我正在尝试使用RecyclerView实施CardView。但是，当我从云加载数据时，我需要使用notifyDataSetChanged()方法来更新视图，但这样做会导致我的自定义适配器类中出现NullPointerException。
以下是我在主要活动中的表现：
public class BigBoard extends ActionBarActivity {
private List<Person> persons;
private RecyclerView rv;
RVAdapter adapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Parse.initialize(this, "app-id", "client-key");
setContentView(R.layout.activity_big_board);
adapter = new RVAdapter(persons);
rv=(RecyclerView)findViewById(R.id.rv);
LinearLayoutManager llm = new LinearLayoutManager(this);
rv.setLayoutManager(llm);
rv.setHasFixedSize(true);
initializeData();
initializeAdapter();
}
private void initializeData(){
persons = new ArrayList<>();
ParseQuery<ParseObject> query = new ParseQuery<ParseObject>("Credentials");
query.findInBackground(new FindCallback<ParseObject>() {
public void done(List<ParseObject> credentialList, ParseException e) {
if (e == null) {
for(int i=0;i<credentialList.size();i++)
{
persons.add(new Person(credentialList.get(i).getString("Name"), credentialList.get(i).getString("SurName")));
Log.d("OUT", "So the Val::------> " + persons);
adapter.notifyDataSetChanged();// <---- Problem line
}
} else {
Log.d("score", "Error: " + e.getMessage());
}
}
});
}
private void initializeAdapter(){
rv.setAdapter(adapter);
}
以下是Adapter类的代码：
public class RVAdapter extends RecyclerView.Adapter<RVAdapter.PersonViewHolder> {
public static class PersonViewHolder extends RecyclerView.ViewHolder {
CardView cv;
TextView personName;
TextView personAge;
//ImageView personPhoto;
PersonViewHolder(View itemView) {
super(itemView);
cv = (CardView)itemView.findViewById(R.id.cv);
personName = (TextView)itemView.findViewById(R.id.person_name);
personAge = (TextView)itemView.findViewById(R.id.person_age);
//personPhoto = (ImageView)itemView.findViewById(R.id.person_photo);
}
}
List<Person> persons;
RVAdapter(List<Person> persons){
this.persons = persons;
}
@Override
public void onAttachedToRecyclerView(RecyclerView recyclerView) {
super.onAttachedToRecyclerView(recyclerView);
}
@Override
public PersonViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.item, viewGroup, false);
PersonViewHolder pvh = new PersonViewHolder(v);
return pvh;
}
@Override
public void onBindViewHolder(PersonViewHolder personViewHolder, int i) {
personViewHolder.personName.setText(persons.get(i).name);
personViewHolder.personAge.setText(persons.get(i).surname);
// personViewHolder.personPhoto.setImageResource(persons.get(i).photoId);
}
@Override
public int getItemCount() {
return persons.size();
}
我相信return persons.size();这是每次调用notifyDataSetChanged（）时导致NullPointerException的行，因为我猜这是空的或者是这样！ 问题是如何解决这个问题？
这里也是logcat：
> java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at obx.com.futurister.RVAdapter.getItemCount(RVAdapter.java:58)
at android.support.v7.widget.RecyclerView.onMeasure(RecyclerView.java:2357)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1112)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:632)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.support.v7.internal.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:124)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643)
at android.view.View.measure(View.java:18788)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2100)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1216)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1452)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
at android.view.Choreographer.doCallbacks(Choreographer.java:670)
at android.view.Choreographer.doFrame(Choreographer.java:606)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
答案 0 :(得分：2)
在调用任何方法之前，您需要初始化列表persons。
您只需更改以下
即可List<Person> persons;
到
List<Person> persons = new ArrayList<Person>();
OR
您可以将现有的getItemCount方法更新为以下
@Override
public int getItemCount() {
if(persons!=null){
return persons.size();
} else{
return 0;
}
}