Question

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

因此，logcat和Debugger告诉我在使用dbHelper类中的createDatabase（）方法时会抛出异常。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）。这个帮助器类在具有相同.db文件的另一个应用程序上工作正常，所以我认为该错误在其他地方。

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) {
    }
}

主要活动：

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); 
      }
 }
}

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();
            }


        }
    }
}

我一直在试图全面支持这个问题。有人看到了解决方案吗？

Answer 1

您永远不会在CurrentDay中初始化dbHelper。所以dbHelper.createDataBase();就是NPE。

Answer 2

您有2个dbHelper对象。 MainActivity中的1，设置正确。而CurrentDay中的另外一个会引发您的错误。

试试这个：

day.dbHelper = dbHelper;
day.loadPresentations();

Answer 3

DatabaseHelper dbHelper;

你从不初始化它，就像这个

DatabaseHelper dbHelper = new DatabaseHelper(context);

或者，我认为，你可以创建createDataBase（）和checkDataBase（）静态方法

Answer 4

初始化dbHelper。 dbHelper =新的dbHelper（上下文）;

尝试创建SQL数据库时出现NullpointerException

4 个答案: