Android- Null Pointer Exception in Implementing Interfaces during Communicating with Other Fragments

Question

Been trying to implement Communicating with Other Fragments . There are errors during the selId.onIdSelected(pid.toString()) , and getting null pointer exception on this. I think I am not sure on what is the next thing to do. I am sure that when I click the current position has data but sending it to the other fragment is another thing.

  public  static class SectionFragment extends ListFragment {
     OnSelectedIdListener selId;

      public interface OnSelectedIdListener {
            public void onIdSelected(String position);
        }


        public void IdSelected(String id) {
            selId.onIdSelected(id);
        }

      public void onAttach(Activity activity) {
            super.onAttach(activity);

            try {
                 selId = (OnSelectedIdListener) activity;
            } catch (ClassCastException e) {
                Log.d("error!: ",
                        String.format(e.toString(), "helow"));
            }
      }  

      ....

            @Override
            public void onItemClick(AdapterView<?> parent, View view,
                    int position, long id) {

            if(pid!=null){
                   Log.d("if not null here", "pid? "+ pid.toString());
                   selId.onIdSelected(pid.toString());
               }
            else{Log.d("Null selected id", ""+position);}

              }
  } 

Fragment B

public class PlayerFragment extends ListFragment implements
                 SectionFragment.OnSelectedIdListener{

                @Override
                public void onIdSelected(String position) {
                    setSelectedId(position);
                }

}

Answer 1

1. It is good to follow naming conventions. The interface names begin with a capital letter and the method names begin with a small letter. So it's OnSelectedListener and idSelected respectively.
2. While using listener, you should always check whether the listener is not null .
3. You are implementing the interface in the Fragment whereas in your code, you are casting the Activity as the listener.