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How to call method in thread time after time?

I have a method that loads data from Firebase into ArrayList. After this,I use that ArrayList to construct RecyclerView. I've decided to load data on another thread. Below is my code:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_just);
citiesRecyclerView =
(RecyclerView)findViewById(R.id.recyclerView);
handler = new Handler()
{
#Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
if(msg.what==1)
{
cityAdapter = new
CityAdapter(MainActivity.this,cities) ;
citiesRecyclerView.setAdapter(cityAdapter);
}
}
};
t = new Thread(new Runnable() {
#Override
public void run() {
//method that loads data into List.If this method was
//successfully done,then I send message 1 to Handler
loadDataFromFirebase();
}
});
t.start();
//other operations below
}
Hope,that everything understandable. Code works fine. And my problem is that I need to use loadDataFromFirebase method in thread again. I wanted to call t.start() again in order to call loadDataFromFirebase method,but there was error that thread already started. I checked that by writing this code:
if(t.getState()== Thread.State.NEW)
t.start();
else
someMethod();
else statement worked above.
And my questions are:
1) Does loadDataFromFirebase method work really on another thread by this way?
2) How to call loadDataFromFirebase method again in another thread, if something happened? Do I need to create another variable for Thread again?

It's not a good idea to handle all low-level thread work by your own.
Accroding to Android you could:
Use AsyncTask (but notice that they have many drawbacks such as context leak in some cases etc),
I could suggest you to get into RxJava - it's a painless way to use async work in your app.
To 'download' data from Firebase you could probably use FCM (push notifications) to load data on demand.
And what about your question:
"It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution."(c) http://docs.oracle.com/javase/6/docs/api/java/lang/Thread.html#start()

If you are using firebase SDK you can use realtime database feature, so do not need to query it each time.
You should just subscribe one time and get updates. For example:
firebaseReference.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
// This method is called once with the initial value and again
// whenever data at this location is updated.
YourDataObject value = dataSnapshot.getValue(YourDataObject.class);
Log.d(TAG, "Value is: " + value);
}
#Override
public void onCancelled(DatabaseError error) {
// Failed to read value
Log.w(TAG, "Failed to read value.", error.toException());
}
});
You can read docs here.

Retrofit "IllegalStateException: Already executed"

I have a Retrofit network call that id like to run every 5 seconds. My current code:
Handler h = new Handler();
int delay = 5000; //milliseconds
h.postDelayed(new Runnable() {
public void run() {
call.enqueue(new Callback<ApiResponse>() {
#Override
public void onResponse(Response<ApiResponse> response) {
Log.d("api", "response: " + response.body().getPosition().getLatitude().toString());
}
#Override
public void onFailure(Throwable t) {
}
});
h.postDelayed(this, delay);
}
}, delay);
This runs once, but then throws the following:
java.lang.IllegalStateException: Already executed.
at retrofit2.OkHttpCall.enqueue(OkHttpCall.java:52)
at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall.enqueue(ExecutorCallAdapterFactory.java:57)
at orbyt.project.MyFragment$1.run(MyFragment.java:93)
Whats the issue here?
As a bonus: whats a better way to handle this? Ill be updating a map every update. I was thinking about trying to use Rx but not sure if this is an appropriate use-case, or how to implement it.

A Call can only be used once. Its documentation tells you how to use one multiple times:
Use clone() to make multiple calls with the same parameters to the same webserver; this may be used to implement polling or to retry a failed call.
So use call.clone().enqueue(..) for Asynchornous and call.clone().execute() for Synchornous respectively to ensure that you have a fresh, unexecuted Call for each request.

Asynctask and queue OR delay in execution

Scenario:
The user has a list of items, let's say 10 items. Each item has an Operation button, which calls an AsyncTask which makes a web call. When a call is made, the item displays a spinner during the execution of the task
Problem:
Some of the users abuse this, and press quickly more Operation buttons, quicklt one after another, executing the web calls too often. So I want to be able to somehow, execute each of the AsyncTasks one after another with a delay of 2 seconds between executions. I do not want to switch to something else from AsyncTask if possible. So basically if there are 3 Operation buttons pressed, the execution should be:
-> Operation 1
-> 2 seconds delay
-> Operation 2
-> 2 seconds delay
-> Operation 3
-> ....
What would be the best way to do this in Android?
LE:
I have just realized something, for executing my task I ran the following code:
myTask = new MyTask();
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
myTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
} else {
myTask.execute();
}
Well, I've been using this code for a lot of time now, knowing that after honeycomb the tasks were not executed in parallel anymore without using an Executor. So it seems that only doing a simple myTask.execute() and adding a Thread.sleep() makes my AsyncTasks execute, one after another just as expected.

You will need to maintain a list of the operations that needs to be performed.
on click of the button add the task in the list, call a method which check the list for the task and executes it if there is no other task is running..
in onPostExecute method call the same method to check if there is any other task / operation that needs to be performed..
It may not be the full code you require... but may give you some idea..
public class TestActivity extends AppCompatActivity {
private static boolean isTaskRunning =false;
static ArrayList<CustomTask> customTaskList = new ArrayList();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test);
}
public void onBtnClick(View view)
{
// create custom task with required values and actions
CustomTask customTask = new CustomTask();
customTaskList.add(customTask);
checkAndExecuteTask();
}
private static void checkAndExecuteTask()
{
//checks if there is any task in the list and is there any other running task
if(customTaskList.size()>0 && !isTaskRunning) {
new MyAsync(customTaskList.get(0)).execute();
}
}
static class MyAsync extends AsyncTask<Void,Void,Void>
{
CustomTask currentCustomTask;
public MyAsync(CustomTask customTask)
{
currentCustomTask = customTask;
}
#Override
protected void onPreExecute() {
super.onPreExecute();
isTaskRunning= true;
}
#Override
protected Void doInBackground(Void... voids) {
// do your stuff
return null;
}
#Override
protected void onPostExecute(Void aVoid) {
super.onPostExecute(aVoid);
customTaskList.remove(currentCustomTask);
isTaskRunning =false;
checkAndExecuteTask(); // task is completed so check for another task and execute (if any).
}
}
class CustomTask
{
// create class with required fields and method
}
}

There are a number of ways you can do this in android.
One way is to use a handler.
What you need to do is to, create a seperate thread and run handler.postDelayed in it.
private void startWebCall() {
Thread thread = new Thread() {
public void run() {
Looper.prepare();
final Handler handler = new Handler();
handler.postDelayed(new Runnable() {
#Override
public void run() {
// Do your web calls here
handler.removeCallbacks(this);
Looper.myLooper().quit();
}
}, 2000);
Looper.loop();
}
};
thread.start();
}
You should call above method whenever user clicks a item.
Another way that I can think of is using an IntentService
An IntentService is a service that is used for doing asynchronous tasks in background. It maintains a queue of the tasks it needs to do. It is different from the above approach in the sense that it executes these tasks in a sequential order. So when you make requests to it to make web calls it will queue them, make the first call and then after it finishes it will make the second call. So the different web calls will not execute in parallel. They will execute in a sequential order but in a different thread. Also it is a service so it can run even in the background, i.e if user closes the app.
This is a good tutorial to get start with IntentService.
AsyncTaks should be generally avoided unless the work one needs to do is quite trivial. This blog explains its pitfalls.

synchronized blocks android

I have one function: fillFromDB which fills an instances of ArrayList called list from the SQLite DB.
I also have another Thread which is suppose to clear that list and re-populate it.
I'm having some issues with the list, since sometimes the other Thread clear the list while the first one is still filling it, causing IndexOutOfBoundsException on an Adapter.
// This occurs onCreate()
fillFromDb(list);
downloadAndRepopulateList(list);
private void downloadAndRepopulateList(List list) {
new Thread(new Runnable() {
#Override public void run() {
list.clear();
// Some other clode
}
}
}
I know that the issue is cause they are not Thread-safe, but I have never used synchronized before.
So my question is:
if I change the download function like this:
private void downloadAndRepopulateList(List list) {
new Thread(new Runnable() {
#Override public void run() {
synchronized(list) {
list.clear();
// Some other clode
}
}
}
}
will it wait for the UIThread to finish filling the list on fillFromDb() and then proceed to clear and repopulate it?
if not, how should I do this?

Though I would rather recommend using LinkedBlockingQueue. i.e.
list = new LinkedBlockingQueue<SomeType>();
fillFromDb(list);
downloadAndRepopulateList(list);
private void downloadAndRepopulateList(List list) {
new Thread(new Runnable() {
#Override public void run() {
list.clear();
// Some other clode
}
}
}
LinkedBlockingQueue will itself manage synchronization for you and you dont need to hold any lock etc.
If you wish to make it using Arraylist, you will have to make it atomic from both the sides. i.e. to access list, they should first hold the lock to object and the proceed. If someone else holds the lock, then wait.
synchonized(list){
fillFromDb(list);
downloadAndRepopulateList(list);
}
private void downloadAndRepopulateList(List list) {
new Thread(new Runnable() {
#Override public void run() {
synhronized(list){
list.clear();
}
// Some other clode
}
}
}

To make it work you also have to make synchronization on your fillFromDb method. So in the end you would have two synchronized parts, one that synchronizes your filling of the list and another synchronizes clear of the list. Also you can use your list as a mutex, but I would recommend you to have a separate object as mutex, it could me just like Object mutex = new Object();.

How can I run my code after Activity is made visible?

I have an Activity with 3 spinners. These spinners get their data from a web-service by a method that takes about 1 minute to be completed.
I want to load the Activity first and after it is made visible, call that web-service method and load data. I have tested the following codes separately but none of them solved my problem. In these samples application goes into a black screen and when the web-service operation completed, it is made visible.
#Override
protected void onCreate() {
//.........
final Runnable r = new Runnable()
{
public void run()
{
loadMyData();
}
};
Utilities.performOnBackgroundThread(r);
}
#Override
protected void onResume() {
new Thread() {
#Override
public void run() {
loadMyData();
}
}.start();
super.onResume();
}
#Override
protected void onStart() {
if (comesFromOnCreateMethod)
{
final Runnable r = new Runnable()
{
public void run()
{
loadMyData();
}
};
Utilities.performOnBackgroundThread(r);
}
comesFromOnCreateMethod = false;
super.onStart();
}
#Override
protected void onResume() {
if (comesFromOnCreateMethod)
{
final Runnable r = new Runnable()
{
public void run()
{
loadMyData();
}
};
Utilities.performOnBackgroundThread(r);
}
comesFromOnCreateMethod = false;
}

If you are getting a black screen, then I would assume your code is being run on the UI thread and not on the background, causing the UI to hang until the work is completed.
One of the best solutions to doing background work is an AsyncTask. Using this, you can call it in your onCreate() method, and when its done, it will post a callback to the UI thread for you in which you can display you data.
If you want this method to run everytime this Activity displays, then call it in onResume(). Otherwise, call it in onCreate().

In your onCreate, make the async tasks as the others have advised. Make sure you generate the content for the app first and then call the asyncTasks. You can control your spinners from the callback.

First of all, you might want to increase your accept rate, 39% is pretty low.
Anyway, you might want to check AsyncTask, it should do the thing. http://developer.android.com/reference/android/os/AsyncTask.html
Typically, you will want to initialize in onPreExecute, do the networking in the doInBackGround, and set the result to the UI thread on the OnPostExecute. Hope this will help.

Use AssynchTask() and you should call super.onResume() or any lifecycle method in respective life cycle method first then other specific method you want to do....