android viewpager - How to write variable of activity that contains fragment class from inside current Fragment? -

i have activity indefinite number of fragments. inside fragments there variable need access in activity call intend activity bar correctly. variable of current fragment on screen. because call depends of fragment are. due fragment manager creates 3 fragment @ time can't write variable calling inside fragment because don't know last writes variable. need identify fragment displayed. tried inside control visibility overriding setuservisiblehint(boolean b) , write variable when true same happends. not guaranteed current fragment last writting. tried getting current fragment activity call fragment method , variable. difficult identify current fragment. tried this:

getsupportfragmentmanager()      .findfragmentbytag("android:switcher:"+r.id.myviewpager+":"+myviewpager.getcurrentitem()).getvariable(); 

but have null pointer exception.