android預填充數據庫程序中的nullPointerException

Question

我正在修改訪問預填充數據庫的代碼。我遇到了一個nullPointerException錯誤，我無法弄清楚發生了什麼事情。我將發佈我的代碼，如果有人可以幫助，我將不勝感激。

活性：

public class Activity1c extends ListActivity { 


IngredientHelper mDbHelper=null; 
int char_property; 
int char_name; 
long a=1; 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.act1c); 
    mDbHelper = new IngredientHelper(this); 

    Bundle extras = getIntent().getExtras(); 
    char_name = extras.getInt("char_name"); 
    char_property = extras.getInt("char_property"); 
    char_name+=1; 
    char_property+=1; 

    Toast.makeText(Activity1c.this, 
      "The char_name you picked was " + char_name + " and the property was "+char_property, 
      Toast.LENGTH_LONG).show(); 



    Cursor notesCursor = mDbHelper.fetchNote(a); 
    startManagingCursor(notesCursor); 

    // Create an array to specify the fields we want to display in the list (only TITLE) 
    String[] from = new String[]{IngredientHelper.COLUMN_TITLE,IngredientHelper.COLUMN_TITLE2}; 

    // and an array of the fields we want to bind those fields to (in this case just text1) 
    int[] to = new int[]{R.id.craft_type_display,R.id.spec_type_display}; 




    // Now create a simple cursor adapter and set it to display 
    SimpleCursorAdapter notes = 
     new SimpleCursorAdapter(this, R.layout.finalresult1, notesCursor, from, to); 
    setListAdapter(notes); 


}///end main 




    }///end class 

XML/act1c

 <?xml version="1.0" encoding="utf-8"?> 
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 


    android:layout_width="fill_parent" 
    android:layout_height="fill_parent"> 

<ListView android:id="@+id/android:list" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content"/> 
<TextView android:id="@+id/android:empty" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     /> 
    </LinearLayout> 

XML/finalresult1

<?xml version="1.0" encoding="utf-8"?> 
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:orientation="vertical" > 

    <TextView 
    android:id="@+id/craft_type_display" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" /> 

<TextView 
    android:id="@+id/spec_type_display" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" /> 

    </LinearLayout> 

類/ IngredientHelper

public class IngredientHelper extends SQLiteOpenHelper{ 

//we declare a bunch of useful constants 
//the should be pretty obvious what they are! 
private static final String DATABASE_PATH = "/data /data/com.android.Database_Practice_3_31_12/databases/"; 
private static final String DATABASE_NAME="infotest.sqlite"; 
private static final int SCHEMA_VERSION=1; 
public static final String TABLE_NAME = "info"; 
public static final String COLUMN_ID = "_id"; 
public static final String COLUMN_TITLE = "name"; 
public static final String COLUMN_TITLE2 = "phone"; 
public static final String COLUMN_TITLE3 = "fav_color"; 
public static final String COLUMN_TITLE4 = "home_city"; 

public SQLiteDatabase dbSqlite; 

private final Context myContext; 

public IngredientHelper(Context context) { 
    super(context, DATABASE_NAME, null, SCHEMA_VERSION); 
    this.myContext = context; 
    // check if exists and copy database from resource 
    //createDB(); 
} 

@Override 
public void onCreate(SQLiteDatabase db) { 
    // check if exists and copy database from resource 

} 

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

} 
public void createDatabase() { 
    createDB(); 
} 

private void createDB() { 

    boolean dbExist = DBExists(); 

    if (!dbExist) { 

     //By calling this method we create an empty database into the default system location 
     //We need this so we can overwrite that database with our database. 
     this.getReadableDatabase(); 
     //now we copy the database we included! 
     copyDBFromResource(); 

    } 

}  

private boolean DBExists() { 

    SQLiteDatabase db = null; 

    try { 
     String databasePath = DATABASE_PATH + DATABASE_NAME; 
     db = SQLiteDatabase.openDatabase(databasePath, null, 
       SQLiteDatabase.OPEN_READWRITE); 
     db.setLocale(Locale.getDefault()); 
     db.setLockingEnabled(true); 
     db.setVersion(1); 

    } catch (SQLiteException e) { 

     Log.e("SqlHelper", "database not found"); 

    } 

    if (db != null) { 

     db.close(); 

    } 

    return db != null ? true : false; 
} 

private void copyDBFromResource() { 

    InputStream inputStream = null; 
    OutputStream outStream = null; 
    String dbFilePath = DATABASE_PATH + DATABASE_NAME; 

    try { 

     inputStream = myContext.getAssets().open(DATABASE_NAME); 

     outStream = new FileOutputStream(dbFilePath); 

     byte[] buffer = new byte[1024]; 
     int length; 
     while ((length = inputStream.read(buffer)) > 0) { 
      outStream.write(buffer, 0, length); 
     } 

     outStream.flush(); 
     outStream.close(); 
     inputStream.close(); 

    } catch (IOException e) { 

     throw new Error("Problem copying database from resource file."); 

    } 

} 

public void openDataBase() throws SQLException { 

    String myPath = DATABASE_PATH + DATABASE_NAME; 
    dbSqlite = SQLiteDatabase.openDatabase(myPath, null, 
      SQLiteDatabase.OPEN_READWRITE); 

} 

@Override 
public synchronized void close() { 

    if (dbSqlite != null) 
    { 
     dbSqlite.close(); 
    } 
    super.close(); 

} 


public Cursor getCursor() { 

    SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder(); 

    queryBuilder.setTables(TABLE_NAME); 

    String[] asColumnsToReturn = new String[] { COLUMN_ID,COLUMN_TITLE}; 

    //make sure you get your search by string pass correctly! 
    Cursor mCursor = queryBuilder.query(dbSqlite, asColumnsToReturn, null, 
      null, null, null, null); 

    return mCursor; 
} 

public String getName(Cursor c) { 
    return(c.getString(1)); 
} 

public Cursor fetchNote(long rowId) throws SQLException { 

    Cursor mCursor = 

     dbSqlite.query(true, TABLE_NAME, new String[] { 
       COLUMN_ID, COLUMN_TITLE,COLUMN_TITLE2,COLUMN_TITLE3,COLUMN_TITLE4}, COLUMN_ID + "=" + rowId, null, 
       null, null, null, null); 
    if (mCursor != null) { 
     mCursor.moveToFirst(); 
    } 
    return mCursor; 

} 

}

logcat的

04-04 19:54:50.764: E/AndroidRuntime(454): Uncaught handler: thread main exiting due to uncaught exception 
04-04 19:54:50.774: E/AndroidRuntime(454): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android.Database_Practice_3_31_12/com.android.Database_Practice_3_31_12.Activity1c}: java.lang.NullPointerException 
04-04 19:54:50.774: E/AndroidRuntime(454):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496) 
04-04 19:54:50.774: E/AndroidRuntime(454):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512) 
04-04 19:54:50.774: E/AndroidRuntime(454):  at android.app.ActivityThread.access$2200(ActivityThread.java:119) 
04-04 19:54:50.774: E/AndroidRuntime(454):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863) 
04-04 19:54:50.774: E/AndroidRuntime(454):  at android.os.Handler.dispatchMessage(Handler.java:99) 
04-04 19:54:50.774: E/AndroidRuntime(454):  at android.os.Looper.loop(Looper.java:123) 
04-04 19:54:50.774: E/AndroidRuntime(454):  at android.app.ActivityThread.main(ActivityThread.java:4363) 
04-04 19:54:50.774: E/AndroidRuntime(454): at java.lang.reflect.Method.invokeNative(Native Method) 
04-04 19:54:50.774: E/AndroidRuntime(454): at java.lang.reflect.Method.invoke(Method.java:521) 
04-04 19:54:50.774: E/AndroidRuntime(454): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860) 
04-04 19:54:50.774: E/AndroidRuntime(454): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) 
04-04 19:54:50.774: E/AndroidRuntime(454): at dalvik.system.NativeStart.main(Native Method) 
04-04 19:54:50.774: E/AndroidRuntime(454): Caused by: java.lang.NullPointerException 
04-04 19:54:50.774: E/AndroidRuntime(454): at com.android.Database_Practice_3_31_12.IngredientHelper.fetchNote(IngredientHelper.java:173) 
04-04 19:54:50.774: E/AndroidRuntime(454): at com.android.Database_Practice_3_31_12.Activity1c.onCreate(Activity1c.java:49) 
04-04 19:54:50.774: E/AndroidRuntime(454): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
04-04 19:54:50.774: E/AndroidRuntime(454):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459) 

Answer 1

Ooof。很難遵循這一切。

首先，您會在fetchNote（）中看到錯誤。在堆棧跟蹤中，從底部開始的第5行顯示了這一點。您可以使用最後的行號來接近，其中發生了。

我的猜測是查詢失敗。

我的進一步猜測是，這是因爲你試圖通過從其他地方將byte-for-byte複製到「還原」數據庫。我不相信這會起作用。在應用程序中提供預填充數據庫的最佳方式是將其數據放在一個平面文件中，然後在應用程序首次運行時創建數據庫並將數據寫入其中。

在開發人員指南中，示例應用程序SearchableDictionary中的源文件DictionaryDatabase.java爲此提供了示例代碼。