Develop Reference

How long does SharedPreferences save data?

I am making an Android app, and I will do some process at the first launch of the app.After the first launch I will not these process again. So, for doing this, I want to use SharedPrefences.
How long does it save data? When and how does its saved datas remove?

Data in SharedPrefences is saved until the user uninstalls the app, or clears the app cache

Data in SharedPrefences is saved as file in your app internal storage, its be removed in case:
unInstall app
clear you app data
call yourSharedPreferences.edit().clear().apply()
hope this helps

When you clear the cash memory form settings all the information will be deleted and start fully new. so it not exactly that you need to uninstall your app.
So sharedpreferences is basically store data in primitive way which mean it doesn't contain data directly instead it's reference.

Ways to store variables throughout the life time of android app in phone

I am new to Android. I am building a test application where I need to store a variable's value say, strength of the user. I want to increase or decrease this strength's value whenever user uses the app. And after user closes the app and reopens it on next day, he should find the same value of strength.
One way I can think of is to store a local db in phone and read/write each time into that, since there are hardly 3 to 4 such variables. So db is not a good option I guess.
Other one I thought was to use android.app.Application class but I am not able to get what I want from that. Can we actually do it using android.app.Application? Or then any other method for 3 to 4 variables.

You can use SharedPreferences to store your variable inside shared preferences. For example, set it like:
SharedPreferences sharedPreferences = context.getSharedPreferences("DATA",Context.MODE_PRIVATE);
sharedPreferences.edit().putString("STRENGTH",yourVar).apply();
Then get it out using:
SharedPreferences sharedPreferences = context.getSharedPreferences("DATA",Context.MODE_PRIVATE);
strength = sharedPreferences.getString("STRENGTH",null);

Use SharedPreferences. SharedPreference provides an easy mechanism to persist a value across the life of an app.

Write an XML file in the application directory that the application can read on startup.

Use application class when our app is open read last state and store your data in shared preference, and you are right db is not good choice.

Like you suggested, you can use an SQLite database to store a table regarding the users. Although, you might only have one variable, strength, associated with each user now but if there's potential for more associated values that you want to store regarding the user, I'll recommend using an SQLite database.
Another option you can use is SharedPreferences, it allows you to store key value pairs. So in your case, each strength value can correspond to a username.
If you haven't learned the different storage options for Android, I'll recommend you taking a look at this.
Hope this helps!

Is Shared Preferences safe for private datas?

Im saving datas from my db/user into a gson formated ArrayList in SharedPreferences. Now my question :
Is it safe to save these datas (or data in general) into Sharedpreferences. Are users able to read these gson Arraylists out ? Maybe from SD card ,in a folder or somewhere else.
Thank you !

They are stored as xml files in your app directory, with permissions that allow only your app to access them. But on rooted device they are easily accessible. If you are concerned with security then you may use encryption, those projects might be usefull to you:
https://github.com/rtoshiro/SecureSharedPreferences
https://github.com/sveinungkb/encrypted-userprefs
still those projects does not give you 100% guarantee, hacker may decompile your apk and find keys used to encrypt shared preferences. So if your data is of use only for short time then remember to remove it from your device once user has finished using it. You may for example keep data on server and download it only when needed, caching locally only for short time - when its needed.

SharedPreferences is just a file located in phone private memory. So user can't access it but root can. Root can everything and many users have root's nowadays. You shouldn't store fragile data there
Android SharedPreference security

You can read all shared preferences Data
The SharedPreferences class provides a general framework that allows
you to save and retrieve persistent key-value pairs of primitive data
types.
To see the information in the store you need to know the important thing from the data. This will make reading through the information super easy. But as simple as it's to keep a tiny bit of data as difficult it's to keep and browse large structured data since you need to define key for every data, in addition you can't really search inside the data except you've got a certain concept for naming the secrets.
Please read Android SharedPreference security

Saving preference that can be read and modified by another Application

I built an application that will download and install an APK from a WebService using the technique described here:
Install Application programmatically on Android
During this installation, the Webservice sends a 'flag' that indicates if the SQLite database from the application that is being updated should be deleted or not during it´s first run.
Is there any way to set a "Global Preference" that could be read (if the flag is true, the database should be deleted) and cleared (it should be set to false after deletion to avoid deleting the database all times that app is started) during the first usage of the updated app, without saving it to the SDCard?
I know how to read the preferences from the app that is being updated but, I did´t realize how to modify these preferences from another app.
Thanks a lot.

SharedPreferences are unique to each App/APK - no way to share them that I'm aware of and no 'Global' equivalent.
If you want to share data, the solution is usually some sort of ContentProvider, but that relies on both apps running at the same time.
If you only want to hand-over a token or state, I'd suggest writing a file onto the SDCARD is probably the simplest option?

Here is a tutorial on how to do it.
Basically you have to use MODE_WORLD_WRITEABLE for the prefs file.
To get the context for the other package you use createPackageContext()

android bestway to keep temp data

hi in my application i have temp data , i need to keep the data only up to a particular condition , when app meet this condition,i don't need the data anymore. my problem how to keep the data,upon resuming the application

Using shared preference is the best way

You could try storing data in a sqlite database - there are lots of examples around the internet about how you can use sqlite database in an android application :
http://developer.android.com/guide/topics/data/data-storage.html
http://www.screaming-penguin.com/node/7742
or, you could store stuff in xml files
http://www.ibm.com/developerworks/opensource/library/x-android/
if you store data in xml or an sqlite database - the data is persistent until you specifically remove it.