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copied!<p>Here a compilation of <a href="http://developer.android.com/guide/faq/framework.html#3" rel="noreferrer">most common ways to achieve this</a>:</p> <ul> <li>Send data inside intent</li> <li>Static fields</li> <li>HashMap of <code>WeakReferences</code></li> <li>Persist objects (sqlite, share preferences, file, etc.)</li> </ul> <p><strong>TL;DR</strong>: 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 <code>Parcelable</code>). If passing complex objects save an instance in a singleton somewhere else and access them from the launched activity.</p> <p>Some examples of how and why to implement each approach:</p> <h2>Send data inside intents</h2> <pre><code>Intent intent = new Intent(FirstActivity.this, SecondActivity.class); intent.putExtra("some_key", value); intent.putExtra("some_other_key", "a value"); startActivity(intent); </code></pre> <p>On the second activity:</p> <pre><code>Bundle bundle = getIntent().getExtras(); int value = bundle.getInt("some_key"); String value2 = bundle.getString("some_other_key"); </code></pre> <p>Use this method <strong>if you are passing primitive data or Strings</strong>. You can also pass objects that implements <code>Serializable</code>.</p> <p>Although tempting, you should think twice before using <code>Serializable</code>: it's error prone and horribly slow. So in general: <strong>stay away from <code>Serializable</code></strong> if possible. If you want to pass complex user-defined objects, <strong>take a look at the <code>Parcelable</code> interface</strong>. It's harder to implement, but it has considerable speed gains compared to <code>Serializable</code>.</p> <h1>Share data without persisting to disk</h1> <p>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.</p> <p><strong>Note:</strong> 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 <code>Application</code>) 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.</p> <h2>Use a singleton class</h2> <p>Have a class to hold the data:</p> <pre><code>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;} } </code></pre> <p>From the launched activity:</p> <pre><code>String data = DataHolder.getInstance().getData(); </code></pre> <h3>Use application singleton</h3> <p>The application singleton is an instance of <code>android.app.Application</code> which is created when the app is launched. You can provide a custom one by extending <code>Application</code>:</p> <pre><code>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;} } </code></pre> <p>Before launching the activity:</p> <pre><code>MyApplication app = (MyApplication) getApplicationContext(); app.setData(someData); </code></pre> <p>Then, from the launched activity:</p> <pre><code>MyApplication app = (MyApplication) getApplicationContext(); String data = app.getData(); </code></pre> <h3>Static fields</h3> <p>The idea is basically the same as the singleton, but in this case you provide static access to the data:</p> <pre><code>public class DataHolder { private static String data; public static String getData() {return data;} public static String setData(String data) {DataHolder.data = data;} } </code></pre> <p>From the launched activity:</p> <pre><code>String data = DataHolder.getData(); </code></pre> <h3>HashMap of <code>WeakReferences</code></h3> <p>Same idea, but allowing the garbage collector to removed unreferenced objects (e.g. when the user quits the activity):</p> <pre><code>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(); } } </code></pre> <p>Before launching the activity:</p> <pre><code>DataHolder.getInstance().save(someId, someObject); </code></pre> <p>From the launched activity:</p> <pre><code>DataHolder.getInstance().retrieve(someId); </code></pre> <p>You may or may not have to pass the object id using the intent’s extras. It all depends on your specific problem.</p> <h1>Persist objects to disk</h1> <p>The idea is to save the data in disk before launching the other activity.</p> <p><strong>Advantages:</strong> you can launch the activity from other places and, if the data is already persisted, it should work just fine.</p> <p><strong>Disadvantages:</strong> 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.</p> <p>Some of the ways to persist objects include:</p> <ul> <li><a href="http://developer.android.com/guide/topics/data/data-storage.html#pref" rel="noreferrer">Save them to the shared preferences</a></li> <li><a href="http://developer.android.com/guide/topics/data/data-storage.html#db" rel="noreferrer">Save them to a sqlite database</a></li> <li>Save them to a file (I’d avoid this one)</li> </ul>

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