从数据库问题中删除行

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();
    }

我正在从表名为 mygrades 的数据库中删除行。有一个 contextMenu 我有两个选项查看和删除。当我按下删除程序停止。错误日志显示我对 processDelete(long rowId) 有疑问。我的论点是否以正确的方式通过？我将在下面发布错误日志：-

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

你有一个空指针。我的猜测是：

• dbh 未初始化
• c 未初始化

Answer 2

我有一个很棒的数据库问题教程。试试这个 DatabaseHelper 代码，它会对你有用。

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.
        }
    }
}