How to populate listview adapter from outside Activity in a TabActivity

I have the following problem.

I am in a tabActivity with 2 tabs. Every tab will launch the same activity, but there is a onTabChangeListener.
In this tabchange listener i need to set up the listview adapter that is different for each activity.
So i will have two different lists, and i want to change their adapter from TabActivity, from this onTabchangeListener, but i get the following error: system services not available to activities before oncreate()
The code for tabs is:

  host = getTabHost();
 
     host.addTab(host.newTabSpec(TAG_AVAILABLE).setIndicator("First Tab")
             .setContent(new Intent(this, MyActivity.class)));
     host.addTab(host.newTabSpec(TAG_DOWNLOAD).setIndicator("Second Tab ")
             .setContent(new Intent(this, MyActivity.class)));
     host.setOnTabChangedListener(new TabHost.OnTabChangeListener() {
         @Override
         public void onTabChanged(String s) {
             MyActivity myActivity = new MyActivity();
             int i = getTabHost().getCurrentTab();
             if (i == FIRST_TAB) {
                 setAdapterForMyActivity(firstAdapter);       
             }
             }
             if (i == SECOND_TAB) {
                  setAdapterForMyActivity(secondAdapter);        
             }
             }
         }
     });
 
 }
 

The main question is how to set adapter of an listView which is in an Activity from TabActivity.
Thank you very much.

---