Q:

How do I communicate between fragments as if they were all on one activity? 

I am using the android studio fragments sample.  My goal is to set the value of EditText et1 in fragment1 by using code in fragment2.  Also I want to update the ListView lv, that is in fragment1 by using code in fragment2. 
The example does some back and forth sending of ARG_SECTION_NUMBER, but that is just text and won't work for a listview.  What does not work is this
private View rvza, rvl, rvea;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    rvza = inflater.inflate(R.layout.fragment_zeit_und_aktion, container, false);
    rvl = inflater.inflate(R.layout.fragment_liste, container, false);
    rvea = inflater.inflate(R.layout.fragment_eintragen_und_aendern, container, false);
    ....
    }
...      
Button btnNeu = (Button) rvea.findViewById(R.id.btnNeu); //this line is somewhere...

What also doesn't work is this
new MainActivity().myfunction((ListView)rvl.findViewById(R.id.listView));

it gives an Attempt to invoke interface method 'int
  android.widget. ListAdapter.getCount()' on a null object reference at

public void myfunction(ListView lv)
{
   int count = lv.getAdapter().getCount();

So how do I communicate between fragments having one parent activity?  Is there a helper class? 

A:

If you want to access any method or variable, that has an use by more than one child, make parent activity singleton and access the desired method or variable in child fragment.  Also if you want to access anything within fragment from anther, don't forgot to make and init an object of that fragment in parent activity and use that object further to add or replace fragment in fragment manager.  By this you have access to every child through the parent activity 
How to make singleton?  See below example.  
public class SingletonExample {
// Static member holds only one instance of the
// SingletonExample class
private static SingletonExample singletonInstance;
// SingletonExample prevents any other class from instantiating
private SingletonExample() {
}
// Providing Global point of access
public static SingletonExample getSingletonInstance() {
    if (null == singletonInstance) {
        singletonInstance = new SingletonExample();
    }
    return singletonInstance;
}
public void printSingleton(){
    System.out.println("Inside print Singleton");
}

}

