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Having an a fragment communicate via interface to activity that did not launch it

I have a fragment that communicates to an activity via an interface. This is cool and all but, is it possible to have the fragment communicate with an activity that did not launch it?
The reason why is I don't want one activity to be a million lines long of code implementing all interface methods for the fragments when I could just create "helper" activities to implement all the interfaces.
Currently I am using the Google Navigation drawer template so, maybe I could create new activities and group fragments around them. Im not sure if it will break my navigation drawer if I try to launch new activites.

Because of the activity lifecycle parameters of an activity are usually separated of the activity. You can store them in the savedInstancestate and retrieve them back. This way you make sure that your activity has the right information when it is restored.
If content changes somewhere during the app lifecycle it is good to store that content somewhere permanently (SharedPreferences, Database, File).
If you want to notify several parts of your app of an event a good way to go is a Local Broadcast.
Having said that it seems strange to me that another activity than the currently running one (and the one containing your fragment) should be notified of an event. When it is resumed it would collect the necessary information and update itself.

You're breaking android development practices. Fragments are encapsulated within an Activity. And an Activity is encapsulated within itself. An Activity should not communicate with another Activity via references.
Activities are communicating via references here, so I totally would not recommend doing this. But here's how you can do what you're asking.
class HelperActivity { // implement here
public static HelperActivity context = this;
public MyFragment myFragment = new MyFragment(this); // cast to implementation
}
class NormalActivity {
void onCreate {
getSupportFragmentManager().beginTransaction()
.add(R.id.fragment_attach, HelperActivity.myFragment)
.commit();
}
}

Android - Communicating between Fragments

I'm trying to access a function in one Fragment (f1) from another Fragment (f2). It seems that i have to make my function in f1 public and static to be able to access it from f2.
I've read that it's not a good idea to access one Fragment from another, so i've tried to make f2 access my Activity, which then connects to the function in f1. Although, even doing it this way, i still have to make my function in f1 public and static.
I don't reuse the Fragments, i simply have them in a ViewPager for swiping back and forth through the Fragments.
At the moment i have a lot of static variables because i have to make my functions static to access them from other Fragments.
Am i going about this the wrong way? Is there a better way?
Thanks in advance.

You're going down a road of much pain unless you are very controlled about what you do.
First of all, sharing state through static methods and variables is a fairly awful way of doing things, and static members won't be able to access anything in the instance of the fragment. If you really think you need to use static methods, don't bother putting them in the fragment classes. They don't provide an advantage there. Put them in a common class that they all reference.
Second of all, if you're using a ViewPager with fragments, you can't be guaranteed that any given fragment's view hierarchy even exists at any moment in time. This is because ViewPager typically only keeps fragment views alive that are on the current page or on +/- 1 offset from the visible page. So if you need to tell a fragment at offset +2 from the current fragment, you won't be able to make visible changes to it.
Instead, the easiest thing to do is create an object that maintains whatever state you want to share between the fragments, and have them all make changes to that one object. If you need instant changes to other fragments, you can use something like an event bus to have those changes communicated to other active fragments.
What you're trying to do is in general fairly complex. Expect to spend a lot of time designing a correct solution, and be sure to learn how fragments and ViewPager works very well.

You got two questions
1. Am i going about this the wrong way?
Yes, its not recommended to have the methods & variables declared static just to make them used to access from outside of your class.
2. Is there a better way?
Yes, declare the methods as public (but not static) inside its fragments.
From the activity (which is hosting the fragments) get the reference to the fragments using FragmentManager classes methods findFragmentById() or findFragmentByTag() then call the methods.
Sample :
Fragment fragment =
fragmentManager.findFragmentByTag.findFragmentById
(R.id.fragment);
or
Fragment currentFragment =
fragmentManager.findFragmentByTag("fragmentTag");

If you really require both of your functions to be isolated, you can use Broadcasts. Just send a Broadcast from one fragment and have the Broadcast receiver in the other fragment.

Share Headless Fragment Between Activities

For some reason my understanding was that a headless Fragment lives for the duration of your application. With this understanding, in my attempt to persist an object between startActivityForResult() I put the object in a Headless Fragment like this
private HeadlessFragment modelFragment;
modelFragment = (HeadlessFragment)
getSupportFragmentManager().findFragmentByTag(Constants.HEADLESS_FRAGMENT_TAG);
if (modelFragment == null){
modelFragment = new HeadlessFragment();
}
modelFragment.setInvoice(invoice);
I can confirm that the custom object was set, however when I go to the next activity and try to get the same object by calling findFragmentByTag with same tag the object is null.
Does a Headless Fragment survive between two Activities life cycle? I did set setRetainInstance(true) on that Headless Fragment. I was hoping that I will not have to implement Parceable on my custom object.

For some reason my understanding was that a headless Fragment lives for the duration of your application.
No. Fragments are owned by activities and are not application-wide constructs.
I can confirm that the custom object was set, however when I go to the next activity and try to get the same object by calling findFragmentByTag with same tag the object is null.
There are at least two reasons for this:
First, at least in the code that you are showing, you never add the fragment to the FragmentManager via a FragmentTransaction. As such, the activity that created the fragment will not be able to find the fragment via findFragmentByTag(), because the FragmentManager does not know about it.
Second, each activity has its own FragmentManager, and fragments from one activity are not accessible in another activity.
I was hoping that I will not have to implement Parceable on my custom object.
Then don't pass the object. Pass the information (e.g., a key or ID) by which the other activity can retrieve the object (from a singleton POJO cache, by querying the database, etc.).
Or, do not make them separate activities, but have them as separate (regular) fragments in one activity.
Or, implement Serializable, though Parcelable executes more quickly.

Populating fragment fields from activity

I have a ViewPager of 3 fragments. All 3 fragments are of the same type, with identical layouts, but they are to hold different (text) information. I am trying to create my activity, where I create the fragments and prepare the text data that I will populate my fragments with. However, I can't seem to work with the fragments from within my activity. All the activity's lifecycle methods are executed before the fragment lifecycle methods. So if I try to update a textview in a fragment from within my activity, it won't work, because the textview is null in the fragment.
I'm going to need to make periodic updates to the fragments, so passing the data as a bundle is not an option. Plus, since I'm passing lots of text, I'm using a StringBuilder object, which is not something I can pass in a bundle (unless I make it Parcelable, which I don't want)
I think I could run a method from within my fragment class that would execute during fragment creation, but that means all 3 fragments would run this method. That's not really the level of control I'm looking for.
Is there a neat way to make this work?
Thanks

Keep references to your fragments, and let them all implement an interface with a common update-method. As an example, let's make it super clear and call the interface Updatable with one method called 'update':
public interface Updatable {
public void update(String text);
}
Now, in your Activity's onCreate, save references to your Updatables there (i.e. when you lookup or instantiate your Fragments).
It should now be trivial to update your Fragments when necessary from the Activity. Needless to say, the fragment implementation of the update code needs to do the actual update of the TextView(s).
If the update implementation is exactly the same for all your Fragments, your could save some lines of code and make a base class which implements Updatable and extends Fragment.

you might able to populating fragment fields during onActivityCreated(Bundle savedInstanceState).
Refer to this site for more information about fragments' life cycle:
http://developer.android.com/guide/components/fragments.html

You should consider using Observer pattern... there is a really great implementation which you can include as gradle dependency called EventBus:
https://github.com/greenrobot/EventBus

You can use Otto Bus to send data to your fragments from your activity.
http://square.github.io/otto/
Create a new bus in you application class
Bus bus = new Bus();
Create an event that contains your data which you'll pass to fragments.
bus.post(new MyDataEvent(data));
Register your fragment in your fragment's onResume() (Do not forget to unregister in your fragment's onPause())
bus.register(this);
And get data with subscribe in your fragment
#Subscribe
public void onDataReceived(MyDataEvent event) {
// TODO Do what ever you want
}
I hope this'll help you.

Fragment has target not in Fragment Manager

I'm new to android programming.
I just try to save the state of the ListView in my fragment. For that I follow headless fragments (fragment which has no UI). In this fragment, I save the data, used in the ListView, and starting the headless fragment from the main fragment (the one which has the UI).
Now I got the exception:
java.lang.IllegalStateException: Failure saving state: RetainedFragment{4161f850 #1 work} has target not in fragment manager: JobOpeningFramgent{41601c00}
As far my concern, this is happening when I'm trying to replace the fragments with another one in the DrawerLayout.
Please temme the cause of this exception, for better understanding.
Thanks.
Boopathy.

Here's a workaround:
put this in the fragment that causes the problems:
#Override
public void onSaveInstanceState(final Bundle outState) {
setTargetFragment(null, -1);
...
and remember to set it to the real target fragment when you need it.

I'm not sure what do you want to save and where do you want to save it.
The official docs state that: "A Fragment represents a behavior or a portion of user interface in an Activity."
Using a Fragment as a container of another Fragment's UI state is generally a bad idea.
If you want to persist some values throughout the activity lifecycle (that includes screen rotations) just override onSaveInstanceState method. If you want to store some variables even after activity life-time use singelton class or Preferences, and if you want to store your values even after app life-time use SharedPreferences
Please elaborate on what do you exacly want