我编写了此程序，该程序在recyclerview中显示了表格数据，但是出现以下运行时错误，但我无法修复它， 我已经检查了是否可以通过从方法返回非null ArrayList来解决问题，但是仍然会出现相同的错误，
java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.lifecycle.LiveData.observeForever(androidx.lifecycle.Observer)' on a null object reference
这是我的Recyclerview片段，其中包含有问题的代码，如所示
public class RcyFrag extends Fragment {
Button b1,b2;
EditText nem1,werk1;
PersonDatabase pdd;
APPmodel apd;
File jap;
Observer<ArrayList<Datap>> ppl;
RcyAdap dddp;
DatabaseService ddbs;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
jap=new File(getActivity().getExternalFilesDir(null),"Data");
apd= ViewModelProviders.of(getActivity()).get(APPmodel.class);
pdd= Room.databaseBuilder(getActivity().getApplicationContext(),
PersonDatabase.class, jap.toString()).build();
ddbs=new DatabaseService("Nameei",pdd);
apd.getPersonList(pdd);
return inflater.inflate( R.layout.rcyfragm, container, false);
}
@Override
public void onStart()
{
super.onStart();
Person ppk=new Person();
Intent it=new Intent(getActivity(), DatabaseService.class);
it.putExtra("Name","Aditya");
it.putExtra("Work","Engineer");
it.putExtra("ID","add");
getActivity().startService(it);
b2=(Button) getView().findViewById(R.id.buttonadd);
final RecyclerView recw = (RecyclerView) getView().findViewById(R.id.recw);
ppl=new Observer<ArrayList<Datap>>() {
@Override
public void onChanged(ArrayList<Datap> dataps) {
dddp= new RcyAdap(dataps);
recw.setLayoutManager(new LinearLayoutManager(getActivity()));
recw.setAdapter(dddp);
Toast.makeText(getActivity(),"Change Happened",Toast.LENGTH_LONG).show();
}
};
?? apd.getPersonList().observe( this, ppl);// Problematic code here <-<-<-<-<-<-<-<-<-<-<-<-<-<-<-<-
nem1 = (EditText) getView().findViewById(R.id.nem1);
werk1 = (EditText) getView().findViewById(R.id.werk1);
b1 = (Button) getView().findViewById(R.id.buttonad);
b1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent it=new Intent(getActivity(), DatabaseService.class);
it.putExtra("Name",nem1.getText().toString());
it.putExtra("Work",werk1.getText().toString());
it.putExtra("ID","add");
getActivity().startService(it);
}
});
recw.addOnItemTouchListener(new CustomRVItemTouchListener(getActivity(), recw, new RecyclerViewItemClickListener() {
@Override
public void onClick(View view, int position) {
}
@Override
public void onLongClick(View view, int position) {
dddp.remelem(position);
Intent it=new Intent(getActivity(),DatabaseService.class);
it.putExtra("Name",nem1.getText().toString());
it.putExtra("Work",werk1.getText().toString());
it.putExtra("ID","delete");
getActivity().startService(it);
Toast.makeText(getActivity(),"Element deletd",Toast.LENGTH_LONG).show();
}
}));
b2.setOnClickListener(new View.OnClickListener() {
@RequiresApi(api = Build.VERSION_CODES.M)
@Override
public void onClick(View v) {
File aha= new File(getActivity().getExternalFilesDir(null),"Here.txt");
try {
aha.createNewFile();
} catch (IOException e) {
e.printStackTrace();
}
nem1.setText(aha.toString());
}
});
}
}
这是我的名为APPmodel的View模型
public class APPmodel extends ViewModel {
private LiveData<List<Person>> PersonList;
private LiveData<ArrayList<Datap>> DatapList= Transformations.map(PersonList, persons -> {
ArrayList<Datap> pp = null;
for(int i=0; i<persons.size();i++)
{
pp.add(new Datap(persons.get(i).name, persons.get(i).work));
}
return pp;
});
public LiveData<ArrayList<Datap>> getPersonList(PersonDatabase pd)
{
if(PersonList==null)
{
PersonList = pd.personDao().getAll();
}
return DatapList;
}
public LiveData<ArrayList<Datap>> getPersonList() {
return DatapList;
}
}
有人可以帮我吗？
