Why extend the Android Application class

Question

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

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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

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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

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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

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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

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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

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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  amp  vice-versa   There is also an event called onLowMemory   that is triggered when the Android device is low on memory

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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

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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

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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

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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

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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

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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

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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

[android] Why extend the Android Application class?

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