我是Android新手。 我试图从列表中的SQlite检索所有位置，但我面临错误。 实际上，我想在列表中显示存储在数据库中的纬度，经度和缩放值。 救救我！
Logcat错误
10-10 00:52:35.257: E/AndroidRuntime(9868): FATAL EXCEPTION: main
10-10 00:52:35.257: E/AndroidRuntime(9868): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.haider.trymap/com.haider.trymap.SavedLocations}: java.lang.NullPointerException
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2309)
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.app.ActivityThread.access$700(ActivityThread.java:157)
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1289)
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.os.Handler.dispatchMessage(Handler.java:99)
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.os.Looper.loop(Looper.java:176)
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.app.ActivityThread.main(ActivityThread.java:5317)
10-10 00:52:35.257: E/AndroidRuntime(9868): at java.lang.reflect.Method.invokeNative(Native Method)
10-10 00:52:35.257: E/AndroidRuntime(9868): at java.lang.reflect.Method.invoke(Method.java:511)
10-10 00:52:35.257: E/AndroidRuntime(9868): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
10-10 00:52:35.257: E/AndroidRuntime(9868): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
10-10 00:52:35.257: E/AndroidRuntime(9868): at dalvik.system.NativeStart.main(Native Method)
10-10 00:52:35.257: E/AndroidRuntime(9868): Caused by: java.lang.NullPointerException
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:236)
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
10-10 00:52:35.257: E/AndroidRuntime(9868): at com.haider.trymap.LocationsDB.<init>(LocationsDB.java:40)
10-10 00:52:35.257: E/AndroidRuntime(9868): at com.haider.trymap.SavedLocations.<init>(SavedLocations.java:11)
10-10 00:52:35.257: E/AndroidRuntime(9868): at java.lang.Class.newInstanceImpl(Native Method)
10-10 00:52:35.257: E/AndroidRuntime(9868): at java.lang.Class.newInstance(Class.java:1319)
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.app.Instrumentation.newActivity(Instrumentation.java:1071)
10-10 00:52:35.257: E/AndroidRuntime(9868): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2171)
10-10 00:52:35.257: E/AndroidRuntime(9868): ... 11 more
LocationsDB.java
public class LocationsDB extends SQLiteOpenHelper{
/** Database name */
private static String DBNAME = "locationmarkersqlite";
/** Version number of the database */
private static int VERSION = 1;
/** Field 1 of the table locations, which is the primary key */
public static final String FIELD_ROW_ID = "_id";
/** Field 2 of the table locations, stores the latitude */
public static final String FIELD_LAT = "lat";
/** Field 3 of the table locations, stores the longitude*/
public static final String FIELD_LNG = "lng";
/** Field 4 of the table locations, stores the zoom level of map*/
public static final String FIELD_ZOOM = "zom";
/** A constant, stores the the table name */
private static final String DATABASE_TABLE = "locations";
/** An instance variable for SQLiteDatabase */
private SQLiteDatabase mDB;
/** Constructor */
public LocationsDB(Context context) {
super(context, DBNAME, null, VERSION);
this.mDB = getWritableDatabase();
}
/** This is a callback method, invoked when the method getReadableDatabase() / getWritableDatabase() is called
* provided the database does not exists
* */
@Override
public void onCreate(SQLiteDatabase db) {
String sql = "create table " + DATABASE_TABLE + " ( " +
FIELD_ROW_ID + " integer primary key autoincrement , " +
FIELD_LNG + " double , " +
FIELD_LAT + " double , " +
FIELD_ZOOM + " text " +
" ) ";
db.execSQL(sql);
}
/** Inserts a new location to the table locations */
public long insert(ContentValues contentValues){
long rowID = mDB.insert(DATABASE_TABLE, null, contentValues);
return rowID;
}
/** Deletes all locations from the table */
public int del(){
int cnt = mDB.delete(DATABASE_TABLE, null , null);
return cnt;
}
/** Returns all the locations from the table */
public Cursor getAllLocations(){
return mDB.query(DATABASE_TABLE, new String[] { FIELD_ROW_ID, FIELD_LAT , FIELD_LNG, FIELD_ZOOM } , null, null, null, null, null);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
public ArrayList<String> getAllLocationz() {
Cursor mcursor = mDB.rawQuery("SELECT contentValues FROM "
+ DATABASE_TABLE, null);
ArrayList<String> result = new ArrayList<String>();
if(mcursor!=null)
{
//move cursor to first result record
mcursor.moveToFirst();
do {
result.add(mcursor.getString(mcursor.getColumnIndex("contentValues")));
} while (mcursor.moveToNext());
}
return result;
}
}
SavedLocations.java
public class SavedLocations extends ListActivity{
LocationsDB lb;
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.saved_locations);
ArrayList<String> data = lb.getAllLocationz();
lb.close();
setListAdapter(new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_1,
data));
}