An activity is a single, focused thing that the user can do. Almost all activities interact with the user, so the Activity class takes care of creating a window for you in which you can place your UI with setContentView(View).

An activity has essentially four states:

  • If an activity in the foreground of the screen (at the top of the stack), it is active or running.
  • If an activity has lost focus but is still visible (that is, a new non-full-sized or transparent activity has focus on top of your activity), it is paused. A paused activity is completely alive (it maintains all state and member information and remains attached to the window manager), but can be killed by the system in extreme low memory situations.
  • If an activity is completely obscured by another activity, it is stopped. It still retains all state and member information, however, it is no longer visible to the user so its window is hidden and it will often be killed by the system when memory is needed elsewhere.
  • If an activity is paused or stopped, the system can drop the activity from memory by either asking it to finish, or simply killing its process. When it is displayed again to the user, it must be completely restarted and restored to its previous state.

public class Activity extends ApplicationContext {
     protected void onCreate(Bundle savedInstanceState);

     protected void onStart();
     protected void onRestart();

     protected void onResume();

     protected void onPause();

     protected void onStop();

     protected void onDestroy();

Process Lifecycle

  1. The foreground activity (the activity at the top of the screen that the user is currently interacting with) is considered the most important. Its process will only be killed as a last resort, if it uses more memory than is available on the device. Generally at this point the device has reached a memory paging state, so this is required in order to keep the user interface responsive.
  2. visible activity (an activity that is visible to the user but not in the foreground, such as one sitting behind a foreground dialog) is considered extremely important and will not be killed unless that is required to keep the foreground activity running.
  3. background activity (an activity that is not visible to the user and has been paused) is no longer critical, so the system may safely kill its process to reclaim memory for other foreground or visible processes. If its process needs to be killed, when the user navigates back to the activity (making it visible on the screen again), its onCreate(Bundle) method will be called with the savedInstanceState it had previously supplied inonSaveInstanceState(Bundle) so that it can restart itself in the same state as the user last left it.
  4. An empty process is one hosting no activities or other application components (such as Service or BroadcastReceiver classes). These are killed very quickly by the system as memory becomes low. For this reason, any background operation you do outside of an activity must be executed in the context of an activity BroadcastReceiver or Service to ensure that the system knows it needs to keep your process around.

    Use Intent to start a new activity:


    Intent intent = new Intent(Intent.ACTION_DIAL, Uri.parse("tel://"+ phoneNumber));


    2. Contain return data:

    Sometimes you want to get a result back from an activity when it ends. To do this, you call the startActivityForResult(Intent, int) version with a second integer parameter identifying the call. The result will come back through your onActivityResult(int, int, Intent) method.

    When an activity exits, it can call setResult(int) to return data back to its parent. It must always supply a result code, which can be the standard results RESULT_CANCELED, RESULT_OK, or any custom values starting at RESULT_FIRST_USER. In addition, it can optionally return back an Intent containing any additional data it wants.

     public class MyActivity extends Activity {
         static final int PICK_CONTACT_REQUEST = 0;
         protected boolean onKeyDown(int keyCode, KeyEvent event) {
             if (keyCode == KeyEvent.KEYCODE_DPAD_CENTER) {
                 // When the user center presses, let them pick a contact.
                     new Intent(Intent.ACTION_PICK,
                     new Uri("content://contacts")),
                return true;
             return false;
         protected void onActivityResult(int requestCode, int resultCode,
                 Intent data) {
             if (requestCode == PICK_CONTACT_REQUEST) {
                 if (resultCode == RESULT_OK) {
                     // A contact was picked.  Here we will just display it
                     // to the user.
                     startActivity(new Intent(Intent.ACTION_VIEW, data));

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