it. In the OnCreate method for this Activity, I am filling a spinner for "Mentors" for the course and a spinner for which "Term" the course belongs in. This information is being pulled from a Room DB.
I have a seperate activity for a new course and for editing a course. For the "new course" activity, everything works fine. I getAllMentors() or getAllTerms() successfully and fill the spinner list.
For the "Edit Course" Activity, there's an extra step involved and it seems to be causing me some issues.
When editing a course, I pass the intent from the originating Activity with all the necessary EXTRAS. This is successful.
In OnCreate for EditCourseActivity, I do the following:
I get the mentorID from the EXTRA that's passed in from the originating Activity.
I access my MentorViewModel and call my getAllMentors() method which returns LiveData> of all mentors in the db.
because it returns LiveData, I use an observer and loop through the LiveData adding the Name of each mentor to a List and the
entire mentor to a List.
I populate my spinner with the information in List full of mentor names.
then I do a for loop, looping through List looking for one that has the same id as what I grabbed form the EXTRA in step 1.
If I find a match in that list, I call a getMentorName() method to snag their name as a string.
I have a methond getIndex(spinner, string) that will loop through the provided spinner, trying to find a match for the string that's
passed in (mentors name) that I grabbed that should match the ID of
the mentor assigned to the course. This method returns index location
of the matched string in the spinner.
I set the spinner selection to the index found.
I do basically the same process for term.
Me being a new developer, I'm not used to OnCreate running the code synchronously.
Because of this, it appears that I have a race condition somewhere between populating the List of mentor names that populates the spinner, and calling my getIndex() method.
Sometimes the spinner is populated and getIndex works properly and sets the correct mentor. Sometimes the spinner is empty and my getIndex() returns -1 (which it should do in a no-find situation) that populates the spinner with the first item in the list (once it's populated).
protected void onCreate(Bundle savedInstanceState) {
//////////////////////////Handling Mentor spinner menu/////////////////////////////////////////////////
int mentorId = courseData.getIntExtra(EXTRA_COURSE_MENTOR_ID, -1);
final ArrayAdapter<String> sp_CourseMentorAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_item, mentorNameList);
sp_CourseMentorAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
sp_CourseMentor.setAdapter(sp_CourseMentorAdapter);
final MentorViewModel mentorViewModel = ViewModelProviders.of(this).get(MentorViewModel.class);
//Mentor test = mentorViewModel.getMentorById(mentorId);
mentorViewModel.getAllMentors().observe(this, new Observer<List<Mentor>>() {
#Override
public void onChanged(#Nullable List<Mentor> mentorList) {
if (mentorList != null) {
for (Mentor m : mentorList) {
mentorNameList.add(m.getMentor_name());
mentorListMentor.add(m);
}
}
sp_CourseMentorAdapter.notifyDataSetChanged();
}
});
for(Mentor m: mentorListMentor){
if (m.getMentor_id()==mentorId){
String test = m.getMentor_name();
int spinnerSelectionM2 = getIndexM(sp_CourseMentor, test);
sp_CourseMentor.setSelection(spinnerSelectionM2);
}
}
Is there a way to get them to run asynchronously? Somehow to get the observer doing my getAllMentors() to complete first and populate the spinner, THEN have the for loop run?
Or a better way to handle this?
Thanks in advance.

Room always runs the code on a separated thread, not the Main/UI thread. You can change that behavior with
allowMainThreadQueries()
after initializating your database. This will make the query run first, populate your list and then run your for-loop code. I do not recommend this approach, since it is a bad practice to make queries on the UI thread.
You have two options:
Change your foor loop to a function and call it after adding the values from the observer:
mentorViewModel.getAllMentors().observe(this, new Observer<List<Mentor>>() {
#Override
public void onChanged(#Nullable List<Mentor> mentorList) {
if (mentorList != null) {
for (Mentor m : mentorList) {
mentorNameList.add(m.getMentor_name());
mentorListMentor.add(m);
}
lookForMentor();
}
}
});
private void lookForMentor() {
for(Mentor m: mentorListMentor){
if (m.getMentor_id()==mentorId){
String test = m.getMentor_name();
int spinnerSelectionM2 = getIndexM(sp_CourseMentor, test);
sp_CourseMentor.setSelection(spinnerSelectionM2);
}
}
}
Put the for inside the observer, change the Room DAO to return a List and use LiveData on your own viewmodel:
MentorViewModel.java:
MentorViewModel extends ViewModel {
private MutableLiveData<List<Mentor>> _mentorsLiveData = new MutableLiveData<List<Mentor>>();
public LiveData<List<Mentor>> mentorsLiveData = (LiveData) _mentorsLiveData;
void getAllMentors(){
//room db query
_mentorsLiveData.postValue(mentorsList);
}
}
EditActivity.java:
mentorsViewModel.getAllMentors();
mentorViewModel.mentorsLiveData.observe(this, new Observer<List<Mentor>>() {
#Override
public void onChanged(#Nullable List<Mentor> mentorList) {
mentorsListMentor.addAll(mentorList);
sp_CourseMentorAdapter.notifyDataSetChanged();
for(Mentor m: mentorListMentor){
if (m.getMentor_id()==mentorId){
String test = m.getMentor_name();
int spinnerSelectionM2 = getIndexM(sp_CourseMentor, test);
sp_CourseMentor.setSelection(spinnerSelectionM2);
}
}
}
}
});

Editing and saving an ArrayList<> that has been passed to another activity

I am making a frisbee logger and have an ArrayList of Team objects. Each Team has an ArrayList of Player objects. Everything is using Serializable properly to be sent using Intent.
In my main activity I am displaying the list of Team objects in a ListView and an option to add another Team (only a name is needed). Once a Team is selected I pass the object to another activity using Intent. On this second activity I have it display the list of Player objects and have fields to enter another player object into the passed list.
When I return to the main activity and go back to the add Player activity, what I have added is gone.
I cannot use static because there is obviously more than one Team object. I think passing back the changed ArrayList could work but that seems a little lame, time-consuming, and frustrating.
Is there a built-in way in Android Studio that does this or am I on my own?
Note: I am not using SQLite as suggested in the comments
There's not a whole lot to show on this but here it is I guess:
MainActivity.java
private static ArrayList<Team> listOfTeams = new ArrayList<>();
private static ArrayList<Game> listOfGames = new ArrayList<>();
private ListView gameList, teamList;
.....
teamList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
#Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Team t = (Team)teamList.getItemAtPosition(position);
viewTeam(t);
}
});
.....
//Item select in teamList. Start the TeamViewActivity
public void viewTeam(Team t)
{
Intent i = new Intent(this, TeamViewActivity.class);
i.putExtra("teamView",t);
startActivity(i);
}
TeamViewActivity.java
private Team team;
private ListView rosterList;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_team_view);
rosterList = (ListView) findViewById(R.id.playerList);
Intent i = getIntent();
Bundle extras = i.getExtras();
if(extras!=null)
{
if(extras.get("teamView")!=null)
{
team = (Team) extras.get("teamView");
}
}
populateRosterList(team.getRoster());
}
public void addPlayerToRoster(View view)
{
String checkFirst = ((EditText) findViewById(R.id.firstText)).getText().toString();
String checkLast = ((EditText) findViewById(R.id.lastText)).getText().toString();
String checkNumber = ((EditText) findViewById(R.id.numberText)).getText().toString();
if(!checkNumber.equals(""))
{
team.addPlayer(checkFirst, checkLast, Integer.parseInt(checkNumber));
((EditText) findViewById(R.id.firstText)).setText("");
((EditText) findViewById(R.id.lastText)).setText("");
((EditText) findViewById(R.id.numberText)).setText("");
populateRosterList(team.getRoster());
}
}
public void returnToMain(View view)
{
Intent i = new Intent(this, MainActivity.class);
i.putExtra("teamView", team);
startActivity(i);
}
private void populateRosterList(ArrayList<Player> list)
{
ArrayAdapter<Player> adapter = new ArrayAdapter<>(this,
R.layout.activity_list, R.id.genericText, list);
rosterList.setAdapter(adapter);
}

Consider your concept:
You serialize an object, i.e. you transform it into a transferrable format which is then copied over to the other activity and reconstructed as a new instance.
Consequently, you alter another instance, which is not available in the previous activity, if you do not return it - again, serialized - and finally reconstruct and copy it back into the respective instance.
What you need is a shared memory storage in your application, which can alter and retrieve data cross-activity OR a proper data routing using Intents w/ ISerializable.
Options:
Always serialize objects and pass and copy them around.
-> No multithreaded alteration, possibly slow, unbeautiful
Singleton application with global data storage ir Context Object (I do NOT recommend the due to memory management and Garbage
Collection inbetween Activity Switches BUT for consistency I'd
wanted to mention this option)
SQLite3
-> Quick, Simple and Scalable, But a bit cumbersome to get started with
Any other file-structure stored and maintained in the data folder
-> I'd expect a lot of boilerplate code here, and low performance
Webservice and remote database
Proper component setup, i.e. initialize all accessing components in your software with the appropriate reference to the data structs using for example fragments (Thanks to #mismanc, I actually missed that option initially)
In general you could abstract all that away using services and repositories, which allows you to under-the-hood test options like 3. 4. And 5. and find your best solution, and in addition, keeo the accessing code simple and clean.

in your case, you can use startActivityForResult instead of startActivity, then get your modified Team object from onActivityResult(int requestCode, int resultCode, Intent data) to update your list.
startActivityForResult example

You can use fragments. You hold the list in the MainActivity and pass its reference to ShowListFragment and AddPlayerFragment by interfaces. And you can also do other operations over them. If you dont want to use json or sqlite it can be a good way for you.
MainActivity.java
public class MainActivity extends Activity implements ShowListener{
public interface ShowListener{
ArrayList<Team> getTeamList();
}
private ArrayList<Team> listOfTeams = new ArrayList<>();
#Override
public ArrayList<Team> getTeamList() {
return listOfTeams;
}
}
ShowListFragment.java
public class ShowListFragment extends Fragment {
private ArrayList<Team> listOfTeams;
private ShowListener listener;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
listener = (ShowListener)getActivity();
listOfTeams = listener.getTeamList();
}
}

As #Kingfisher Phuoc mentioned you could use srartActivityForResult in case you don't want to change your approach.
Otherwise I will suggest you use either :
SharedPreference to store your arraylist object (by converting the arraylist to json then store it as string in json format). In the PlayerActivity you retrieve the data, manipulate it then save it. see this post
SQLite

Android: Create new object and manipulate/call new methods serially with Synchronized

I am attempting to have a user create a new Flow object and add it to an ArrayList to keep track when they press then "+" on the tool bar.
I am struggling with the multithreading of Java, because my methods that require the object and its properties, are running before the object is instantiated caused all sorts of problems
I want my methods to execute serially (ie. show dialog, get name, use object constructor, add new object to list) which is why I've attempted to use the Synchronized action on an object which I declared but did not instantiate.
This strategy can't seem to work because the object locked onto cannot be null.
java.lang.NullPointerException: Null reference used for synchronization (monitor-enter)
Any thoughts on how I could make my methods run in serial like this pseudo code:
private Flow newFlow; //Blank flow object declared.
private static List<Flow> flowsInStream = new ArrayList<Flow>();
synchronized (newFlow) {
flowDialog();
// presents user a dialog box to receive input.
// takes user input, invokes separate method to actually instantiate
// the newFlow object using the user input.
// Originally blank newFlow object now has:
// newFlow.name = userInput
// --X END X--
addToStream(newFlow);
// adds the newly instantiated newFlow object to the flowsInStream
// array to keep track of them.
// --X END X--
executedCorrectly();
// displays log message showing both the newFlow.name & the current
// elements in the flowsInStream array.
// --X END X--
} // end of synchronized
TheStream.java
public class TheStream extends AppCompatActivity {
private static final String TAG = TheStream.class.getName();
private Toolbar streamToolbar;
private Flow theFlow; //Blank flow object declared.
private static List<Flow> flowsInStream = new ArrayList<Flow>();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_the_stream);
streamToolbar = (Toolbar) findViewById(R.id.streamToolbar);
setSupportActionBar(streamToolbar);
}
#Override
public boolean onPrepareOptionsMenu(final Menu menu) {
getMenuInflater().inflate(R.menu.menu_thestream, menu);
return super.onCreateOptionsMenu(menu);
}
#Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.action_settings:
// User chose the "Settings" item, show the app settings UI...
return true;
case R.id.action_newFlow:
flowDialog();
addToStream(theFlow);
executedCorrectly();
return true;
default:
// If we got here, the user's action was not recognized.
// Invoke the superclass to handle it.
return super.onOptionsItemSelected(item);
}
}
public void flowDialog() {
//Creates dialog box asking for name for the new flow
AlertDialog.Builder newFlowDialog = new AlertDialog.Builder(TheStream.this);
LinearLayout layout = new LinearLayout(this);
layout.setOrientation(LinearLayout.VERTICAL);
LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(
LinearLayout.LayoutParams.MATCH_PARENT,
LinearLayout.LayoutParams.MATCH_PARENT);
params.setMarginStart(70);
params.setMarginEnd(150);
//Create edit text field for name entry
final EditText nameInputET = new EditText(TheStream.this);
//Sets maximum length of the EditText
nameInputET.setFilters(new InputFilter[]{new InputFilter.LengthFilter(30)});
//Adds the ET and params to the layout of the dialog box
layout.addView(nameInputET, params);
newFlowDialog.setTitle("Name your new Flow.");
newFlowDialog.setIcon(R.drawable.new_flow);
newFlowDialog.setView(layout);
newFlowDialog.setPositiveButton("Lets Roll",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int whichButton) {
if (nameInputET.getText().toString().equals("")) {
Toast.makeText(TheStream.this, "Every Flow deserves a good name :(", Toast.LENGTH_LONG).show();
flowDialog(); //Recall the dialog
} else {
// Sets name of flow object
theFlow = instantiateFlow(nameInputET.getText().toString());
}
}
});
newFlowDialog.setNegativeButton("Nevermind",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int whichButton) {
dialog.dismiss();
}
});
//Display Alert
newFlowDialog.show();
}
protected Flow instantiateFlow(String userInput) {
//Instantiates (Constructor) the newFlow object.
Flow newFlow = new Flow(userInput);
Log.d(TAG, "Your flow's name is " + newFlow.getFlowName());
/** Returns errors attached below */
return newFlow;
}
public void addToStream(Flow flow) {
flowsInStream.add(flow);
}
public void executedCorrectly() {
Log.d(TAG, "The synchronized activity executed correctly because the new Flow object's name is " + theFlow.getFlowName());
Log.d(TAG, "The new Flow list is also updated check it out: " + flowsInStream);
}
}
Flow.java
public class Flow {
private String flowName;
public Flow() {
} // End of default constructor
public Flow(String flowName) {
this.flowName = flowName;
} // End of constructor
/** Getters & Setters **/
public void setFlowName(String flowName) {
this.flowName = flowName;
}
public String getFlowName() {
return this.flowName;
}
If any additional code would help, just let me know and I'd be happy to post some. And if possible in your answer please mention where my technical understanding was lacking in attempting this.
ERROR RECEIVED:
java.lang.NullPointerException: Attempt to invoke virtual method
'java.lang.String nhacks16.flow.Main.Flow.getFlowName()' on a null object
reference

Yoy are using synchronized (newFlow), when newFlow is still null. You can't use synchronized on a null reference. If you really want to synchronize, create a different Object (any Object will do) and synchronize on that one, or synchronize on this (just using synchronized { without parenthesis). Which one is correct, depends on what kind of parallelism you want to guard against, which brings me to the next point:
I don't see any multithreading, so I'm not sure, if you even need synchronization.

#mastov was completely correct in that there doesn't appear to be any multi threading in the code and my newFlow object was null. But I just wanted to clarify what my own technical mistake was after reading his comments and a friend of mine pointed it out, in case someone else finds useful!
I was under the impression that that the dialog box FREEZES all activity (ie. the methods: addToStream(theFlow); and executedCorrectly();will wait until flowDialog()finished before executing themselves).
Thus, because the methods seemed to execute before the dialog was gone, I was under the impression that they were running on different threads.
The reason for the nullPointerException was that the object was not instantiated until user clicked on the button and input text. So once the flowDialog was set up, the next method would run, but because the flow object was not instantiated it threw the null exception!

Passing a String Array Between Java Classes Android App

I am writing an Android app where I need to pass a string array between two classes. The string initializes fine and I can output the contents of the string fine in the one class but as I try to pass it to another class I get a Null Pointer Exception error. The following is the stripped down version of my code:
accelerometer.java:
public class accelerometer extends Service {
public String movement[];
public void onCreate() {
movement = new String[1000000];
}
public void updatearray() {
movement[arraypos]=getCurrentTimeString();
//Toast.makeText(this, movement[arraypos] , Toast.LENGTH_SHORT).show(); //this correctly displays each position in the array every time it updates so I know the array is working correctly in this file
arraypos+=1;
}
public String[] getmovement(){
return movement;
}
}
wakeupalarm.java:
public class wakeupalarm extends Activity {
private TextView herestext_;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.wakeup);
herestext_ = (TextView) findViewById(R.id.TextView01);
accelerometer accelerometercall = new accelerometer();
String movearray[] = accelerometercall.getmovement();
herestext_.setText(movearray[2]);
}
}
I have a feeling I'm missing something very simple but any help would be greatly appreciated!
Thanks,
Scott

You're creating a new accelerometer class, which is completely uninitialized since there is no constructor, then you access its member. Of course it'll be null.
Not sure how your two classes are related, but if the activity is called by the service, then you need to pass the string through the intent (through an extra, for example).
Side note: Class names should always start with a capital letter. Method/variable names should have camel case, i.e. "updateArray". Also, you can format your code here by selecting it and pressing CTRL+K.

Your first problem, I think, is that you are creating an array with a million slots in it. Do you really mean to be doing that? It's going to take a lot of memory---quite possibly more than is available. You should instead look to having a Vector of Strings that you extend as necessary.

How to link classes and menus

I am really struggling with linking menus together. The app I want to create os a collection of menus that leads to url links to various sites I plan to open within the application. I have created a list activity menu with 8 options and I have eight classes with further options. My problem is how to link the menus together.
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
// Create an array of Strings, that will be put to our ListActivity
String[] names = new String[] { "P", "Ch", "Le", "Le", "B", "Sk", "Awa", "Tra"};
// Create an ArrayAdapter, that will actually make the Strings above
// appear in the ListView
this.setListAdapter(new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_checked, names));
}
#Override
protected void onListItemClick(ListView l, View v, int position, long id) {
super.onListItemClick(l, v, position, id);
// Get the item that was clicked
Object o = this.getListAdapter().getItem(position);
String keyword = o.toString();
Toast.makeText(this, "You selected: " + keyword, Toast.LENGTH_LONG)
.show();
}
}
At the moment all this does is print the selection using the toast method but how do I get it to switch to the p.java class when I have selected it. In basic I would take the names variable and say if names = p goto p.java, I have googled and although I get part of the answer I cannot figure out how to implement it.
Many Thanks In Advance.

I suspect that rather than a class, what you want is an instance of the class in question. One way to do that would be with a Map:
Map<String, Runner> runners = new HashMap<String, Runner>();
runners.put("P", new P());
runners.put("Ch", new Ch());
// etc.
(where Runner is an interface that all your classes implement). Then, inside your onListItemClick() method, where you have the toast:
runners.get(keyword).run();
(where run() is the method you want to launch).
Update (to address your comment)
It's hard to say exactly where to place which bits of code, but based on your question:
You could make runners a field in your Activity, and initialize it in your same onCreate function. So that part's handled.
The Runner interface could be as simple as this (in its own file):
public interface Runner {
public void run();
}
and each of your classes (P, Ch, Le, etc.) would have an implements bit in the constructor:
public class P implements Runner {
And would have to include a run() method (which could simply call whatever existing method you want called for the URL):
public void run() {
// do whatever you want done here
}