出于某种原因，当它确实不为空时，我得到指向 videosFound 的空指针异常。即使传入的参数 adapter 也不是null。我只是通过打印出来检查并验证这是否为null（两者都不为空）。那么有人可以告诉我我是如何得到这个错误的吗？
private ListView videosFound;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_search);
searchInput = (EditText)findViewById(R.id.search_input);
videosFound = (ListView)findViewById(R.id.videos_found);
handler = new Handler();
addClickListener();
searchInput.setOnEditorActionListener(new TextView.OnEditorActionListener() {
@Override
public boolean onEditorAction(TextView v, int actionId, KeyEvent event) {
if (actionId == EditorInfo.IME_ACTION_DONE) {
searchOnYoutube(v.getText().toString());
return false;
}
return true;
}
});
}
private void updateVideosFound(){
ArrayAdapter<VideoItem> adapter = new ArrayAdapter<VideoItem>(getApplicationContext(), R.layout.video_item, searchResults){
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if(convertView == null){
convertView = getLayoutInflater().inflate(R.layout.video_item, parent, false);
}
ImageView thumbnail = (ImageView)convertView.findViewById(R.id.video_thumbnail);
TextView title = (TextView)convertView.findViewById(R.id.video_title);
TextView description = (TextView)convertView.findViewById(R.id.video_description);
VideoItem searchResult = searchResults.get(position);
Picasso.with(getApplicationContext()).load(searchResult.getThumbnailURL()).into(thumbnail);
title.setText(searchResult.getTitle());
description.setText(searchResult.getDescription());
return convertView;
}
};
if(videosFound == null){
System.out.println("videoFound is null***********");
}else{
System.out.println("videoFound is NOT null***********");
}
if(adapter == null){
System.out.println("adapter is null***********");
}else{
System.out.println("adapter is NOT null***********");
}
videosFound.setAdapter(adapter);//ERROR HERE. neither videosFound no adapter are null
}
private void addClickListener(){
videosFound.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> av, View v, int pos,
long id) {
Intent intent = new Intent(getApplicationContext(), PlayerActivity.class);
intent.putExtra("VIDEO_ID", searchResults.get(pos).getId());
startActivity(intent);
}
});
}
XML
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="@string/search"
android:id="@+id/search_input"
android:singleLine="true" />
<ListView
android:id="@+id/videos_found"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:dividerHeight="5dp" />
</LinearLayout>
控制台：
03-24 18:04:31.833 17014-17440/com.example.shawn.myyoutube D/YC: Could not search: com.google.api.client.googleapis.json.GoogleJsonResponseException: 403 Forbidden
{
"code" : 403,
"errors" : [ {
"domain" : "usageLimits",
"message" : "There is a per-IP or per-Referer restriction configured on your API key and the request does not match these restrictions. Please use the Google Developers Console to update your API key configuration if request from this IP or referer should be allowed.",
"reason" : "ipRefererBlocked",
"extendedHelp" : "https://console.developers.google.com"
} ],
"message" : "There is a per-IP or per-Referer restriction configured on your API key and the request does not match these restrictions. Please use the Google Developers Console to update your API key configuration if request from this IP or referer should be allowed."
}
03-24 18:04:31.833 17014-17014/com.example.shawn.myyoutube I/System.out: videoFound is NOT null***********
03-24 18:04:31.833 17014-17014/com.example.shawn.myyoutube I/System.out: adapter is NOT null***********
03-24 18:04:31.843 17014-17014/com.example.shawn.myyoutube D/AndroidRuntime: Shutting down VM
03-24 18:04:31.843 17014-17014/com.example.shawn.myyoutube E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.shawn.myyoutube, PID: 17014
java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:330)
at android.widget.ListView.setAdapter(ListView.java:502)
at com.example.shawn.myyoutube.SearchActivity.updateVideosFound(SearchActivity.java:98)
at com.example.shawn.myyoutube.SearchActivity.access$200(SearchActivity.java:23)
at com.example.shawn.myyoutube.SearchActivity$2$1.run(SearchActivity.java:61)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:5951)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
答案 0 :(得分：4)
在堆栈跟踪中我看到了：
java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
适配器正在尝试在空size()上调用List。确保您的列表searchResults不为空。
答案 1 :(得分：2)
NullPointerException因为searchResults未初始化且为null。
关于你在评论中提出的问题 -
每当您致电你知道为什么错误实际上并没有指向那里的行 searchResults是？
ArrayAdapter或使用getCount()刷新已创建的适配器时， setAdapter()会在内部调用notifyDataSetChanged()。
在您致电setAdapter()或notifyDataSetChanged()之前，ArrayAdapter不会为您创建视图，为了创建该视图，它需要检查您传递给它的数据的大小。
为此，它调用getCount()
public int getCount() {
return mObjects.size();
}
其中mObjects初始化为您在创建适配器时传递的数据列表。
由于mObjects.size()为NullPointerException，显然mObjects会抛出null，因为searchResults为空。