我想创建一个从db中获取Mysql数据的android应用程序，并将其显示在列表中。我得到的问题是当应用程序运行时它显示不幸它停止。 Logcat显示问题出在这一行
list.setAdapter(adapter);
无论在哪里，我都尽力搜索，但它不起作用。由于我是Android开发的初学者，请帮助我
Logcat：
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at com.example.abdul.zx.MainActivity.showList(MainActivity.java:81)
at com.example.abdul.zx.MainActivity$1GetDataJSON.onPostExecute(MainActivity.java:129)
at com.example.abdul.zx.MainActivity$1GetDataJSON.onPostExecute(MainActivity.java:90)
at android.os.AsyncTask.finish(AsyncTask.java:632)
at android.os.AsyncTask.access$600(AsyncTask.java:177)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
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:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
主要Activity.java
public class MainActivity extends ActionBarActivity {
String myJSON;
private static final String TAG_RESULTS="result";
private static final String TAG_ID = "id";
private static final String TAG_NAME = "name";
private static final String TAG_ADD ="address";
JSONArray peoples = null;
ArrayList <HashMap <String, String> > personList;
ListView list;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
list = (ListView) findViewById(R.id.listView);
personList = new ArrayList<HashMap<String,String>>();
getData();
}
protected void showList(){
try {
JSONObject jsonObj = new JSONObject(myJSON);
peoples = jsonObj.getJSONArray(TAG_RESULTS);
for(int i=0;i<peoples.length();i++){
JSONObject c = peoples.getJSONObject(i);
String id = c.getString(TAG_ID);
String name = c.getString(TAG_NAME);
String address = c.getString(TAG_ADD);
HashMap<String,String> persons = new HashMap<String,String>();
persons.put(TAG_ID,id);
persons.put(TAG_NAME,name);
persons.put(TAG_ADD,address);
personList.add(persons);
}
ListAdapter adapter = new SimpleAdapter
(
MainActivity.this, personList, R.layout.list_item,
new String[]{TAG_ID,TAG_NAME,TAG_ADD},
new int[]{R.id.id, R.id.name, R.id.address}
);
list.setAdapter(adapter);
} catch (JSONException e) {
e.printStackTrace();
}
}
public void getData(){
class GetDataJSON extends AsyncTask<String, Void, String>{
@Override
protected String doInBackground(String... params) {
DefaultHttpClient httpclient = new DefaultHttpClient(new BasicHttpParams());
HttpPost httppost = new HttpPost("http://10.0.2.2/in.php");
// Depends on your web service
httppost.setHeader("Content-type", "application/json");
InputStream inputStream = null;
String result = null;
try {
HttpResponse response = httpclient.execute(httppost);
HttpEntity entity = response.getEntity();
inputStream = entity.getContent();
// json is UTF-8 by default
BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"), 8);
StringBuilder sb = new StringBuilder();
String line = null;
while ((line = reader.readLine()) != null)
{
sb.append(line + "\n");
}
result = sb.toString();
} catch (Exception e) {
// Oops
}
finally {
try{if(inputStream != null)inputStream.close();}catch(Exception squish){}
}
return result;
}
@Override
protected void onPostExecute(String result){
myJSON=result;
showList();
}
}
GetDataJSON g = new GetDataJSON();
g.execute();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
activity_main.xml中
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:padding="10dp"
android:paddingLeft="10dp"
android:paddingRight="10dp" >
<TextView
android:id="@+id/id"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:paddingBottom="2dip"
android:paddingTop="6dip"
android:textStyle="bold" />
<TextView
android:id="@+id/name"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:paddingBottom="2dip"
android:textStyle="bold"/>
<TextView
android:id="@+id/address"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="left"
android:textStyle="bold" />
list_item.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">
<ListView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/listView" />
</LinearLayout>