What s the best way to share data between activities

Question

I have one activity which is the main activity used throughout the app and it has a number of variables  I have two other activities which I would like to be able to use the data from the first activity  Now I know I can do something like this   GlobalState gs    GlobalState  getApplication    String s   gs getTestMe      However I want to share a lot of variables and some might be rather large so I don t want to be creating copies of them like above   Is there a way to directly get and change the variables without using get and set methods  I remember reading an article on the Google dev site saying this is not recommended for performance on Android

User · Answer

There is a new and better way to share data between activities, and it is LiveData. Notice in particular this quote from the Android developer's page:

The fact that LiveData objects are lifecycle-aware means that you can share them between multiple activities, fragments, and services. To keep the example simple, you can implement the LiveData class as a singleton

The implication of this is huge - any model data can be shared in a common singleton class inside a LiveData wrapper. It can be injected from the activities into their respective ViewModel for the sake of testability. And you no longer need to worry about weak references to prevent memory leaks.

User · Answer

All of the aforementioned answers are great    I m just adding one no one had mentioned yet about persisting data through activities and that is to use the built in android SQLite database to persist relevant data    In fact you can place your databaseHelper in the application state and call it as needed throughout the activates   Or just make a helper class and make the DB calls when needed    Just adding another layer for you to consider    But all of the other answers would suffice as well   Really just preference

User · Answer

What you can use    passing data between activities  like Cristian said  using a class with a lot of static variables  so you can call them without an instance of the class and without using getter setter  Using a database Shared Preferences   What you choose depends on your needs  Probably you will use more than one way when you have  a lot of

User · Answer

If your intention is to call other Activities from the current Activity  you should use Intents  Your focus could be less on persisting data than on sharing it on an as-needed basis   However  if you really need to persist these values then you could persist them in some kind of structured text file or database on local storage  A properties file  XML file  or JSON file could store your data and be easily parsed during activity creation  Don t forget also that you have SQLite on all Android devices  so you could store them in a database table  You could also use a Map to store key-value pairs and serialize the map to local storage  but this might be  too cumbersome to be useful for simple data structures

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Do what google commands you to do  here   http   developer android com resources faq framework html 3     Primitive Data Types Non-Persistent Objects Singleton class - my favorite  D A public static field method A HashMap of WeakReferences to Objects Persistent Objects  Application Preferences  Files  contentProviders  SQLite DB

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Assuming you are calling activity two from activity one using an Intent  You can pass the data with the intent putExtra     Take this for your reference  Sending arrays with Intent putExtra  Hope that s what you want

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However I want to share alot of   variables and some might be rather   large so I don t want to be creating   copies of them like above     That doesn t make a copy  especially with String  but even objects are pass by value of the reference  not the object itself  and getter s like that are fine to use -- arguably better to use than other means because they are common and well understood   The older  performance myths   such as not using getters and setters  still have some value  but have also been updated in the docs    But if you don t want to do that  you could also just make the variables public or protected in GlobalState and access them directly  And  you can make a static singleton as the Application object JavaDoc indicates      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    Using Intent data  as other answers here note is another way to pass data  but it s usually used for smaller data and simple types  You can pass larger more complex data  but it s more involved than just using a static singleon  The Application object is still my personal favorite for sharing larger more complex non persistent data between Android application components though  because it has a well defined lifecycle in an Android app     Also  as others have noted  if the data gets very complex and needs to be persistent then you can use SQLite or the filesystem too

User · Answer

There are various way to share data between activities  1  Passing data between activities using Intent  Intent intent new Intent this  desirableActivity class   intent putExtra  KEY    Value    startActivity intent    2  Using static keyword    define variable as public static and use any where in project        public static int sInitialValue 0    use anywhere in project using classname variableName   3   Using Database  but its bit lengthy process  you have to use query for inserting data and iterate data using cursor when need  But there are no chance of losing data without cleaning cache   4  Using shared Preferences  much easier than database  but there is some limitation you  can not save ArrayList  List and custome objects   5  Create getter setter in Aplication class and access any where in project         private String data        public String getData               return data                 public void setData String data              this data   data            here set and get from activities             YourApplicationClass getApplicationContext    setData  abc               String data   YourApplicationClass getApplicationContext    getData

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Using the hashmap of weak reference approach  described above  and in http   developer android com guide faq framework html seems problematic to me  How are entire entries reclaimed  not just the map value  What scope do you allocate it in  As the framework is in control of the Activity lifecycle  having one of the participating Activities own it risks runtime errors when the owner is destroyed in advance of its clients  If the Application owns it  some Activity must explicitly remove the entry to avoid the hashmap from holding on to entries with a valid key and a potentially garbaged collected weak reference  Furthermore  what should a client do when the value returned for a key is null   It seems to me that a WeakHashMap owned by the Application or within a singleton is a better choice  An value in the map is accessed via a key object  and when no strong references to the key exist  i e  all Activities are done with the key and what it maps to   GC can reclaim the map entry

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Here a compilation of most common ways to achieve this    Send data inside intent Static fields HashMap of WeakReferences Persist objects  sqlite  share preferences  file  etc     TL DR  there are two ways of sharing data  passing data in the intent s extras or saving it somewhere else  If data is primitives  Strings or user-defined objects  send it as part of the intent extras  user-defined objects must implement Parcelable   If passing complex objects save an instance in a singleton somewhere else and access them from the launched activity   Some examples of how and why to implement each approach   Send data inside intents  Intent intent   new Intent FirstActivity this  SecondActivity class   intent putExtra  some key   value   intent putExtra  some other key    a value    startActivity intent     On the second activity   Bundle bundle   getIntent   getExtras    int value   bundle getInt  some key    String value2   bundle getString  some other key      Use this method if you are passing primitive data or Strings  You can also pass objects that implements Serializable   Although tempting  you should think twice before using Serializable  it s error prone and horribly slow  So in general  stay away from Serializable if possible  If you want to pass complex user-defined objects  take a look at the Parcelable interface  It s harder to implement  but it has considerable speed gains compared to Serializable   Share data without persisting to disk  It is possible to share data between activities by saving it in memory given that  in most cases  both activities run in the same process   Note  sometimes  when the user leaves your activity  without quitting it   Android may decide to kill your application  In such scenario  I have experienced cases in which android attempts to launch the last activity using the intent provided before the app was killed  In this cases  data stored in a singleton  either yours or Application  will be gone and bad things could happen  To avoid such cases  you either persist objects to disk or check data before using it to make sure its valid   Use a singleton class  Have a class to hold the data   public class DataHolder     private String data    public String getData    return data     public void setData String data   this data   data      private static final DataHolder holder   new DataHolder      public static DataHolder getInstance    return holder       From the launched activity   String data   DataHolder getInstance   getData      Use application singleton  The application singleton is an instance of android app Application which is created when the app is launched  You can provide a custom one by extending Application   import android app Application  public class MyApplication extends Application     private String data    public String getData    return data     public void setData String data   this data   data       Before launching the activity   MyApplication app    MyApplication  getApplicationContext    app setData someData     Then  from the launched activity   MyApplication app    MyApplication  getApplicationContext    String data   app getData      Static fields  The idea is basically the same as the singleton  but in this case you provide static access to the data   public class DataHolder     private static String data    public static String getData    return data     public static void setData String data   DataHolder data   data       From the launched activity   String data   DataHolder getData      HashMap of WeakReferences  Same idea  but allowing the garbage collector to removed unreferenced objects  e g  when the user quits the activity    public class DataHolder     Map lt String  WeakReference lt Object gt  gt  data   new HashMap lt String  WeakReference lt Object gt  gt        void save String id  Object object        data put id  new WeakReference lt Object gt  object           Object retrieve String id        WeakReference lt Object gt  objectWeakReference   data get id       return objectWeakReference get            Before launching the activity   DataHolder getInstance   save someId  someObject     From the launched activity   DataHolder getInstance   retrieve someId     You may or may not have to pass the object id using the intent   s extras  It all depends on your specific problem   Persist objects to disk  The idea is to save the data in disk before launching the other activity   Advantages  you can launch the activity from other places and  if the data is already persisted  it should work just fine   Disadvantages  it   s cumbersome and takes more time to implement  Requires more code and thus more chance of introducing bugs  It will also be much slower   Some of the ways to persist objects include    Save them to the shared preferences Save them to a sqlite database Save them to a file  I   d avoid this one

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And if you wanna work with data object  this two implements very important   Serializable vs Parcelable   Serializable is a marker interface  which implies the user cannot marshal the data according to their requirements  So when object implements Serializable Java will automatically serialize it   Parcelable is android own serialization protocol  In Parcelable  developers write custom code for marshaling and unmarshaling  So it creates less garbage objects in comparison to Serialization The performance of Parcelable is very high when comparing to Serializable because of its custom implementation It is highly recommended to use Parcelable implantation when serializing objects in android    public class User implements Parcelable  check more in here

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Sharing data between activites  example passing an email after login    email  is the name that can be used to reference the value on the activity that s being requested  1 Code on the login page  Intent openLoginActivity   new Intent getBaseContext    Home class       openLoginActivity putExtra  email   getEmail     2 code on the home page  Bundle extras   getIntent   getExtras        accountEmail   extras getString  email

User · Answer

Well I have a few ideas  but I don t know if they are what your looking for   You could use a service that holds all of the data and then just bind your activities to the service for data retrival   Or package your data into a serializable or parcelable and attach them to a bundle and pass the bundle between activities   This one may not be at all what your looking for  but you could also try using a SharedPreferences or a preference in general   Either way let me know what you decide

User · Answer

You could extend the Application class and tag on any objects you want there  they are then available anywhere in your application

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