Android is particularly designed to make pause to an application when the home button is used.

Because of this, there are two methods to manage what should happen when someone uses home or navigates out of the application: onPause () and onResume () .

You can learn more about how they work in Pausing and Resuming an Activity .

By way of example, you can make a class of your that extends Activity to perform certain tasks when the application enters pause :

public class BaseActivity extends Activity {
    @Override
    protected void onPause() {
        // raio, usaram o botão 'home', deixa-me proteger os ficheiros!
    }

    @Override
    protected void onResume() {
        // epá, utilizador voltou, deixa desproteger os ficheiro!
    }
}

public class Activity1 extends BaseActivity {
    // ...
}

Also for your case, setting android:clearTaskOnlaunch="true" can be useful. so that activities is terminated when someone returns to the application.

Check if application in background

It may also be useful to know if the application is going to background , which allows us to react in a more supportive way.

The function below lets you check if the application is going to background . It can be called in the onPause() method of all Activity of the application:

 /**
   * Verifica se o aplicativo está sendo enviado para segundo plano
   * (ou seja, para trás da atividade de outra aplicação).       * 
   *
   * @param context O contexto
   * @return <code>true</code> se outro aplicativo vai estar acima deste.
   */
  public static boolean isApplicationSentToBackground(final Context context) {
    ActivityManager am = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
    List<RunningTaskInfo> tasks = am.getRunningTasks(1);
    if (!tasks.isEmpty()) {
      ComponentName topActivity = tasks.get(0).topActivity;
      if (!topActivity.getPackageName().equals(context.getPackageName())) {
        return true;
      }
    }

    return false;
  }

The function lacks extra permissions so you should include the following in your AndroidManifest.xml :

<uses-permission android:name="android.permission.GET_TASKS" />

Credits of this function for the user @peceps in SOEN in this answer .