Develop Reference

Why does SharedPreferences need two keys?

I'm trying to build a single object that handles all my SharedPreferences, since they are mostly used in the whole app, and I don't quite Understand why it takes two keys to get a value.
The call looks like this:
context.getSharedPreferences(FirstKey, Context.MODE_PRIVATE).getString(SecondKey, default)
I get that its basically built up as a two dimensional array.
The FirstKey gives me a collection of key-value pairs I can use my SecondKey on to get my value. And I get that If I have dozens of SharedPreferences this might come in handy to manage them and prevent mixups/unwanted overwriting.
But is this necessary If I only have like 10 preferences I save anyway or is it reasonable to just use one FirstKey for all of my preferences?

But is this necessary If I only have like 10 preferences I save anyway
or is it reasonable to just use one FirstKey for all of my
preferences?
So for this case you can avoid the use of that FirstKey by using getDefaultSharedPreferences() like this:
SharedPreferences.Editor editor = PreferenceManager.getDefaultSharedPreferences(context).edit();
editor.putString("myKey", "myValue");
editor.apply();
or read already set preferences:
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
String myPref = prefs.getString("myKey", "myDefaultValue");

I guess it is to encapsulate, organize and manage better groups (first key values) of data you want to store (second keys values). So in case you want for example to retrieve all settings preferences you can group them by a Settings file (first key). Or in case you would like to delete all stored values regarding a user preferences (preferred language, preferred currency.. ) then you can organize those data within a "UserPref" file (first key) and then you can iterate within it to either delete all of them when you logout or whatever pogic you see useful for your user experience.

First key
Retrieve and hold the contents of the preferences file
Second key
Retrieve and hold value in this file

You can use only your "FirstKey" for all your preferences and normally dev us it like this only. You can create multiple instances of shared preference by changing the "First key" at the time of getSharedPreference(). Suppose you want two different shared preference for two different modules in your project then change the "FirstKey" parameter, in this case, you have two be careful while storing and fetching data from preference as you have two different shared preference.

As you rightfully stated, the first key represents a group of key-value pairs (which is actually very similar to a single file) and the second key helps you fetch the values in that file/group.
I personally believe this design is great especially in cases where you may want to separate all your values into different categories. If your app is "small", you can save all your values in one single file/group. Otherwise, you can split all your values into separate files/groups.
I hope this helps.. Merry coding!

can i store two or more values with same key using SharedPreferences in android?

can i store two or more values with same key using SharedPreferences in android? If no, please tell me how to store values of username, first name, password etc when many users register in registration app?
Ex:
person A registered with username="john12", first name="john" and DOB="06/06/2000".
person B registered with username="arun89", first name="arun" and DOB="08/11/1989".
Now, I want to store these values in SharedPreferences and retrieve them later. Is it possible using SharedPreferences? If not, Please tell me how to do in other way.
Thank you in advance.

I woud consider creating a JSONObject and add the fields you want to store as a key:value pair.
json.putString(key, value);
You can then store the json object in it's string representation with json.toString() and restore it later with
JSONObject jo = new JSONObject(jsonString);
String value = jo.getString(key);
JSONObject also offeres different data types beside strings.
It really depends on how much data you want to store. Depending on that I would choose SharedPreferences or a SQLite implementation.

You cannot store these values directly (as ones added latter will overwrite previously added) but you can always store Parcelable and put your data into it

For your case it is better use SQLIte database.But if you want to use shared preference it is still possible.You have to use a key with additional index to remember different user like
UserName1:arun
UserName2:john
You have to remember the total number of user.Then can maintain all of them.you can also use other data structure like hashmap to maintain data for the shared preference.

I dont't think it is possible, as you don't know the number of users.
You could try to separate the users with commas, but that's lame.
You should consider using SQLite database.
http://developer.android.com/reference/android/database/sqlite/SQLiteDatabase.html

Then you have to store a array List of user objects first create a class userInfo then create a array List type of userInfo then store the data in this list and put a serialize-able object in SharedPreferences.

You can also store them on a single key called "registers" as string. Concatenate each register to preference. Put ";" (or any other characther you want) between each register. Then parse the string and use the values.
Key: registers
Value: "username=john12, first name=john, DOB=06/06/2000;username=mike12, first name=mike, DOB=06/07/2012"
Using split method of String will give you a list of registers as String.
registers.split(";");
Splitting again with "," will give you properties of each register.

Keeping data when Android does an onExit()

I have a String array that I want to keep to display in a ListView, but every time I exit the activity, the global String array is deleted and the ListView doesn't display. Is there anyway that I can keep a String array in the application's storage for the next use?

It's actually pretty easy for just a string. You can store it in shared preferences. See this link: http://developer.android.com/guide/topics/data/data-storage.html#pref

Is there simple way to create user defaults values for TextView in Android?

I'm looking for simple way to create user defaults values for TextView in Android: I need the standart solution that user is seeing the previous value, that was entered by him to the TextView. Sure, there is way to store value of TextView at somewhere and then read it again while starting activity, but, may be, there is any simple tool to get what I wish ?

You can use SharedPreferences to store previous values and then get previous values from shared preferences to set as default values

Just to get your question correctly:
You want to present the values the user selected/set in his last visit?
You can save those values in the SharedPreferences - That's what they're made for.
You can give the google prediction api a try.

Android storing string array

I've been looking for over the past day and a half at several websites about how to store strings/string arrays/etc. and I can't seem to figure it out. I'm not sure if I'm just not quite comprehending how to implement data storage..or what. But here is my problem, simply put, I have two activity pages (we'll name 'A,B' respectively) . All I want to do is get a string from a text view in Activity B, store it in an array, and then have it accessed by clicking a button in Activity A.
I know it is simple, but I hit a block for some reason... I am trying to use SharedPreferences but how would I obtain the string from Activity B, store it in a global array, and let it be accessed by a different activity (Activity A) ?

Just store it into shared preferences (usually in onPause()):
SharedPreferences.Editor editor = getPreferences(MODE_PRIVATE).edit();
editor.putString(GAME_STATE, writer.toString());
editor.commit();
in one activity and load in another (usually in onResume()):
SharedPreferences preferences = getPreferences(MODE_PRIVATE);
String jsonState = preferences.getString(GAME_STATE, null);
And nothing prevents you from using public static variables

You can't store an array directly in shared preferences, but you can store a set.
See putStringSet and getStringSet. You can add all the items from your array to a LinkedHashSet (as long as they are unique) if you wish to store them in SharedPreferences. See Konstantin's answer for the general idea on how to use SharedPreferences.

Yes you can obtain and store value in global array but you will loss everything once you close the application. So its better you create a table and store/get values from the table whenever you required.
Here is an example for creating SQLite example.