從數據庫刪除行問題

Question

public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) { 
     super.onCreateContextMenu(menu, v, menuInfo); 
     MenuInflater inflater = getMenuInflater(); 
     inflater.inflate(R.menu.context_menu, menu); 
     } 

    public boolean onContextItemSelected(MenuItem item) { 
     //AdapterContextMenuInfo info = (AdapterContextMenuInfo) item.getMenuInfo(); 
     //String[] names = getResources().getStringArray(R.array.names); 
     switch(item.getItemId()) 
     { 
     case R.id.viewData: 
     { Intent r=new Intent(Database.this,MainMenu.class); 
      startActivity(r); 
      return true; 
     } 

     case R.id.DelData: 
     { 

      AdapterView.AdapterContextMenuInfo info1= 
      (AdapterView.AdapterContextMenuInfo)item.getMenuInfo(); 

     delete(info1.id); 
     return true; 

     } 
     } 
     return(super.onOptionsItemSelected(item)); 

    } 
    private void delete(final long rowId) { 
     if (rowId>0) { 
      new AlertDialog.Builder(this) 
       .setTitle(R.string.delete_title) 
       .setPositiveButton(R.string.ok, 
                 new DialogInterface.OnClickListener() { 
        public void onClick(DialogInterface dialog, 
                   int whichButton) { 
         processDelete(rowId); 
        } 
       }) 
       .setNegativeButton(R.string.cancel, 
                 new DialogInterface.OnClickListener() { 
        public void onClick(DialogInterface dialog, 
                   int whichButton) { 
        // ignore, just dismiss 
        } 
       }) 
       .show(); 
     } 
    } 

    private void processDelete(long rowId) { 
     String[] args={String.valueOf(rowId)}; 

     //DatabaseHelper dbh = new DatabaseHelper(this); 
     dbh.getWritableDatabase().delete("mygrades", "_ID=?", args); 
     c.requery(); 
    } 

我正在從表名爲mygrades的數據庫中刪除行。有一個contextMenu，我有兩個選項查看和刪除。當我按刪除程序停止。錯誤日誌說我有processDelete（long rowId）的問題。我的論點是否通過了正確的方式？我將發佈以下錯誤日誌： -

04-02 03:38:29.450: W/dalvikvm(541): threadid=1: thread exiting with uncaught exception (group=0x409c01f8) 
04-02 03:38:29.489: E/AndroidRuntime(541): FATAL EXCEPTION: main 
04-02 03:38:29.489: E/AndroidRuntime(541): java.lang.NullPointerException 
04-02 03:38:29.489: E/AndroidRuntime(541): at nidhin.survey.Database.processDelete(Database.java:139) 
04-02 03:38:29.489: E/AndroidRuntime(541): at nidhin.survey.Database.access$0(Database.java:135) 
04-02 03:38:29.489: E/AndroidRuntime(541): at nidhin.survey.Database$1.onClick(Database.java:121) 
04-02 03:38:29.489: E/AndroidRuntime(541): at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:166) 
04-02 03:38:29.489: E/AndroidRuntime(541): at android.os.Handler.dispatchMessage(Handler.java:99) 
04-02 03:38:29.489: E/AndroidRuntime(541): at android.os.Looper.loop(Looper.java:137) 
04-02 03:38:29.489: E/AndroidRuntime(541): at android.app.ActivityThread.main(ActivityThread.java:4424) 
04-02 03:38:29.489: E/AndroidRuntime(541): at java.lang.reflect.Method.invokeNative(Native Method) 
04-02 03:38:29.489: E/AndroidRuntime(541): at java.lang.reflect.Method.invoke(Method.java:511) 
04-02 03:38:29.489: E/AndroidRuntime(541): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 
04-02 03:38:29.489: E/AndroidRuntime(541): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 
04-02 03:38:29.489: E/AndroidRuntime(541): at dalvik.system.NativeStart.main(Native Method) 
04-02 03:38:32.388: I/Process(541): Sending signal. PID: 541 SIG: 9 
04-02 03:39:09.788: D/dalvikvm(586): GC_FOR_ALLOC freed 66K, 4% free 9065K/9347K, paused 82ms 
04-02 03:39:09.788: I/dalvikvm-heap(586): Grow heap (frag case) to 9.452MB for 556016-byte allocation 
04-02 03:39:09.930: D/dalvikvm(586): GC_CONCURRENT freed <1K, 4% free 9607K/9927K, paused 5ms+5ms 
04-02 03:39:10.259: D/gralloc_goldfish(586): Emulator without GPU emulation detected. 

Answer 1

我有一個很好的數據庫問題教程。試試這個DatabaseHelper代碼，它對你很有用。

public class DatabaseHelper 
{ 

    Context context; 
    private SQLiteDatabase db;  
    private final String DB_NAME = "database_name"; 
    private final int DB_VERSION = 1; 

    private final String TABLE_NAME = "database_table"; 
    private final String TABLE_ROW_ID = "id"; 
    private final String TABLE_ROW_ONE = "table_row_one"; 
    private final String TABLE_ROW_TWO = "table_row_two"; 

    public DatabaseHelper(Context context) 
    { 
     this.context = context; 

     CustomSQLiteOpenHelper helper = new CustomSQLiteOpenHelper(context); 
     this.db = helper.getWritableDatabase(); 
    } 




    /********************************************************************** 
    * ADDING A ROW TO THE DATABASE TABLE 
    * 
    * This is an example of how to add a row to a database table 
    * using this class. You should edit this method to suit your 
    * needs. 
    * 
    * the key is automatically assigned by the database 
    * @param rowStringOne the value for the row's first column 
    * @param rowStringTwo the value for the row's second column 
    */ 
    public void addRow(String rowStringOne, String rowStringTwo) 
    { 

     ContentValues values = new ContentValues(); 
     values.put(TABLE_ROW_ONE, rowStringOne); 
     values.put(TABLE_ROW_TWO, rowStringTwo); 


     try{db.insert(TABLE_NAME, null, values);} 
     catch(Exception e) 
     { 
      Log.e("DB ERROR", e.toString()); 
      e.printStackTrace(); 
     } 
    } 



    /********************************************************************** 
    * DELETING A ROW FROM THE DATABASE TABLE 
    * 
    * This is an example of how to delete a row from a database table 
    * using this class. In most cases, this method probably does 
    * not need to be rewritten. 
    * 
    * @param rowID the SQLite database identifier for the row to delete. 
    */ 
    public void deleteRow(long rowID) 
    { 
     // ask the database manager to delete the row of given id 
     try {db.delete(TABLE_NAME, TABLE_ROW_ID + "=" + rowID, null);} 
     catch (Exception e) 
     { 
      Log.e("DB ERROR", e.toString()); 
      e.printStackTrace(); 
     } 
    } 

    /********************************************************************** 
    * UPDATING A ROW IN THE DATABASE TABLE 
    * 
    * This is an example of how to update a row in the database table 
    * using this class. You should edit this method to suit your needs. 
    * 
    * @param rowID the SQLite database identifier for the row to update. 
    * @param rowStringOne the new value for the row's first column 
    * @param rowStringTwo the new value for the row's second column 
    */ 
    public void updateRow(long rowID, String rowStringOne, String rowStringTwo) 
    { 

     ContentValues values = new ContentValues(); 
     values.put(TABLE_ROW_ONE, rowStringOne); 
     values.put(TABLE_ROW_TWO, rowStringTwo); 

     // ask the database object to update the database row of given rowID 
     try {db.update(TABLE_NAME, values, TABLE_ROW_ID + "=" + rowID, null);} 
     catch (Exception e) 
     { 
      Log.e("DB Error", e.toString()); 
      e.printStackTrace(); 
     } 
    } 

    /********************************************************************** 
    * RETRIEVING A ROW FROM THE DATABASE TABLE 
    * 
    * This is an example of how to retrieve a row from a database table 
    * using this class. You should edit this method to suit your needs. 
    * 
    * @param rowID the id of the row to retrieve 
    * @return an array containing the data from the row 
    */ 
    public ArrayList<Object> getRowAsArray(long rowID) 
    { 
     // create an array list to store data from the database row. 
     // I would recommend creating a JavaBean compliant object 
     // to store this data instead. That way you can ensure 
     // data types are correct. 
     ArrayList<Object> rowArray = new ArrayList<Object>(); 
     Cursor cursor; 

     try 
     { 
      // this is a database call that creates a "cursor" object. 
      // the cursor object store the information collected from the 
      // database and is used to iterate through the data. 
      cursor = db.query 
      (
        TABLE_NAME, 
        new String[] { TABLE_ROW_ID, TABLE_ROW_ONE, TABLE_ROW_TWO }, 
        TABLE_ROW_ID + "=" + rowID, 
        null, null, null, null, null 
      ); 

      // move the pointer to position zero in the cursor. 
      cursor.moveToFirst(); 

      // if there is data available after the cursor's pointer, add 
      // it to the ArrayList that will be returned by the method. 
      if (!cursor.isAfterLast()) 
      { 
       do 
       { 
        rowArray.add(cursor.getLong(0)); 
        rowArray.add(cursor.getString(1)); 
        rowArray.add(cursor.getString(2)); 
       } 
       while (cursor.moveToNext()); 
      } 

      // let java know that you are through with the cursor. 
      cursor.close(); 
     } 
     catch (SQLException e) 
     { 
      Log.e("DB ERROR", e.toString()); 
      e.printStackTrace(); 
     } 

     // return the ArrayList containing the given row from the database. 
     return rowArray; 
    } 




    /********************************************************************** 
    * RETRIEVING ALL ROWS FROM THE DATABASE TABLE 
    * 
    * This is an example of how to retrieve all data from a database 
    * table using this class. You should edit this method to suit your 
    * needs. 
    * 
    * the key is automatically assigned by the database 
    */ 

    public ArrayList<ArrayList<Object>> getAllRowsAsArrays() 
    { 
     // create an ArrayList that will hold all of the data collected from 
     // the database. 
     ArrayList<ArrayList<Object>> dataArrays = new ArrayList<ArrayList<Object>>(); 

     // this is a database call that creates a "cursor" object. 
     // the cursor object store the information collected from the 
     // database and is used to iterate through the data. 
     Cursor cursor; 

     try 
     { 
      // ask the database object to create the cursor. 
      cursor = db.query(
        TABLE_NAME, 
        new String[]{TABLE_ROW_ID, TABLE_ROW_ONE, TABLE_ROW_TWO}, 
        null, null, null, null, null 
      ); 

      // move the cursor's pointer to position zero. 
      cursor.moveToFirst(); 

      // if there is data after the current cursor position, add it 
      // to the ArrayList. 
      if (!cursor.isAfterLast()) 
      { 
       do 
       { 
        ArrayList<Object> dataList = new ArrayList<Object>(); 

        dataList.add(cursor.getLong(0)); 
        dataList.add(cursor.getString(1)); 
        dataList.add(cursor.getString(2)); 

        dataArrays.add(dataList); 
       } 
       // move the cursor's pointer up one position. 
       while (cursor.moveToNext()); 
      } 
     } 
     catch (SQLException e) 
     { 
      Log.e("DB Error", e.toString()); 
      e.printStackTrace(); 
     } 

     // return the ArrayList that holds the data collected from 
     // the database. 
     return dataArrays; 
    } 




    /********************************************************************** 
    * THIS IS THE BEGINNING OF THE INTERNAL SQLiteOpenHelper SUBCLASS. 
    * 
    * I MADE THIS CLASS INTERNAL SO I CAN COPY A SINGLE FILE TO NEW APPS 
    * AND MODIFYING IT - ACHIEVING DATABASE FUNCTIONALITY. ALSO, THIS WAY 
    * I DO NOT HAVE TO SHARE CONSTANTS BETWEEN TWO FILES AND CAN 
    * INSTEAD MAKE THEM PRIVATE AND/OR NON-STATIC. HOWEVER, I THINK THE 
    * INDUSTRY STANDARD IS TO KEEP THIS CLASS IN A SEPARATE FILE. 
    *********************************************************************/ 

    /** 
    * This class is designed to check if there is a database that currently 
    * exists for the given program. If the database does not exist, it creates 
    * one. After the class ensures that the database exists, this class 
    * will open the database for use. Most of this functionality will be 
    * handled by the SQLiteOpenHelper parent class. The purpose of extending 
    * this class is to tell the class how to create (or update) the database. 
    * 
    * @author Randall Mitchell 
    * 
    */ 
    private class CustomSQLiteOpenHelper extends SQLiteOpenHelper 
    { 
     public CustomSQLiteOpenHelper(Context context) 
     { 
      super(context, DB_NAME, null, DB_VERSION); 
     } 

     @Override 
     public void onCreate(SQLiteDatabase db) 
     { 
      // This string is used to create the database. It should 
      // be changed to suit your needs. 
      String newTableQueryString = "create table " + 
             TABLE_NAME + 
             " (" + 
             TABLE_ROW_ID + " integer primary key autoincrement not null," + 
             TABLE_ROW_ONE + " text," + 
             TABLE_ROW_TWO + " text" + 
             ");"; 
      // execute the query string to the database. 
      db.execSQL(newTableQueryString); 
     } 


     @Override 
     public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) 
     { 
      // NOTHING TO DO HERE. THIS IS THE ORIGINAL DATABASE VERSION. 
      // OTHERWISE, YOU WOULD SPECIFIY HOW TO UPGRADE THE DATABASE. 
     } 
    } 
} 

Answer 2

您有一個空指針。我的猜測是兩種：

• 胸徑未初始化
• c未初始化