java - ArrayAdapter.clear() 上的 NPE

Question

在初始化之后，我在 if(adapter.getCount() != 0)adapter.clear(); 行得到了一个 NPE。我的代码哪里错了？我认为因为我刚刚初始化了它，所以不应该有 NPE，但我错了..这里我放了3个与适配器相关的方法:

    package com.example.app.Fragment;
public class FragmentGestioneCorsa extends Fragment implements AdapterView.OnItemSelectedListener{

@SuppressWarnings("unused")
private static final String TAG = FragmentGestioneCorsa.class.getSimpleName();

private Context context;
private DatabaseLocale db;
private ArrayAdapter<String> adapter;
private  Spinner spnLinee;
private Button aggiornamento;


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    linee = new ArrayList<HashMap<String, String>>();
    context = getActivity();
    return inflater.inflate(R.layout.fragment_gestione_corsa, container, false);
}

@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);

    spnLinee = (Spinner) getView().findViewById(R.id.spinnerLinee);
    ArrayAdapter<String> adapter =new ArrayAdapter<String>(context, android.R.layout.simple_spinner_item);
    spnLinee.setAdapter(adapter);
    spnLinee.setOnItemSelectedListener(this);
    popolamentoSpinner();

    aggiornamento = (Button) getView().findViewById(R.id.buttonAggiornamento);
    aggiornamento.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            // Se scrivo DownloadLinee.execute() ottengo l'errore:
            // If I write DownloadLinee.execute() I get the error:
            // Non-static method cannot be referenced from a static contecxt
            DownloadLinee l = new DownloadLinee();
            l.execute();
        }
    });
}

private void popolamentoSpinner(){
    if(adapter.getCount() != 0) adapter.clear();
    db = new DatabaseLocale(context);
    SQLiteDatabase dbLeggibile = db.getReadableDatabase();
    String[] colonne = {DatabaseLocale.getTagCodiceLinea(), DatabaseLocale.getTagNomeLinea()};
    String tabella = DatabaseLocale.getTableName();
    Cursor cursore = dbLeggibile.query(tabella, colonne, null, null, null, null, null);
    while(cursore.moveToNext()){
        adapter.add(cursore.getString(0) + " " + cursore.getString(1));
    }
    cursore.close();
    db.close();
}
}

为什么是这个 NPE？提前致谢

Answer 1

您声明了两次ArrayAdapter<String> adapter ，一旦进入onViewCreated方法，并作为类的成员一次。

当你在 onViewCreated 中初始化它时，事实上它正在初始化一个局部变量，该变量在 popolamentoSpinner 中不可见。 。基本上，这两个函数中的变量不是相同的，因此实际上您永远不会初始化名为 adapter 的变量。您在 popolamentoSpinner 中使用的!

一种解决方案是删除 onViewCreated 中的声明所以你确定你初始化了成员变量:

@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);

    spnLinee = (Spinner) getView().findViewById(R.id.spinnerLinee);
    adapter =new ArrayAdapter<String>(context, android.R.layout.simple_spinner_item);
    spnLinee.setAdapter(adapter);
    ...