What's the best way to share data between activities?

Question

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<String, WeakReference<Object>> data = new HashMap<String, WeakReference<Object>>();

  void save(String id, Object object) {
    data.put(id, new WeakReference<Object>(object));
  }

  Object retrieve(String id) {
    WeakReference<Object> 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)

What you can use:

1. passing data between activities (like Cristian said)
2. 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)
3. Using a database
4. Shared Preferences

What you choose depends on your needs. Probably you will use more than one way when you have "a lot of"

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)

"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.

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

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.

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();