Android - NullPointerException getReadableDatabase（）

Question

我正在寫一個應用程序，檢查數據庫中的第一個值是否與字符串匹配。當我嘗試連接到數據庫時，我得到一個NullPointerException。這發生在兩個getReadableDatabase()和getWritableDatabase()

這裏是我的代碼：

public class DataManager extends SQLiteOpenHelper 
{ 
    private final static String DB_TABLE = "Nums"; 
    private final static String COL_KEY  = "KEY"; 
    private final static String COL_VALUE= "VALUE"; 
    private static Context context; 
    private ContentValues initialValues; 
    private SQLiteDatabase db; 
    private static DataManager dm; 

    public static DataManager getInstance(Context _context) 
    { 
     if (dm==null) 
     {dm=new DataManager(_context);} 
     return dm; 
    } 

    private DataManager() 
    { 
     super(context, DB_TABLE, null, 1); 
    } 
    private DataManager(Context _context) 
    { 
     super(_context, DB_TABLE, null, 1); 
     context=_context; 
     initialValues = new ContentValues(); 
     if (db==null) 
     {db=getWritableDatabase();} 
    } 

    @Override 
    public void onCreate(SQLiteDatabase db) 
    { 
     StringBuilder Query = new StringBuilder(); 
     Query.append("CREATE TABLE IF NOT EXISTS "); 
     Query.append(DB_TABLE); 
     Query.append('('); 
     Query.append(COL_KEY); 
      Query.append(" TEXT PRIMARY KEY,"); 
     Query.append(COL_VALUE); 
      Query.append(" TEXT);"); 
     Log.i(Constants.TAG,"CREATE STRING: "+Query.toString()); 
     db.execSQL(Query.toString()); 

     if(tableEmpty()) 
     {setDefault();} 
    } 

    /** 
    * Populate the database with numbers 1->MAXNUM giving each a value of 0. 
    */ 
    private void setDefault() 
    { 
     for (int i=1;i<=Constants.MAXNUM;i++) 
     { 
      setValue(String.valueOf(i),"0"); 
     } 
    } 

    /** 
    * Method to get the values, ordered by frequency 
    * @return Comma seperated 
    */ 
    public String[] getAllValues() 
    { 
     Cursor c=null; 
     int counter=0; 
     String[] val = new String[Constants.MAXNUM]; 

     try 
     { 
      c = getReadableDatabase().query(DB_TABLE, null,null, null, null, null, COL_VALUE); 
      c.moveToFirst(); 
      //Ensure there is something in the database 
      if(c.getCount()>0) 
       { // Make sure the cursor never goes over the edge 
        while(!c.isAfterLast()) 
        { // Append each value in order, seperated by a comma 
         val[counter++]=c.getString(1); 
         c.moveToNext(); 
        } 
       } 
     } 
     catch(SQLiteException e){ 
      Log.e(Constants.TAG,"getValue::SQLiteException::"+e.getMessage()); 
      e.printStackTrace(); 
     } 
     finally { 
      c.close(); // Tidy up 
     } 
     return val; 
    } 

    public String getValueByKey(String _key) 
    { 
     String val = ""; 
     try 
     { 
      Log.i(Constants.TAG,"key is: "+_key); 
      Cursor c=getReadableDatabase().query(DB_TABLE,new String[]{COL_VALUE},COL_KEY + " LIKE ?",new String[]{_key},null,null,null); 
      c.moveToFirst(); 
      if(c.getCount()>0) 
       { val = c.getString(0); } 
      c.close(); 
     } 
     catch(SQLiteException e) 
     { 
      Log.e(Constants.TAG,"SQLiteException::"+e.getMessage()); 
      e.printStackTrace(); 
     } 

     return val; 
    } 

    /** 
    * Method checks to see if there are any records in the database 
    * @return Boolean true if empty, false if not empty 
    */ 
    private boolean tableEmpty() 
    { 
     boolean result = false; 

     if(!(getValueByKey("1") == "0")) 
     { 
      result=true; 
     } 

     return result; 
    } 

    @Override 
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) 
    {} 
} 

我的數據庫有2列。左欄有鍵，右面有值。我正在嘗試獲取第一條記錄的值並將其返回。錯誤消息是一個通用的NullPointerError，所以我對這個錯誤瞭解不多，除了與getReadableDatabase()有關的事實。任何人都可以看到我做錯了什麼？

謝謝。

編輯：我已添加完整的代碼。這裏是堆棧跟蹤：

09-23 15:16:27.450: ERROR/AndroidRuntime(11825): FATAL EXCEPTION: main 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825): java.lang.NullPointerException 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:118) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at com.eoinzy.myApp.MyClass.openDatabase(MyClass.java:137) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at com.eoinzy.myApp.MyClass.getFrequency(MyClass.java:204) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at com.eoinzy.myApp.MyClass.tableEmpty(MyClass.java:252) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at com.eoinzy.myApp.MyClass.getAllValues(MyClass.java:169) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at com.eoinzy.myApp.MyClass.setupNumbers(MyClass.java:48) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at com.eoinzy.myApp.MyClass.<init>(MyClass.java:38) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at com.eoinzy.myApp.ButtonControl.onClick(ButtonControl.java:56) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at android.view.View.performClick(View.java:2501) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at android.view.View$PerformClick.run(View.java:9107) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at android.os.Handler.handleCallback(Handler.java:587) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at android.os.Handler.dispatchMessage(Handler.java:92) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at android.os.Looper.loop(Looper.java:130) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at android.app.ActivityThread.main(ActivityThread.java:3835) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at java.lang.reflect.Method.invokeNative(Native Method) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at java.lang.reflect.Method.invoke(Method.java:507) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:847) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:605) 
09-23 15:16:27.450: ERROR/AndroidRuntime(11825):  at dalvik.system.NativeStart.main(Native Method) 

Answer 1

試試這種編碼方式，以便您可以輕鬆地調試您的錯誤。

另外Null Pointer來很多的原因爲Null values。

private Cursor cursor_value; 
private DbHelper mDbHelper; 
mDbHelper = new DbHelper(this); 
mDbHelper.open(); 

try{ 
    String cursor_link_value,cursor_time_value,cursor_name_value; 
    cursor_value = mDbHelper.fetch_message_values(); 
    Log.v("cursor_value", ""+cursor_value.getCount()); 
    startManagingCursor(cursor_value); 
    if (cursor_value.moveToFirst()) { 
    do { 
     value =cursor_value.getString(cursor_value.getColumnIndex(Column_Name)); 
    } while (cursor_value.moveToNext()); 
    } 
} 
catch(Exception e){ 
    Log.v("Excepqqqqqqqqqqqqqqqqqqqq", ""+e); 
} 

對於DB：

寫您的查詢DbHelper類裏面：

public Cursor fetch_message_values() { 
    // TODO Auto-generated method stub 
    Cursor c=mDb.rawQuery("Your Query",null); 
    Log.v("Cursor Count for Draft Table", ""+c.getCount()); 
    return c; 
    } 

備查檢查此鏈接爲Notepad Example for Database