My App shoult load entries from a Sql Database in a list, but when i execute it i just get a NullPointerException:
我的应用程序应该从列表中的Sql数据库加载条目,但是当我执行它时,我只得到一个NullPointerException:
01-16 18:35:30.239: E/AndroidRuntime(2254): FATAL EXCEPTION: main
01-16 18:35:30.239: E/AndroidRuntime(2254): Process: com.mobilecartography.tudresdenspeechorganiser, PID: 2254
01-16 18:35:30.239: E/AndroidRuntime(2254): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mobilecartography.tudresdenspeechorganiser/com.mobilecartography.tudresdenspeechorganiser.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.mobilecartography.tudresdenspeechorganiser.DatabaseHelper.createDataBase()' on a null object reference
01-16 18:35:30.239: E/AndroidRuntime(2254): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
01-16 18:35:30.239: E/AndroidRuntime(2254): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
01-16 18:35:30.239: E/AndroidRuntime(2254): at android.app.ActivityThread.access$800(ActivityThread.java:144)
01-16 18:35:30.239: E/AndroidRuntime(2254): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
01-16 18:35:30.239: E/AndroidRuntime(2254): at android.os.Handler.dispatchMessage(Handler.java:102)
01-16 18:35:30.239: E/AndroidRuntime(2254): at android.os.Looper.loop(Looper.java:135)
01-16 18:35:30.239: E/AndroidRuntime(2254): at android.app.ActivityThread.main(ActivityThread.java:5221)
01-16 18:35:30.239: E/AndroidRuntime(2254): at java.lang.reflect.Method.invoke(Native Method)
01-16 18:35:30.239: E/AndroidRuntime(2254): at java.lang.reflect.Method.invoke(Method.java:372)
01-16 18:35:30.239: E/AndroidRuntime(2254): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
01-16 18:35:30.239: E/AndroidRuntime(2254): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
01-16 18:35:30.239: E/AndroidRuntime(2254): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.mobilecartography.tudresdenspeechorganiser.DatabaseHelper.createDataBase()' on a null object reference
01-16 18:35:30.239: E/AndroidRuntime(2254): at com.mobilecartography.tudresdenspeechorganiser.CurrentDay.loadPresentations(CurrentDay.java:39)
01-16 18:35:30.239: E/AndroidRuntime(2254): at com.mobilecartography.tudresdenspeechorganiser.MainActivity.setCurrentWeek(MainActivity.java:70)
01-16 18:35:30.239: E/AndroidRuntime(2254): at com.mobilecartography.tudresdenspeechorganiser.MainActivity.onCreate(MainActivity.java:32)
01-16 18:35:30.239: E/AndroidRuntime(2254): at android.app.Activity.performCreate(Activity.java:5933)
01-16 18:35:30.239: E/AndroidRuntime(2254): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
01-16 18:35:30.239: E/AndroidRuntime(2254): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
01-16 18:35:30.239: E/AndroidRuntime(2254): ... 10 more
So the logcat and the Debugger tells me that it throws the exception when using the method createDatabase() from my dbHelper class. This method is called by loadPresentations() in CurrentDay (although my db file currently just contains dummy data). This helper class works fine on another app with the same .db file, so i´d have thought that the error is somewhere else.
因此,logcat和Debugger告诉我,当我使用dbHelper类中的createDatabase()方法时,它会引发异常。此方法由CurrentDay中的loadPresentations()调用(尽管我的db文件当前只包含虚拟数据)。这个助手类在具有相同.db文件的另一个应用程序上工作正常,所以我原以为错误在其他地方。
Anyway here is my dbHelper Class (the .db file is stored in the project under assets/databases/):
无论如何这里是我的dbHelper类(.db文件存储在assets / databases /下的项目中):
package com.mobilecartography.tudresdenspeechorganiser;
import android.content.Context;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import com.readystatesoftware.sqliteasset.SQLiteAssetHelper;
public class DatabaseHelper extends SQLiteAssetHelper {
private static String DB_PATH;
private static String DB_PATH_PREFIX = "/data/data/";
private static String DB_PATH_SUFFIX = "/databases/";
private static String DB_NAME = "exercise_database.db";
private SQLiteDatabase myDataBase;
private final Context myContext;
public DatabaseHelper(Context context) {
super(context, DB_NAME, null, 1);
this.myContext = context;
}
public void createDataBase() throws IOException {
DB_PATH = DB_PATH_PREFIX + myContext.getPackageName() + DB_PATH_SUFFIX
+ DB_NAME;
boolean dbExist = checkDataBase();
SQLiteDatabase db_Read = null;
if (dbExist) {
} else {
db_Read = this.getReadableDatabase();
db_Read.close();
}
}
private boolean checkDataBase() {
SQLiteDatabase checkDB = null;
try {
checkDB = SQLiteDatabase.openDatabase(DB_PATH, null,
SQLiteDatabase.NO_LOCALIZED_COLLATORS);
checkDB.close();
return true;
} catch (SQLiteException e) {
return false;
}
}
private void copyDataBase() throws IOException {
InputStream assetsDB = myContext.getAssets().open(DB_NAME);
File directory = new File(DB_PATH);
if (directory.exists() == false) {
directory.mkdir();
}
OutputStream dbOut = new FileOutputStream(DB_PATH);
byte[] buffer = new byte[1024];
int length;
while ((length = assetsDB.read(buffer)) > 0) {
dbOut.write(buffer, 0, length);
}
dbOut.flush();
dbOut.close();
}
public void openDataBase() throws SQLException {
myDataBase = SQLiteDatabase.openDatabase(DB_PATH, null,
SQLiteDatabase.NO_LOCALIZED_COLLATORS);
}
public SQLiteDatabase getDataBase() throws SQLException {
myDataBase = SQLiteDatabase.openDatabase(DB_PATH, null,
SQLiteDatabase.NO_LOCALIZED_COLLATORS);
return myDataBase;
}
@Override
public synchronized void close() {
if (myDataBase != null) {
myDataBase.close();
}
super.close();
}
//@Override
//public void onCreate(SQLiteDatabase db) {
//}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}
Main activity:
package com.mobilecartography.tudresdenspeechorganiser;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.List;
import android.app.Activity;
import android.app.ListActivity;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.widget.ExpandableListView;
import com.mobilecartography.tudresdenspeechorganiser.R;
import com.mobilecartography.tudresdenspeechorganiser.DatabaseHelper;
public class MainActivity extends ListActivity {
ArrayList<CurrentDay> currentweek = new ArrayList<CurrentDay>();
SQLiteDatabase database = null;
Cursor dbCursor;
DatabaseHelper dbHelper;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
this.dbHelper = new DatabaseHelper(this);
setCurrentWeek();
ExpandableListView listView = (ExpandableListView) findViewById(R.id.listView);
MyExpandableListAdapter adapter = new MyExpandableListAdapter(this,
currentweek);
listView.setAdapter(adapter);
}
//fill list with 7 entrys with dates for the next 7 days, for
//each days query presentations for that specific day from sql database
public void setCurrentWeek() {
Calendar cal = new GregorianCalendar();
CurrentDay day = null;
int numDays = cal.getActualMaximum(Calendar.DAY_OF_MONTH);
int today = cal.get(Calendar.DAY_OF_MONTH);
int currentmonth = cal.get(Calendar.MONTH)+1;
int currentyear = cal.get(Calendar.YEAR);
for (int i = 0; i < 7; i++){
if (today+i > numDays){
for (int h = 0; h < 7 - i; h++){
day = new CurrentDay(1+h, currentmonth+1, currentyear);
}
}
else {
day = new CurrentDay(today+i, currentmonth, currentyear);
}
day.loadPresentations();
currentweek.add(day);
}
}
}
Day Object class:
日对象类:
package com.mobilecartography.tudresdenspeechorganiser;
import java.io.IOException;
import java.sql.Date;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.List;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.text.format.DateFormat;
import android.widget.ArrayAdapter;
import com.mobilecartography.tudresdenspeechorganiser.DatabaseHelper;
public class CurrentDay {
public int day;
public int month;
public int year;
DatabaseHelper dbHelper;
public List<String> children;
SQLiteDatabase database = null;
Cursor dbCursor;
public CurrentDay(int day, int month, int year) {
this.day = day;
this.month = month;
this.year = year;
}
public void loadPresentations() {
try {
dbHelper.createDataBase();
} catch (IOException ioe) {
}
try {
children = new ArrayList<String>();java.sql.Date(parsedUtilDate.getTime());
database = dbHelper.getDataBase();
dbCursor = database.rawQuery("SELECT university FROM pres_table",
null);
dbCursor.moveToFirst();
int index = dbCursor.getColumnIndex("title");
while (!dbCursor.isAfterLast()) {
String record = dbCursor.getString(index);
children.add(record);
dbCursor.moveToNext();
}
}
finally {
if (database != null) {
dbHelper.close();
}
}
}
}
I´ve been trying to get behind that problem all day. Does anybody see a solution?
我一直在努力克服这个问题。有人看到了解决方案吗?
3 个解决方案
#1
0
You have 2 dbHelper objects. 1 in MainActivity which is set properly. And another 1 in CurrentDay which throws your error.
你有2个dbHelper对象。 MainActivity中的1是正确设置的。还有一个在CurrentDay中抛出你的错误。
Try this:
day.dbHelper = dbHelper;
day.loadPresentations();
#2
0
You never initialize dbHelper in CurrentDay. So dbHelper.createDataBase(); cases the NPE.
您永远不会在CurrentDay中初始化dbHelper。所以dbHelper.createDataBase();案例NPE。
#3
0
DatabaseHelper dbHelper;
you never initialize it, like this
你永远不会像这样初始化它
DatabaseHelper dbHelper = new DatabaseHelper(context);
Or, I think, you can make createDataBase() and checkDataBase() static methods
或者,我认为,您可以创建createDataBase()和checkDataBase()静态方法
#1
0
You have 2 dbHelper objects. 1 in MainActivity which is set properly. And another 1 in CurrentDay which throws your error.
你有2个dbHelper对象。 MainActivity中的1是正确设置的。还有一个在CurrentDay中抛出你的错误。
Try this:
day.dbHelper = dbHelper;
day.loadPresentations();
#2
0
You never initialize dbHelper in CurrentDay. So dbHelper.createDataBase(); cases the NPE.
您永远不会在CurrentDay中初始化dbHelper。所以dbHelper.createDataBase();案例NPE。
#3
0
DatabaseHelper dbHelper;
you never initialize it, like this
你永远不会像这样初始化它
DatabaseHelper dbHelper = new DatabaseHelper(context);
Or, I think, you can make createDataBase() and checkDataBase() static methods
或者,我认为,您可以创建createDataBase()和checkDataBase()静态方法