Develop Reference

Why extend the Android Application class?

An extended Application class can declare global variables. Are there other reasons?

Introduction:
If we consider an apk file in our mobile, it is comprised of
multiple useful blocks such as, Activitys, Services and
others.
These components do not communicate with each other regularly and
not forget they have their own life cycle. which indicate that
they may be active at one time and inactive the other moment.
Requirements:
Sometimes we may require a scenario where we need to access a
variable and its states across the entire Application regardless of
the Activity the user is using,
An example is that a user might need to access a variable that holds his
personnel information (e.g. name) that has to be accessed across the
Application,
We can use SQLite but creating a Cursor and closing it again and
again is not good on performance,
We could use Intents to pass the data but it's clumsy and activity
itself may not exist at a certain scenario depending on the memory-availability.
Uses of Application Class:
Access to variables across the Application,
You can use the Application to start certain things like analytics
etc. since the application class is started before Activitys or
Servicess are being run,
There is an overridden method called onConfigurationChanged() that is
triggered when the application configuration is changed (horizontal
to vertical & vice-versa),
There is also an event called onLowMemory() that is triggered when
the Android device is low on memory.

Application class is the object that has the full lifecycle of your application. It is your highest layer as an application. example possible usages:
You can add what you need when the application is started by overriding onCreate in the Application class.
store global variables that jump from Activity to Activity. Like Asynctask.
etc

Sometimes you want to store data, like global variables which need to be accessed from multiple Activities - sometimes everywhere within the application. In this case, the Application object will help you.
For example, if you want to get the basic authentication data for each http request, you can implement the methods for authentication data in the application object.
After this,you can get the username and password in any of the activities like this:
MyApplication mApplication = (MyApplication)getApplicationContext();
String username = mApplication.getUsername();
String password = mApplication.getPassword();
And finally, do remember to use the Application object as a singleton object:
public class MyApplication extends Application {
private static MyApplication singleton;
public MyApplication getInstance(){
return singleton;
}
#Override
public void onCreate() {
super.onCreate();
singleton = this;
}
}
For more information, please Click Application Class

Offhand, I can't think of a real scenario in which extending Application is either preferable to another approach or necessary to accomplish something. If you have an expensive, frequently used object you can initialize it in an IntentService when you detect that the object isn't currently present. Application itself runs on the UI thread, while IntentService runs on its own thread.
I prefer to pass data from Activity to Activity with explicit Intents, or use SharedPreferences. There are also ways to pass data from a Fragment to its parent Activity using interfaces.

The Application class is a singleton that you can access from any activity or anywhere else you have a Context object.
You also get a little bit of lifecycle.
You could use the Application's onCreate method to instantiate expensive, but frequently used objects like an analytics helper. Then you can access and use those objects everywhere.

Best use of application class.
Example: Suppose you need to restart your alarm manager on boot completed.
public class BaseJuiceApplication extends Application implements BootListener {
public static BaseJuiceApplication instance = null;
public static Context getInstance() {
if (null == instance) {
instance = new BaseJuiceApplication();
}
return instance;
}
#Override
public void onCreate() {
super.onCreate();
}
#Override
public void onBootCompleted(Context context, Intent intent) {
new PushService().scheduleService(getInstance());
//startToNotify(context);
}

Not an answer but an observation: keep in mind that the data in the extended application object should not be tied to an instance of an activity, as it is possible that you have two instances of the same activity running at the same time (one in the foreground and one not being visible).
For example, you start your activity normally through the launcher, then "minimize" it. You then start another app (ie Tasker) which starts another instance of your activitiy, for example in order to create a shortcut, because your app supports android.intent.action.CREATE_SHORTCUT. If the shortcut is then created and this shortcut-creating invocation of the activity modified the data the application object, then the activity running in the background will start to use this modified application object once it is brought back to the foreground.

I see that this question is missing an answer. I extend Application because I use Bill Pugh Singleton implementation (see reference) and some of my singletons need context. The Application class looks like this:
public class MyApplication extends Application {
private static final String TAG = MyApplication.class.getSimpleName();
private static MyApplication sInstance;
#Contract(pure = true)
#Nullable
public static Context getAppContext() {
return sInstance;
}
#Override
public void onCreate() {
super.onCreate();
Log.d(TAG, "onCreate() called");
sInstance = this;
}
}
And the singletons look like this:
public class DataManager {
private static final String TAG = DataManager.class.getSimpleName();
#Contract(pure = true)
public static DataManager getInstance() {
return InstanceHolder.INSTANCE;
}
private DataManager() {
doStuffRequiringContext(MyApplication.getAppContext());
}
private static final class InstanceHolder {
#SuppressLint("StaticFieldLeak")
private static final DataManager INSTANCE = new DataManager();
}
}
This way I don't need to have a context every time I'm using a singleton and get lazy synchronized initialization with minimal amount of code.
Tip: updating Android Studio singleton template saves a lot of time.

I think you can use the Application class for many things, but they are all tied to your need to do some stuff BEFORE any of your Activities or Services are started.
For instance, in my application I use custom fonts. Instead of calling
Typeface.createFromAsset()
from every Activity to get references for my fonts from the Assets folder (this is bad because it will result in memory leak as you are keeping a reference to assets every time you call that method), I do this from the onCreate() method in my Application class:
private App appInstance;
Typeface quickSandRegular;
...
public void onCreate() {
super.onCreate();
appInstance = this;
quicksandRegular = Typeface.createFromAsset(getApplicationContext().getAssets(),
"fonts/Quicksand-Regular.otf");
...
}
Now, I also have a method defined like this:
public static App getAppInstance() {
return appInstance;
}
and this:
public Typeface getQuickSandRegular() {
return quicksandRegular;
}
So, from anywhere in my application, all I have to do is:
App.getAppInstance().getQuickSandRegular()
Another use for the Application class for me is to check if the device is connected to the Internet BEFORE activities and services that require a connection actually start and take necessary action.

Source: https://github.com/codepath/android_guides/wiki/Understanding-the-Android-Application-Class
In many apps, there's no need to work with an application class directly. However, there are a few acceptable uses of a custom application class:
Specialized tasks that need to run before the creation of your first activity
Global initialization that needs to be shared across all components (crash reporting, persistence)
Static methods for easy access to static immutable data such as a shared network client object
You should never store mutable instance data inside the Application object because if you assume that your data will stay there, your application will inevitably crash at some point with a NullPointerException. The application object is not guaranteed to stay in memory forever, it will get killed. Contrary to popular belief, the app won’t be restarted from scratch. Android will create a new Application object and start the activity where the user was before to give the illusion that the application was never killed in the first place.

To add onto the other answers that state that you might wish store variables in the application scope, for any long-running threads or other objects that need binding to your application where you are NOT using an activity (application is not an activity).. such as not being able to request a binded service.. then binding to the application instance is preferred. The only obvious warning with this approach is that the objects live for as long as the application is alive, so more implicit control over memory is required else you'll encounter memory-related problems like leaks.
Something else you may find useful is that in the order of operations, the application starts first before any activities. In this timeframe, you can prepare any necessary housekeeping that would occur before your first activity if you so desired.
2018-10-19 11:31:55.246 8643-8643/: application created
2018-10-19 11:31:55.630 8643-8643/: activity created

You can access variables to any class without creating objects, if its extended by Application. They can be called globally and their state is maintained till application is not killed.

The use of extending application just make your application sure for any kind of operation that you want throughout your application running period. Now it may be any kind of variables and suppose if you want to fetch some data from server then you can put your asynctask in application so it will fetch each time and continuously, so that you will get a updated data automatically.. Use this link for more knowledge....
http://www.intridea.com/blog/2011/5/24/how-to-use-application-object-of-android

What is the purpose of Application class in Android

What exactly is the purpose of Application class.
what are the benefits of extending it to a custom subclass
Why use it ?
Can global variables be stored in any other class achieve same goal as Application ?

Nice question !
Your application is a context that is always running while your activities and services are running.
It is also the first context to be created and the last to be destroyed.
Thus, it surrounds the life cycle of your app.
You can use the application class as a way to share data or components (for dependency injection for instance). For instance if you want to share a singleton between activities, you can create the instance in the application class and provide a getter, then all other contexts can get the singleton via
((cast to your class)getApplicationContext()).getFoo();
There may be some use cases where you need to do stuff before even your first activity is launched, then do it in the onCreate method of the application class.
On the other hand, you should never relie on the onDestroy method of the Application class, as it is not always called. There is no contract for that on Android.
But this is rare and, usually, you don't need to override the application class though. Dependency injection can be achieved in other ways by RoboGuice or Dagger for instance.

Two things makes this Class very useful:
Application class is instantiated before any other Activity.
It holds the Application Context
Context brings a host of resources for us: we can figure out some device properties, load some resources, initiate a SQLite database etc, etc.
All of this happens before any Activity loads, and all of this is globally available to the Activities.
Simple example of what I mean:
public class App extends Application{
private static Resources sResources;
//--I want to load strings resources from anywhere--
public static String loadStringResource(int resID) {
return sResources.getString(resID);
}
#Override
public void onCreate() {
super.onCreate();
sResources = getResources();
//---I want to load all preferences when my app starts---
PreferenceManager.setDefaultValues(this,R.xml.prefs,false);
}
}

Extending the Application class allows you to integrate into the application's lifecycle.
This is also useful to store global application-level information (though it's usually good to keep your activities 'independent')

The Application class is aware of the Application Context and is loaded when your app is loaded so it holds the proper callbacks for the application lifecycle before your activity starts. You most likely would not want to extend this class.
From the API docs:
There is normally no need to subclass Application. In most situation, static singletons can provide the same functionality in a more modular way. If your singleton needs a global context (for example to register broadcast receivers), the function to retrieve it can be given a Context which internally uses Context.getApplicationContext() when first constructing the singleton.
http://developer.android.com/reference/android/app/Application.html

Accessing a variable from anywhere in an Application(Android)

Is it possible to create a variable in the starting activity (eg : Book class in Activity 1)
and have it available anywhere in the application (eg Book class in Activity 3 , 4 &5) without actually passing it.
I am asking because my xml handler creates a series of objects be it also updates the xml file after any change is made to the object.

You can create a static variable. As long as it's declared with appropriate access (e.g., public) it will be directly available to any activity in the same process. (This will be the default; you need to do extra work to get an activity into a separate process.)
It's common to separate out such global variables into a separate class.
However, be aware that if your app is pushed into the background, there's a chance that the process will be killed off and re-created. In that case, any data stored in static variables will be lost. Alternatives include using SharedPreferences, a data base, or a ContentProvider.

Implement a class, for example with the name GlobalVariables, which extends Application.
public class GlobalVariables extends Application
In your AndroidManifest.xml do this in the application tag:
<application android:label="#string/YourAppName" android:icon="#drawable/YourIcon"
android:name=".activities.GlobalVariables.">
Don´t forget the package path for declaring your class (similar you do for adding activities to manifest file).
Then you can access this class and its variables or methods from everywhere of your application. Do this in the onCreate method of any Activity:
GlobalVariables globalVariables = (GlobalVariables) getApplicationContext();
The Class extended from Application (e.g. our GlobalVariables here) will be created with the start of your app and will be available until the app will be destroyed.
You can have a HashMap or something, else where you can store your desired variable, in the GlobalVariables class. Push the variable from your first Activity into the GlovalVariables and pull it from the second trough getting an instance to the GlobalVariables. Like this (once again):
GlobalVariables globalVariables = (GlobalVariables) getApplicationContext();

Lifecycle of enum based singleton on Android

A few days ago I've discovered that singleton can become anti-pattern in Android. My singleton (class with private constructor and instance stored in static field) was deleted (instance was deleted despite the fact other activities were still using this singleton (via getInstance() method) so another instance had to be created ) because Activity from which it was first invoked was deleted (after invoking finish for only this one activity).
I've read already how this problem can be resolved however I've also just read "Effective Java". There is said that "Single-element enum type is the bast way to implement a singleton".
So now I'm wondering what would be the lifecycle of singleton created this way in Android application? Would it be the same like in case of "standard singleton implementation" so after destroying activity from which it was invoked the first time it will be destroyed (even if it used also in other activities)?
I'm not asking about proper android singleton implemenation or the singleton pattern itself (is it pattern or anti-pattern etc) but I'd like to know what be the lifecycle of such enum singleton object and when it will be destroyed.

In all cases, the classes you use are tied to the ClassLoader that loaded them. This is true in Java in general, not just Android. Android will isolate activities by using new ClassLoaders each time -- at the least, it doesn't promise it won't, and it does as far as I can tell.
Any singleton, or other class-level state, is tied to the Class which is tied to the ClassLoader. This is why your state "disappears"; it's actually that your calling code is seeing a new Class in a new ClassLoader.
So, your enum-based trick, or anything else along these lines, would have exactly the same behavior. You just can't "persist" activity information this way. You can and should write to a SQLite DB. You could probably stash it in the SharedPreferences too.

The application object is a good location to store information which needs to be accessible to various activity or service instances. You can retrieve it like this (where this is an Activity or Service):
private MyApplication app;
in onCreate(...){
...
this.app = (MyApplication) this.getApplication();
...
}
Remember to set the name also in the manifest:
Set the attribute "name" of the "application" tag:
The value is the path to the class relative to the package of your app.
The application object is created when the app is started, you can initialize like in an activity or service in it's onCreate() method.
One thing to remember: The application object can survive closing your "last" activity. In this case you may get the same application object with the state from the previous interaction with your app. If this is a problem you must somehow detect the initial start of your app (e.g. using a special launcher activity without UI, which initializes the application object and then starts a new intent.
BTW, the same may happen with singletons if they have not yet been lost to garbage collection.

My secure singleton implementation is like that:
I create a singleton class which has an attribute of boolean 'didReceiveMemoryWarning';
public class SingleTon(){
public boolean didReceiveMemoryWarning = true;
...
...
}
In application first screen(It is exactly first launch screen)(I have a splash screen that is visible 3 sec)
SingleTon.getInstance().didReceiveMemoryWarning = false;
And in every Activity's onCreate() method, check this boolean data,
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if(SingleTon.getInstance().didReceiveMemoryWarning){
{ Load your data from local to your SingleTon class,
because your app was released by OS};
}
}
it is my implementation.

getApplication() and spanning Activities within a given Applications

I've spent a while reading up on this and have yet to find an answer.
I have a few activities all within one app. I'd like to share some data between them that is not suitable for intents or serialized data files. I know I can use the overall Application for this purpose, but I'm not fully understanding how it works.
We'll call my overall application class "MyApplication" and the main activity "MyActivity". I would expect that once the app begins from a cold start, it first instantiates and initializes MyApplication (calling onCreate()) and then goes on to instantiate and start my activity (MyActivity) (which was marked with the intent android.intent.action.MAIN). The logs seem to bear this out.
But... if I try to set the following Activity-wide variable, it gets set to null:
private final MyApplication THISAPP = (MyApplication)getApplication();
This is peculiar because the application definitely exists at this point. (Also, I can't delay setting a final variable til MyActivity.onCreate() gets called - that's disallowed ("The final field...cannot be assigned")).
Note that this works if I don't call THISAPP "final" and I assign it in onCreate(). That of course defeats the purpose of protecting a variable with "final" though, and it seems like it should be possible to do.
So, why isn't getApplication() producing a non-null value before onCreate()? Or is this some strange matter of context? (I've found a vague reference to context not being valid til after onCreate() is done, but would that apply to the parent Application's context as seen from a child Activity? I'd expect the parent to already have that set at that point.)
Edit: to emphasize, I can get this to work if I don't try to use final and if I remember to put the "." before the application name in the manifest (a mistake I made and corrected well-before this I wrote this question). It just isn't clear why getApplication() (or getApplicationContext() for that matter) aren't usable before onCreate() in a child Activity... unless "that's just how it is".

I'd like to share some data between them that is not suitable for intents or serialized data files. I know I can use the overall Application for this purpose, but I'm not fully understanding how it works.
I'd just use static data members as a cache for your persistent store, rather than fussing around with Application. Application gives you little advantage over static data members and has one big cost: there can only be one Application, whereas you can organize your static data members/singletons however you like.
I would expect that once the app begins from a cold start, it first instantiates and initializes MyApplication (calling onCreate()) and then goes on to instantiate and start my activity (MyActivity) (which was marked with the intent android.intent.action.MAIN). The logs seem to bear this out.
Correct.
But... if I try to set the following Activity-wide variable, it gets set to null: private final MyApplication THISAPP = (MyApplication)getApplication();
Of course. The activity object has not yet been initialized. You are welcome to use initializers like this for constants or things you get from static methods (e.g., Calendar.getInstance()) or other constructors (e.g., new ArrayList<Foo>()). Do not call a superclass method an expect it to work at this point, since the constructor chain has not yet been called on the object being instantiated. This is just Java, nothing particular to Android.

Have you done it the following way?
Created a class that implements Application
public class MySuperApplication extends Application
{
public String SomeSetting;
}
Defined which is your Application class in manifest (important!)
<application android:name=".MySuperApplication" android:icon="#drawable/icon"
and then accessed it in your activities
app = (MySuperApplication) getApplication();
app.SomeSetting = "Test";
This should work. It does in my case.

AFAIK, You cannot declare a empty final variable and initialize it onCreate. In java it is initialized during declaring the variable or in the constructor.
THISAPP cannot be initialized to Application object during declaration because the Application doesn't exist during compile time. Because onCreate is not a constructor, you cannot declare a empty final variable and initialize it in onCreate.

Instance variables are created when the object (in this case an Activity object) is constructed. For getApplication() to produce a valid value then the Activity object would have to have been provided enough context (little 'c') at instantiation for this to occur.
The source for getApplication() in Activity.java is just this:
/** Return the application that owns this activity. */
public final Application getApplication() {
return mApplication;
}
Here is where it is set:
final void attach(Context context, ActivityThread aThread, Instrumentation instr, IBinder token,
Application application, Intent intent, ActivityInfo info, CharSequence title,
Activity parent, String id, Object lastNonConfigurationInstance,
Configuration config) {
// other code
mApplication = application;
// other code
}
Since mApplication is not set until after attach is called, it won't be available to getApplication() on instantiation of your Activity.