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is fine to open a new fragment in fragment?

I want to separate logic fragment from activity but the problem is I make api call and save data in fragment. And when user click a item in fragment. I need to send parcelable data to other fragment to show detail info about item.
Is launching fragment in fragment anti pattern for android ?
I would like to hear some opinion about this matter.

Yes, is totally an anti-pattern, remember that you need to see the Activity as a container and fragments as independent sub-screens, so is the Activity responsibility to manage the fragments. I.e.: If you have a Post activity you can have a PostText fragment, a PostImage fragment and all of that is manage by the activity, every fragment is attached to an Activity.

It is not a common practice to have a nested fragment inside a fragment even it can be done. However, it would be better to have an activity as the centric container for all your fragments. You can use EventBus (GreenRobot / Otto) to separate the concerns and do all the API calls in another class and send the results by subscribing to this event.

Should I Hold references to fragments within my activity

My app has one MainActivity with three tabs (A, B, C).
Tab A shows FragmentA1. When I click a list entry in this fragment then FragmentA2 is shown (still in tab A). The same applies to the other tabs, some hierarchies go even deeper (FragmentC4).
All the switching and replacing of all the fragments is handled in MainActivity by Listeners. (Edit: I don't define my fragment in XML layouts but in the code only).
My Question is:
Should I hold references to all fragments in MainActivity or should I create them new everytime I need them?
What are the (dis)advantages? Can I reuse fragments by using Alternative 1, instead of recreating them everytime?
Alternative 1:
class MainActivity
private Fragment fgmtA1;
private Fragment fgmtA2;
private Fragment fgmtA3;
...
public onClickItemInA1(int itemId) {
fgmtA2 = new FragmentA2();
// put args
// replace
}
...
}
Alternative 2:
class MainActivity
...
public onClickItemInA1(int itemId) {
FragmentA2 fgmtA2 = new FragmentA2();
// put args
// replace
}
...
}
Alternative 3:
Maybe the best solution is a completely different approach?

Should I hold references to all fragments in MainActivity or should I
create them new everytime I need them?
It depends...
The only two reasons which i can think of are performance and keeping the state of a Fragment.
If you always create a new Fragment the GC will have a lot to do, which could cause some performance issues if you use a lot of bitmaps or huge data. You can reuse a Fragment by holding a reference to it in the Activity or getting the Fragment by tag or id using the methods FragmentManager.findFragmentByTag(String) or FragmentManager.findFragmentById(int). With them you can reuse already created Fragments, which should be done by default.
Furthermore if your Fragments hold some data, you will lose them or you cache it somewhere else to reacreate it if the Fragment is destroyed. While reusing a Fragment you can use onSavedInstanceState() to reacreate your state.
Hence, yes you should reuse a Fragment because it could cause system performance or headaches using anti-patterns to save some data.

What is the correct way to restore a fragment?

First of all I'm sorry if this explanation seems unclear, I'm new to Android.
I have a ViewPager in main activity showing fragments added dynamically by user. Fragments are created initially on activity start up and are added to ViewPager via Adapter i.e. adapter simply returns proper fragment and as I understand correctly fragment's content is created at this time when ViewPager 'retrieves' a fragment first time.
The problem is when main activity gets restored after orientation changing all fragments are resurrected as well and when Adapter tries to return newly created by user Fragment method createView() is no longer called and it fails with NullPointerException. It seems ViewPager retains fragments attached to it initially and doesn't call createView() for newly added ones for the same position.
I have a feeling I'm missing vital point on the Fragment lifecycle. I wouldn't like to change the design. My main question is what the correct way is to return a Fragment added to ViewPage after activity is restored? Is there any way to locate recently attached fragments?

If the fragment already exists, it will be re-used. However the state of the fragment will not. You should take a look at http://developer.android.com/training/basics/activity-lifecycle/recreating.html for more information.
In particular you should look at overriding onSavedInstanceState and onRestoreInstanceState as a method to repopulate the field that is generating that nullpointerexception
To determine if an instance of a fragment has already been created you can use 'findFragmentByTag' like so:
String fragmentTag = MyFramgment.getClass().getName();
MyFragment frag = getFragmentManager().findFragmentByTag(fragmentTag);
if(frag == null)
frag = new MyFragment();
Whatever ends up being referenced in 'frag', show this to the user.

Getting fragment instance in my activity

Is there some way I get already created currently displayed same instance of fragment in my activity. I DON'T to use
findFragmentById(int id), simply I never created that
findFragmentByTag(String tag), because I am not adding tag in every fragment .offcourse due to some requirement.
getFragment(Bundle bundle, String key), because I never am putting in bundle.
Although I may look like fool to mention that, but I want something like this. Is activity keep some fragment instance somewhere.??
What can be the best approach I can take to achieve this requirement.
UPDATE
Okay, so let me tell you why I can't use above methods. If I am adding various fragment in one activity, where I always want to come back to one fragment when back is clicked. (As we have in navigation drawer, u know). And unless there are inner fragment. so for that I don't want to add in the back stack.
Now even if I have the tag associated with my fragments, I cant say for 8 fragment if- else-if-else for getting the tag. That I know is not correct. So first two ways goes out of my option. Now third one. I exactly don't know where to keep it. And even if I keep it where will I get the bundle and key every time I just want my fragment.

You can get from fragment Manager
List<Fragment> fragList=fManager.getFragments();
for(Fragment fr: fragList){
String fragClassName = fr.getClass().getName();
if(fragClassName.equals(Abc.class.getName())){
Log.i("Fragment:","Abc");
}else if (fragClassName.equals(Xyz.class.getName())) {
Log.i("Fragment:","Xyz");
}
}

Communicating between fragments in Android

Based on the example from http://developer.android.com/training/basics/fragments/communicating.html I tried to reproduce the communication between two fragments which are sub fragments of a larger fragment.
In the example, AB activity contains A fragment and B fragment. But I am trying to achieve the same but in my case AB Fragment contains A fragment and B fragment.
The problem is the overridden method in the AB Fragment never gets called. Does this not work because the containing component is a Fragment and not a Activity like in the example? Am I missing out something here?

If you are referring to onClick() or some other onSomething() handler, then these always get called in the Activity class, not the fragment. So in the example you linked, the onArticleSelected() must remain in the Activity, even if you have nested fragments.
To pass info on to the fragment, you have a few options. One, you can keep a reference to the fragment within the activity. This might be lost if your activity recreates (settings event for example).
The second and better way would be to tag your fragments, and then use findFragmentByTag.
When you add your fragment (notice the parameter "my_fragment" which is the tag I gave to the fragment):
getSupportFragmentManager().beginTransaction().add(R.id.fragment_container, myFragment, "my_fragment").commit();
Or when you replace one fragment with another:
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, myFragment, "my_fragment").comit();
Then, when you want to do something in the fragment from within your onArticleSelected of the activity:
Fragment fragment = getSupportFragmentManger().findFragmentByTag("my_fragment");
if (fragment != null) {
fragment.articleSelected(articleId);
}

You can always use an Interface to communicate between fragments. It is the safest way to do so.