Develop Reference

How to retrieve random children from Firebase

Suppose we would like to retrieve 15 random children from questions node having this database structured as below:
1. The first (intuitive and discussed) way of retrieving random children from Firebase is to retrieve the whole required parent node (questions as dataSnapshot) and then select some random children on the client-side. This method has been pointed out in many posts, like in this one here .
Obviously, this method has its downsides; for example when querying through a large sized parent node (e.g. over 10.000 children) retrieving such an amount every time would result in a huge bandwidth usage as well as a client side burden. (when we actually require only a small amount of children)
2. Moving on: another approach, as described here which uses an iterator somehow bypasses the whole client side burden, yet the huge bandwidth usage could still occur as we download the whole parent node every time.
3. An interesting approach is described in Tom's answer in this firebase discussion which proposes:
A hacky way of doing this would be to generate a random key and do a query with startAt().limit(1). I have a feeling this could hurt the performance of your firebase though, so this should not be an operation you perform often. We don't have a real random sample function.
This solution actually sounds pretty good, yet I am not sure how it would indeed impact my Firebase.
4. Another silly solution could actually be naming the question ids manually, so to speak, from 0 to N, therefore handling the random group of ids on the client side and retrieving the questions spot-on by knowing the actual name of nodes.
5. And lastly, I have come up with the following solution to which I ask if is more or less viable than the ones presented above: creating another parent containing the question ids only and when needed, one should retrieve this parent which is much "lighter" than questions parent . From there, I would have the specific random ids and I would only need to snipe for those children. To better understand my meaning, please check the below picture:
Now, from this method arises the following issue: is assigning (let's say) 15 eventListeners good practice? Could this actually slow up things? (Note: this applies to methods 3 and 4 as well)
And ultimately, which method is actually the optimal one when querying from a large database for some random children?

You can use the classic solution as I explained in this answer but if you are afraid of getting huge amount of data then use instead 15 listeners. There is nothing wrong in using listeners as long as you remove them according to the life-cycle of your activity. So, IMHO go ahead with 15 listeners.

We have 2 case here
case 1
If you want to grab all details of the random ids at once, then I suggest 1 listener to the parent node (get value of datasnapshot using pojo class).
case 2
If you want to get the details independently upon request then you will have to attach a listener to each (random id) that you want.
Concerning performance
Try to use only Listener For Single Value Events as they listen one time and then stop (better for performance).
Dont use Value Event Listener (because these listeners keep checking for changes and therefore bad performance as listeners increase).
EDIT
lets say you listened to (questions_ids) node, now you have access to the random id keys, store them in a String variable, and then inside the same listener add another listener to (questions) pointing to the id that you want to grab details
//first listen to question ids ref (the one with 15 ids)
question_ids_ref.addListenerForSingleValueEvent(...{
//grab the key of each (random id) and store in variable
String random_01=......;
//run another listener this time to questions ref
questions_ref.child(random_01).addListenerForSingleValueEvent(..{
//get details of random_01 and so on....
});
});

Android Firebase Database Add Object To List

I have a list of comment objects inside of an object. If I want to add a comment without touching the other comments how would I do that? I have used transactions in the past for editing an integer efficiently but I don't know if I can do that with a list. The end goal is for multiple users to be a ble to add comments at the same time without cutting eachother off (overriding their edits).
Here is an example of my database:

Solved it! Super simple. Just had to add push() to the end of my DatabaseRefercence like so. It gives every comment a key.
commentRef = pollRef.child("comments").push();
commentRef.setValue(comment);

Activity is getting too large and becoming more and more difficult to work with. Solutions to solve this issue?

I am working on my second Android Application, first being, hello world. The application code is quite crazy looking because I love to test new libraries and ideas in it. I have been working on this application for well over 3 months and one of my activities is getting way to large and difficult to work with. I find myself getting lost in the code and it is taking longer to do simple things. There might be simple solutions to solving this issue. I really want to split my activity into two and reference each other if possible. Is there are any suggestions to simplifying and organizing code that would be greatly helpful. Even example will help me very much.
Part of my activity is adding a ton of data into a database and the other part is a long equation with multiple values. Another part is implementing the HoloGraphLibrary (Which I love). It is also implementing a listView with custom adapter. It also has a custom dialog............ I can go on and on. I hope you get my point.
EDIT
Going to work with this.
HoloGraphHelper holoGraph = new HoloGraphHelper();
holoGraph.initialize();

Try creating classes for each responsibility.
A Database Helper that has functions to insert data too:
DatabaseHelper database = new DatabaseHelper();
database .insertData(whatever);
A HoloGraphHelper that initializes the HoloGraph
HoloGraphHelper holoGraph = new HoloGraphHelper();
holoGraph.initialize();
And so on.

Break into multiple files. First classes defined in the Activity like the adapter. Change anonymous classes to classes defined in their own file. Look for ways to break out other related code into a class.

Right click on src folder of your Project and select new - class to create a new class. You can use a class for storing methods but you won't be able to display anything on screen.
To display contents to user, you can create a new Activity bu pressing Ctrl + N and selecting Android - Android Activity.

The best way is modularise your code.
I.e split your code into various related modules, for example a separate class for each part that your testing. So you could have a database entry class, a class for Gui testing, i.e. for your custom dialog. That class does all the work for that test, into various functions, I always try to keep functions as small as possible as they are easy to read.
As an example for your database entry, you could have a function which checks the database if the record already exists and then insert it. But a better way would be your insert function only performs the insert code and instead within this function it calls CheckIfDatAlreadyExists function which can return a bool so you know whether you should go ahead and insert the record. This would keep the code tidy and clean to manage.
Then from your main activity all would need to do is instantiate the relative class and call the relevant method.

SimpleDB - how long after insert until item is available to be read?

I have a Fragment, and once the user presses OK, an Item is added to my database and its ID is added to the ArrayAdapter. Immediately after, the adapter tries to draw the view, but the first time it tries to get its attributes, it returns a null HashMap (it gets drawn properly the following times).
Is there a way to make sure the item is in the table before trying to get its attributes?
Even putting the attribute retrieval into a while loop until it returns a not-null HashMap doesn't work so it doesn't look to be an issue of time.

You need to do Select or GetAttributes with ConsistentRead=true as Amazon SimpleDB supports two read consistency options: eventually consistent read and consistent read. Eventually consistent read is default. For more detail please refer doc. link

Try using AsynTask.
Add item to database in doInBackground.
Read it in postExecute.
You are done.

What is the best way to use threading on a sorting algorithm, that when completed, creates a new activity and gives its data to the new activity?

I will start this by saying that on iOS this algorithm takes, on average, <2 seconds to complete and given a simpler, more specific input that is the same between how I test it on iOS vs. Android it takes 0.09 seconds and 2.5 seconds respectively, and the Android version simply quits on me, no idea if that would be significantly longer. (The test data gives the sorting algorithm a relatively simple task)
More specifically, I have a HashMap (Using an NSMutableDictionary on iOS) that maps a unique key(Its a string of only integers called its course. For example: "12345") used to get specific sections under a course title. The hash map knows what course a specific section falls under because each section has a value "Course". Once they are retrieved these section objects are compared, to see if they can fit into a schedule together based on user input and their "timeBegin", "timeEnd", and "days" values.
For Example: If I asked for schedules with only the Course ABC1234(There are 50 different time slots or "sections" under that course title) and DEF5678(50 sections) it will iterate through the Hashmap to find every section that falls under those two courses. Then it will sort them into schedules of two classes each(one ABC1234 and one DEF5678) If no two courses have a conflict then a total of 2500(50*50) schedules are possible.
These "schedules" (Stored in ArrayLists since the number of user inputs varies from 1-8 and possible number of results varies from 1-100,000. The group of all schedules is a double ArrayList that looks like this ArrayList>. On iOS I use NSMutableArray) are then fed into the intent that is the next Activity. This Activity (Fragment techincally?) will be a pager that allows the user to scroll through the different combinations.
I copied the method of search and sort exactly as it is in iOS(This may not be the right thing to do since the languages and data structures may be fundamentally different) and it works correctly with small output but when it gets too large it can't handle it.
So is multithreading the answer? Should I use something other than a HashMap? Something other than ArrayLists? I only assume multithreading because the errors indicate that too much is being done on the main thread. I've also read that there is a limit to the size of data passed using Intents but I have no idea.
If I was unclear on anything feel free to ask for clarification. Also, I've been doing Android for ~2 weeks so I may completely off track but hopefully not, this is a fully functional and complete app in the iTunes Store already so I don't think I'm that far off. Thanks!

1) I think you should go with AsynTask of Android .The way it handle the View into `UI
threadandBackground threadfor operations (Like Sorting` ) is sufficient enough to help
you to get the Data Processed into Background thread And on Processing you can get the
Content on UI Thread.
Follow This ShorHand Example for This:
Example to Use Asyntask
2) Example(How to Proceed):
a) define your view into onPreExecute()
b) Do your Background Operation into doInBackground()
c) Get the Result into onPostExceute() and call the content for New Activty
Hope this could help...

I think it's better for you to use TreeMap instead of HashMap, which sorts data automatically everytime you mutate it. Therefore you won't have to sort your data before start another activity, you just pass it and that's all.
Also for using it you have to implement Comparable interface in your class which represents value of Map.
You can also read about TreeMap class there:
http://docs.oracle.com/javase/7/docs/api/java/util/TreeMap.html