Develop Reference

Xamarin listview with objects

If I have a class containing of integers, strings and a Date, how should I go about creating, saving and displaying a new instance of this class in my ListViews?
I've used SharedPreferences before for saving local strings but is this possible with objects as well, or should I look into SQLlite?
Edit: Also, if SQLlite is needed, a little steer in the right direction would be appreciated.

You can still use preferences to save your objects but you need to serialize them into strings. And for later use you have to deserialize them from strings into your list of objects. This can easily be done with Json.net but I would recommend to use a database like SQLite. This would make it easy if new requirements like searching or object extension comes up.
A good entry into SQLite and Xamarin can be found here or here.

Session state of ListView

I am new to Android App development, working on an android app which populate a list of numbers, in a listview dynamically, depending on the choice of the user, but, the moment user closes the App, the items in the listview are lost. How can I maintain the state of the listview?
Examples with code would be highly appreciated.
When I open Activity A, it allows users to add friends, and this friend list is shown in the form of items of listview in the same Activity, however, when I move to Activity B, and then come back to Activity A, this friend list disappears. I need to make sure that this friend list should not be lost while moving between activities. Please help.

I think that for your purpose there are 3 main methods, i'll explain them from the easier to the most difficult (in my opinion).
Text File
A way to do this is to create two methods in a class:
one has to create the text file in the storage if it isn't created before and read that, the other has to append a String to a StringBuilder and write it on the previous text file.
For this method you need the uses-permission of reading and writing to storage.
This link can help you: http://developer.android.com/training/basics/data-storage/files.html
JSON (also XML)
With JSON file you can create a list of objects with your data that you can serialize when you update the list and deserialize when you want to read it. For this purpose you have to study JavaScript syntax or, at least, JSON one.
SQLite Database
Android SDK incorporate a class named SQLiteOpenHelper that you can extend to create a database inside your app.
This link can help you: http://developer.android.com/training/basics/data-storage/databases.html
There are also references saving methods but i think that aren't right for your purpose, they work betters to save something like preferences or single data like last login informations.

I went through your comment. I would personally suggest using SQLiteOpenHelper
You might need to understand the use of SQLite, hence the tutorial
Simple Flow. On your Activity 1 where person Add Friends save it to DB
Then refresh the List from the DB. So when you move to Activity 2 and come back again to Activity 1 your List will refresh from DB. Hence no loss of data as you want.
EDIT
As user wanted to know how to use the ListView with DB.
Following are my suggestion
Sai Geetha
Youtube

"global" array with different data types - Android

I'm new in programming for Android so maybe my question will be very easy to solve but still. I'm trying to make an array of different data types :
I have to add there :
int number
String name
int number_2
int time
int total
And my question now is how to implement it in easiest way, and how to get data from it. In case that I have to get a different records for this variables and store it into list .
Also have a question about way how to keep all values which I handle inside of my array.
I have to keep it because in my program I have to go back to other activities go forward to another and again collect data and add it to my array.
What will be the best and easiest solution ?
Thanks in advance for help

You could create the Array as an Array of Objects. All other classes are derived from Object, so you'll be able to store all types of objects in your Array. However, you would have to check the type of an object you get from the Array, before you'd be able to safely interpret as an object of a specific class. Moreover, you would have to use Integer instead of int.
If all (or at least multiple) of your elements you are intending to store in the Array are belonging to one (physical) entity, you could create a custom Class that holds its own properties as class members, and fill your Array with a list of instances of this Class.
Moreover, if you plan to add elements to your Array, you should use a List instead, e.g. an ArrayList.
As for retaining your data, you would have to either store it in a database, or save it to a file. In either way, you will have to save it upon close of the Activity, and load it again once the Activity starts

To pass the data across activities you will need to pass them using objects you can store in an intent. Seems like the best way to handle that is to either create a PREFS file to store the data or to create an object that extends Parcelable like here:
https://stackoverflow.com/questions/18593619/android-parcelable-object-passing-to-another-activity
Parcelables are preferable assuming you need all the data in a single object, you do not want to "putExtra" a bunch of fields and you also want to be sure data can pass from one activity to another. Otherwise, a simple Util class that reads and writes to a PREFS file is the way to go:
android read/write user preferences
A database is always another option, but seems well outside the scope of your question.

Android txt files v's database

My music app is referencing persistently stored data. All are currently stored as text files:
Favorites - single text file array. App starts, reads the text file, stores in memory. Array is checked when ListView is expanded. If array changes text file is rewritten.
Last Played - single text file array list. Updated every 5 seconds. Retains the history of the played songs to allow the user to return to any album and resume position.
Playlists - currently individual text files, one for each list. List of playlists generated from file names when required. Each playlist text file has array list inside it. Read Write when required.
Most Played - single text file array list. Updated once per song played.
I am wondering whether this data would warrant the need to change to a database, or whether I have taken the right approach. I don't foresee the need for adding additional data so this should be the most I would need.
Advice please!

You can store the text files in SharedPreference and it should work well.
Database if preferable if a lot of data needs to be stored. Using a database is more optimal than parsing a string.

The correct approach is to create a class for each of the things you want to represent, e.g. favourites. Each class has a save() and reload() method. The point is that you can change the underlying storage mechanism in the save() and reload() methods without having to change the rest of your code. Imagine in the future, you want to enable saving to Dropbox. You would simply change these methods and your app would just work (OK, you'd need to add stuff to get Dropbox account details but you get the idea).
You could go further and define Interfaces for loading and saving. That interface can use a single class responsible for nothing except saving and loading. As long as any consumer and provider classes adhere to the interface, you can mix and match, and even implement storage approaches yet to be invented, without recoding your app. You only have one class to work with if, and whenever, you change your storage approach.
class StorageManager(){
enum DataType {Favourites, MostPlayed, PlayList };
...
public save(DataType dataType){
if (dataType == DataType.Favourites){
saveFavouritesToDB();
...
...
}
This approach gives you maximum flexibility, maximum future proofing and better maintainability.

I think it would be better to go with Database.
The database should provide you with some optimizations, performance improvements and basically you don't have to reinvent the wheel (doing read / write operations on the disk, use some buffers before rewriting, and the like).
Plus, I think you will love the way all code will be organized and split into its own layers once you start using this way.

Using multiple static ArrayLists through a whole Android application

I am developing my first android Application and hoping to get some tips here.
I am getting a JSONObject from an url which then will be parsed in an ArrayList<MyObject>. The list will be used in multiple tabs and be filtered as needed for the tabs. The objects within these list can be modified by the user and the changes should be synchronized with the lists.
So, to speed up loading time I have created a class DataHolder as a singleton which contains 7 arraylist, based from the one JSONObject in different sorting order and filter criterion. The objects in these lists are references from the original list. Populating the lists works fine.
The lists will be used in different fragments and activities.
Now the problem: the second activity contains tabs with fragments. After initializing the fragment... all arraylists in the DataHolder counts 0! I have to save the JSONObject in SharedPreferences and populate it again to get the List. I can't load the url again because it is slowing down the app to much and using SharedPreferences is not an option (I think) because of the need to synchronized the Lists. I have read that using static variables is not the optimal solution, but it seems to be the easiest way :(
What can I do to solve this problem? Should I use Parcelable Objects and always pass the Lists around? Or maybe use SQLite? Or are there other approaches?

The SQLite way is definitively the correct approach, imho.
You should use the internal database to store such lists :
http://developer.android.com/guide/topics/data/data-storage.html#db

use static list for local operation .when user comes out of that screen(activity) save that changes into database.