我正在Android上编写一个应用程序，我正在使用片段和MySQL，JSON和PHP，但我无法使用数据检索ListView。
我的片段：
public class SecondFragment extends Fragment {
String url = "http://192.168.1.76/Ws_v3/Select.php";
ListView lv;
public SecondFragment() {
// Required empty public constructor
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
Toast.makeText(getContext(),":)",Toast.LENGTH_SHORT).show();
// Inflate the layout for this fragment
View myFragmentView = inflater.inflate(R.layout.activity_main, container, false);
lv = (ListView) myFragmentView.findViewById(R.id.lv);
final Downloader d = new Downloader(getActivity(), url,lv);
d.execute();
return inflater.inflate(R.layout.fragment_second, container, false);
}
}
下载我的数据的类：
public class Downloader extends AsyncTask<Void,Integer,String> {
Context c;
String address;
ListView lv;
ProgressDialog pd;
public Downloader(Context c, String address, ListView lv) {
this.c = c;
this.address = address;
this.lv = lv;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
pd=new ProgressDialog(c);
pd.setTitle("Fetch Data");
pd.setMessage("Fetching Data...Please wait");
pd.show();
}
@Override
protected String doInBackground(Void... params) {
String data=downloadData();
return data;
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
pd.dismiss();;
if(s != null)
{
Parser p=new Parser(c,s,lv);
p.execute();
}else
{
Toast.makeText(c,"Unable to download data",Toast.LENGTH_SHORT).show();
}
}
private String downloadData()
{
//connect and get a stream
InputStream is=null;
String line =null;
try {
URL url=new URL(address);
HttpURLConnection con= (HttpURLConnection) url.openConnection();
is=new BufferedInputStream(con.getInputStream());
BufferedReader br=new BufferedReader(new InputStreamReader(is));
StringBuffer sb=new StringBuffer();
if(br != null) {
while ((line=br.readLine()) != null) {
sb.append(line+"\n");
}
}else {
return null;
}
return sb.toString();
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}finally {
if(is != null)
{
try {
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
return null;
}
}
和我的解析器：
public class Parser extends AsyncTask<Void,Integer,Integer> {
Context c;
ListView lv;
String data;
ArrayList<String> players=new ArrayList<>();
ProgressDialog pd;
public Parser(Context c, String data, ListView lv) {
this.c = c;
this.data = data;
this.lv = lv;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
pd=new ProgressDialog(c);
pd.setTitle("Parser");
pd.setMessage("Parsing ....Please wait");
pd.show();
}
@Override
protected Integer doInBackground(Void... params) {
return this.parse();
}
@Override
protected void onPostExecute(Integer integer) {
super.onPostExecute(integer);
if(integer == 1)
{
//ADAPTER
ArrayAdapter<String> adapter=new ArrayAdapter<String>(c,android.R.layout.simple_list_item_1,players);
//ADAPT TO LISTVIEW
lv.setAdapter(adapter);//AndroidStudio says that this is the line with error
//LISTENET
lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Snackbar.make(view,players.get(position),Snackbar.LENGTH_SHORT).show();;
}
});
}else
{
Toast.makeText(c,"Unable to Parse",Toast.LENGTH_SHORT).show();
}
pd.dismiss();
}
//PARSE RECEIVED DATA
private int parse()
{
try
{
//ADD THAT DATA TO JSON ARRAY FIRST
JSONArray ja=new JSONArray(data);
//CREATE JO OBJ TO HOLD A SINGLE ITEM
JSONObject jo=null;
players.clear();
//LOOP THRU ARRAY
for(int i=0;i<ja.length();i++)
{
jo=ja.getJSONObject(i);
//RETRIOEVE NAME
String name=jo.getString("nombre");
//ADD IT TO OUR ARRAYLIST
players.add(name);
}
return 1;
} catch (JSONException e) {
e.printStackTrace();
}
return 0;
}
}
我的片段XML：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:fab="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<!-- Parte superior -->
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="50">
<ImageView
android:id="@+id/cabecera"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="30"
android:layout_marginBottom="28dp"
android:background="@drawable/material_design_17"/>
<de.hdodenhof.circleimageview.CircleImageView
android:id="@+id/fab"
android:layout_width="40dp"
android:layout_height="40dp"
android:layout_alignParentBottom="true"
android:layout_alignParentLeft="true"
android:layout_gravity="bottom|right"
android:src="@android:drawable/ic_menu_edit"
android:layout_marginLeft="16dp"
android:layout_marginBottom="8dp"/>
<TextView
android:id="@+id/titulo"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Titulo"
android:textAppearance="?android:attr/textAppearanceLarge"
android:layout_marginBottom="48dp"
android:layout_toRightOf="@+id/fab"
android:layout_alignParentBottom="true"
android:layout_marginLeft="16dp"
android:textColor="@android:color/white" />
</RelativeLayout>
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="70"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin">
<include layout="@layout/content_secondfragment" />
</RelativeLayout>
My Content of fragment:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
app:layout_behavior="@string/appbar_scrolling_view_behavior">
<TextView
android:padding="5dp"
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Players List!" />
<ListView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/lv"
android:layout_below="@+id/textView"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
</RelativeLayout>
好吧，我的错误是：
java.lang.NullPointerException：尝试调用虚方法'void android.widget.ListView.setAdapter（android.widget.ListAdapter）'上的一个 null对象引用 在 com.example.promindsoft.theprodigyeye.Parser.onPostExecute（Parser.java:65） 在 com.example.promindsoft.theprodigyeye.Parser.onPostExecute（Parser.java:24） 在android.os.AsyncTask.finish（AsyncTask.java:636） 在android.os.AsyncTask.access $ 500（AsyncTask.java:177） 在 android.os.AsyncTask $ InternalHandler.handleMessage（AsyncTask.java:653） 在android.os.Handler.dispatchMessage（Handler.java:102） 在android.os.Looper.loop（Looper.java:135） 在android.app.ActivityThread.main（ActivityThread.java:5910） at java.lang.reflect.Method.invoke（Native Method） 在java.lang.reflect.Method.invoke（Method.java:372） 在 com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:1405） 在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:1200）
请帮助我，这是一个在学院进行最终测试的项目。