//
public class main_Activity extends Activity{
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Test t = new Test(this);
    t.setText();
  }
  public putText(String s){
    TextView.setText(s);
  }
}
//
public class Test {
  main_Activity a;
  Test(main_Activity a) {
    this.a = a;
  }
  public setText(){
    a.putText("ABC");
  }
}