嗨，我是新的蜜蜂到android。 在执行我的应用程序时，我收到了nullpoint异常错误，不知道它为什么会发生。请任何人帮我摆脱这个问题。提前谢谢。
这是我的logcat：
09-14 12:30:14.027: INFO/System.out(365): Constructor StudentList Adapter...
09-14 12:30:14.027: DEBUG/AndroidRuntime(365): Shutting down VM
09-14 12:30:14.027: WARN/dalvikvm(365): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): FATAL EXCEPTION: main
09-14 12:30:14.047: ERROR/AndroidRuntime(365): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.UserLogin/com.UserLogin.FindPlaces}: java.lang.NullPointerException
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.app.ActivityThread.startActivityNow(ActivityThread.java:2503)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:127)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:339)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:651)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.widget.TabHost.setCurrentTab(TabHost.java:323)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.widget.TabHost$2.onTabSelectionChanged(TabHost.java:129)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.widget.TabWidget$TabClickListener.onClick(TabWidget.java:453)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.view.View.performClick(View.java:2408)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.view.View$PerformClick.run(View.java:8816)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.os.Handler.handleCallback(Handler.java:587)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.os.Handler.dispatchMessage(Handler.java:92)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.os.Looper.loop(Looper.java:123)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.app.ActivityThread.main(ActivityThread.java:4627)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at java.lang.reflect.Method.invokeNative(Native Method)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at java.lang.reflect.Method.invoke(Method.java:521)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at dalvik.system.NativeStart.main(Native Method)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): Caused by: java.lang.NullPointerException
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at com.UserLogin.FindPlaces.displayLocs(FindPlaces.java:35)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at com.UserLogin.FindPlaces.onCreate(FindPlaces.java:30)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
09-14 12:30:14.047: ERROR/AndroidRuntime(365): ... 18 more
09-14 12:30:14.057: WARN/ActivityManager(58): Force finishing activity com.UserLogin/.Innerpage
09-14 12:30:14.594: WARN/ActivityManager(58): Activity pause timeout for HistoryRecord{4506eef0 com.UserLogin/.Innerpage}
09-14 12:30:16.347: INFO/Process(365): Sending signal. PID: 365 SIG: 9
09-14 12:30:16.367: INFO/ActivityManager(58): Process com.UserLogin (pid 365) has died.
09-14 12:30:16.367: INFO/WindowManager(58): WIN DEATH: Window{4506a6e0 com.UserLogin/com.UserLogin.Innerpage paused=false}
09-14 12:30:16.367: INFO/WindowManager(58): WIN DEATH: Window{451492d0 com.UserLogin/com.UserLogin.UserLogin paused=false}
09-14 12:30:16.397: INFO/UsageStats(58): Unexpected resume of com.android.launcher while already resumed in com.UserLogin
Java文件：
import java.util.ArrayList;
import android.app.ListActivity;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
public class FindPlaces extends ListActivity{
private SQLiteDatabase DbLoc;
ListView lv;
private ArrayList<Fields> results = new ArrayList<Fields>();
@Override
public void onCreate(Bundle savedInstance) {
super.onCreate(savedInstance);
//setContentView(R.layout.places);
getallLocs();
displayLocs();
}
private void displayLocs() {
lv = (ListView)findViewById(R.id.listPlaces);
lv.setAdapter(new StudentListAdapter(this, R.layout.placeslist, results));
//lv.setAdapter(new ArrayAdapter<Fields>(this,android.R.layout.simple_list_item_1, results));
}
class StudentListAdapter extends ArrayAdapter<Fields>{
private ArrayList<Fields> locationDetails;
private Context mContext;
public StudentListAdapter(Context context,int textViewResourceId, ArrayList<Fields> results) {
super(context, textViewResourceId, results);
// TODO Auto-generated constructor stub
System.out.println("Constructor StudentList Adapter...");
this.locationDetails = results;
mContext = context;
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return results.size();
}
@Override
public Fields getItem(int position) {
// TODO Auto-generated method stub
return locationDetails.get(position);
}
@Override
public long getItemId(int position) {
// TODO Auto-generated method stub
return super.getItemId(position);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
// TODO Auto-generated method stub
View v = convertView;
if(v == null){
LayoutInflater vl = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
v = vl.inflate(R.layout.placeslist, null);
}
Fields o = results.get(position);
if (o != null) {
TextView iv = (TextView)v.findViewById(R.id.toptext);
TextView tv_sNo = (TextView)v.findViewById(R.id.toptext1);
iv.setText(o.getLocationName());
tv_sNo.setText(o.getLocationImage());
}
DbLoc.close();
return v;
}
}
static class ViewHolder
{
TextView Locationname;
ImageView Locationimage;
}
private void getallLocs() {
// TODO Auto-generated method stub
try {
DatabaseHelper dbHelper = new DatabaseHelper(
this.getApplicationContext());
DbLoc = dbHelper.getWritableDatabase();
Cursor c = DbLoc.rawQuery("SELECT " + DatabaseHelper.LocationName+ " , " + DatabaseHelper.LocationImage + " FROM "
+ DatabaseHelper.LOCATIONTABLE , null);
if (c != null) {
if (c.moveToFirst()) {
do {
String LocationName= c.getString(c.getColumnIndex("LocationName"));
String Mobile = c.getString(c
.getColumnIndex("LocationImage"));
Fields p = new Fields(LocationName, Mobile);
results.add(p);
} while (c.moveToNext());
}
}
} catch (SQLiteException se) {
Log.e(getClass().getSimpleName(),
"Could not create or Open the database");
}
finally { if (DbLoc != null) DbLoc.execSQL("DELETE FROM " +
DatabaseHelper.FRIENDTABLE); DbLoc.execSQL("DELETE FROM " +
DatabaseHelper.LOCATIONTABLE);
DbLoc.execSQL("DROP TABLE IF EXISTS tablename"+ DatabaseHelper.FRIENDTABLE);
DbLoc.execSQL("DROP TABLE IF EXISTS tablename"+ DatabaseHelper.LOCATIONTABLE);
DbLoc.close();
}
}
}
Places.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="fill_parent"
android:layout_height="fill_parent" android:padding="6dip">
<ListView android:id="@+id/listPlaces" android:layout_width="fill_parent"
android:layout_height="wrap_content" />
</LinearLayout>
Placeslist.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="fill_parent"
android:layout_height="wrap_content">
<TextView android:id="@+id/toptext" android:layout_width="fill_parent"
android:layout_height="0dip" android:layout_weight="1"
android:gravity="center_vertical" />
<TextView android:id="@+id/toptext1" android:layout_width="fill_parent"
android:layout_height="0dip" android:layout_weight="1"
android:gravity="center_vertical" />
</LinearLayout>
答案 0 :(得分：4)
取消注释此行
setContentView(R.layout.places);
<强> EDITED ：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="fill_parent"
android:layout_height="fill_parent" android:padding="6dip">
<ListView android:id="@android:id/list" android:layout_width="fill_parent"
android:layout_height="wrap_content" />
</LinearLayout>
将其替换为您的文件（Places.xml）
<强>已更新
public void onCreate(Bundle savedInstance) {
super.onCreate(savedInstance);
//setContentView(R.layout.places);
getallLocs();
setListAdapter(new ArrayAdapter<Fields>(this,android.R.layout.simple_list_item_1, results));
// displayLocs();
}
private void displayLocs() {
lv = (ListView)findViewById(R.id.listPlaces);
lv.setAdapter(new StudentListAdapter(this, R.layout.placeslist, results));
//lv.setAdapter(new ArrayAdapter<Fields>(this,android.R.layout.simple_list_item_1, results));
}
答案 1 :(得分：4)
如果您尚未为活动设置内容视图，则无法使用findViewById()。因此，您的lv将为空。
您正在使用ListActivity。如果您要为其添加自己的样式，则布局中的ListView需要标识android:list：
<ListView android:id="@android:id/list"
...
/>
如图所示here。
如果您使用ListActivity，则不使用findViewById()来获取ListView，您可以使用getListView()-method。