java - cursor.getColumnIndexOrThrow() 抛出 NullPointerException

Question

我有一个 ListView ，当我单击列表项时，用户将被重定向到编辑屏幕，该屏幕将使用光标从数据库中获取数据并填充到编辑文本字段中。但是，当我选择列表项时，我收到 NullPointerException ，并且即使数据库不为空，也会在 cursor.getColumnIndexOrThrow() 处显示错误。

RemindersDbAdapter.java

public class RemindersDbAdapter {

private static final String DATABASE_NAME = "data";
private static final String DATABASE_TABLE = "reminders";
private static final int DATABASE_VERSION = 4;

public static final String KEY_TITLE = "title";
public static final String KEY_BODY = "body";
public static final String KEY_DATE_TIME = "reminder_date_time";
public static final String KEY_ROWID = "_id";

private static final String TAG = "ReminderDbAdapter";
private DatabaseHelper mDbHelper;
private SQLiteDatabase mDb;
Cursor mCursor;
private static final String DATABASE_CREATE = "create table "
           + DATABASE_TABLE + " (" + KEY_ROWID
           + " integer primary key autoincrement, " + KEY_TITLE
            + " text not null, " + KEY_BODY + " text not null, "
           + KEY_DATE_TIME + " text not null);";

private final Context mCtx;

private static class DatabaseHelper extends SQLiteOpenHelper {

    DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {

        db.execSQL(DATABASE_CREATE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                + newVersion + ", which will destroy all old data");
        db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);
        onCreate(db);
    }
}


public RemindersDbAdapter(Context ctx) {
    this.mCtx = ctx;
}


public RemindersDbAdapter open() throws SQLException {
    mDbHelper = new DatabaseHelper(mCtx);
    mDb = mDbHelper.getWritableDatabase();
    return this;
}

public void close() {
    mDbHelper.close();
}


public long createReminder(String title, String body,
        String reminderDateTime) {
    ContentValues initialValues = new ContentValues();
    initialValues.put(KEY_TITLE, title);
    initialValues.put(KEY_BODY, body);
    initialValues.put(KEY_DATE_TIME, reminderDateTime);

    return mDb.insert(DATABASE_TABLE, null, initialValues);
}


public boolean deleteReminder(long rowId) {

    return mDb.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
}

public Cursor fetchAllReminders() {

    return mDb.query(DATABASE_TABLE, new String[] { KEY_ROWID, KEY_TITLE,
            KEY_BODY, KEY_DATE_TIME }, null, null, null, null, null);
}


public Cursor fetchReminder(long rowId) throws SQLException {



    if (mCursor != null && (mCursor.getCount() > 0)) {
        mCursor = mDb.query(true, DATABASE_TABLE, new String[] { KEY_ROWID,
                KEY_TITLE, KEY_BODY, KEY_DATE_TIME }, KEY_ROWID + "="
                + rowId, null, null, null, null, null);
    }
    if (mCursor != null  && mCursor.getCount()>0) 
    { mCursor.moveToFirst(); }

    return mCursor;

}


public boolean updateReminder(long rowId, String title, String body,
        String reminderDateTime) {
    ContentValues args = new ContentValues();
    args.put(KEY_TITLE, title);
    args.put(KEY_BODY, body);
    args.put(KEY_DATE_TIME, reminderDateTime);

    return mDb.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
}
  }

提醒编辑 Activity

public class ReminderEditActivity extends Activity {

// 
// Dialog Constants
//
private static final int DATE_PICKER_DIALOG = 0;
private static final int TIME_PICKER_DIALOG = 1;

// 
// Date Format 
//
   private static final String DATE_FORMAT = "yyyy-MM-dd"; 
   private static final String TIME_FORMAT = "kk:mm";
  public static final String DATE_TIME_FORMAT = "yyyy-MM-dd kk:mm:ss";

private EditText mTitleText;
private EditText mBodyText;
private Button mDateButton;
private Button mTimeButton;
private Button mConfirmButton;
private Long mRowId;
private RemindersDbAdapter mDbHelper;
private Calendar mCalendar;  

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    mDbHelper = new RemindersDbAdapter(this);

    setContentView(R.layout.reminder_edit);

    mCalendar = Calendar.getInstance(); 
    mTitleText = (EditText) findViewById(R.id.title);
    mBodyText = (EditText) findViewById(R.id.body);
    mDateButton = (Button) findViewById(R.id.reminder_date);
    mTimeButton = (Button) findViewById(R.id.reminder_time);

    mConfirmButton = (Button) findViewById(R.id.confirm);

    mRowId = savedInstanceState != null ? savedInstanceState.getLong(RemindersDbAdapter.KEY_ROWID) 
                                        : null;

    registerButtonListenersAndSetDefaultText();
}

private void setRowIdFromIntent() {
    if (mRowId == null) {
        Bundle extras = getIntent().getExtras();            
        mRowId = extras != null ? extras.getLong(RemindersDbAdapter.KEY_ROWID) 
                                : null;

    }
}

@Override
protected void onPause() {
    super.onPause();
    mDbHelper.close(); 
}

@Override
protected void onResume() {
    super.onResume();
    mDbHelper.open(); 
    setRowIdFromIntent();
    populateFields();
}

@Override
protected Dialog onCreateDialog(int id) {
    switch(id) {
        case DATE_PICKER_DIALOG: 
            return showDatePicker();
        case TIME_PICKER_DIALOG: 
            return showTimePicker(); 
    }
    return super.onCreateDialog(id);
}

private DatePickerDialog showDatePicker() {


    DatePickerDialog datePicker = new DatePickerDialog(ReminderEditActivity.this, new DatePickerDialog.OnDateSetListener() {

        @Override
        public void onDateSet(DatePicker view, int year, int monthOfYear, int dayOfMonth) {
            mCalendar.set(Calendar.YEAR, year);
            mCalendar.set(Calendar.MONTH, monthOfYear);
            mCalendar.set(Calendar.DAY_OF_MONTH, dayOfMonth);
            updateDateButtonText(); 
        }
    }, mCalendar.get(Calendar.YEAR), mCalendar.get(Calendar.MONTH), mCalendar.get(Calendar.DAY_OF_MONTH)); 
    return datePicker; 
}

私有(private) TimePickerDialog showTimePicker() {

    TimePickerDialog timePicker = new TimePickerDialog(this, new TimePickerDialog.OnTimeSetListener() {

        @Override
        public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
            mCalendar.set(Calendar.HOUR_OF_DAY, hourOfDay);
            mCalendar.set(Calendar.MINUTE, minute); 
            updateTimeButtonText(); 
        }
    }, mCalendar.get(Calendar.HOUR_OF_DAY), mCalendar.get(Calendar.MINUTE), true); 

    return timePicker; 
}

private void registerButtonListenersAndSetDefaultText() {

    mDateButton.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            showDialog(DATE_PICKER_DIALOG);  
        }
    }); 


    mTimeButton.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            showDialog(TIME_PICKER_DIALOG); 
        }
    }); 

    mConfirmButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View view) {
            saveState(); 
            setResult(RESULT_OK);
            Toast.makeText(ReminderEditActivity.this, getString(R.string.task_saved_message), Toast.LENGTH_SHORT).show();
            finish(); 
        }

    });

      updateDateButtonText(); 
      updateTimeButtonText();
}

private void populateFields()  {



    // Only populate the text boxes and change the calendar date
    // if the row is not null from the database. 
    if (mRowId != null) {
        Cursor reminder = mDbHelper.fetchReminder(mRowId);
        System.out.println("RowID "+mRowId);
        startManagingCursor(reminder);
        mTitleText.setText(reminder.getString(
                reminder.getColumnIndexOrThrow(RemindersDbAdapter.KEY_TITLE)));
        mBodyText.setText(reminder.getString(
                reminder.getColumnIndexOrThrow(RemindersDbAdapter.KEY_BODY)));


        // Get the date from the database and format it for our use. 
        SimpleDateFormat dateTimeFormat = new SimpleDateFormat(DATE_TIME_FORMAT);
        Date date = null;
        try {
            String dateString = reminder.getString(reminder.getColumnIndexOrThrow(RemindersDbAdapter.KEY_DATE_TIME)); 
            date = dateTimeFormat.parse(dateString);
            mCalendar.setTime(date); 
        } catch (ParseException e) {
            Log.e("ReminderEditActivity", e.getMessage(), e); 
        } 
    } else {
        // This is a new task - add defaults from preferences if set. 
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this); 
        String defaultTitleKey = getString(R.string.pref_task_title_key); 
        String defaultTimeKey = getString(R.string.pref_default_time_from_now_key); 

        String defaultTitle = prefs.getString(defaultTitleKey, null);
        String defaultTime = prefs.getString(defaultTimeKey, null); 

        if(defaultTitle != null)
            mTitleText.setText(defaultTitle); 

        if(defaultTime != null)
            mCalendar.add(Calendar.MINUTE, Integer.parseInt(defaultTime));

    }

    updateDateButtonText(); 
    updateTimeButtonText(); 

}

private void updateTimeButtonText() {
    // Set the time button text based upon the value from the database
    SimpleDateFormat timeFormat = new SimpleDateFormat(TIME_FORMAT); 
    String timeForButton = timeFormat.format(mCalendar.getTime()); 
    mTimeButton.setText(timeForButton);
}

private void updateDateButtonText() {
    // Set the date button text based upon the value from the database 
    SimpleDateFormat dateFormat = new SimpleDateFormat(DATE_FORMAT); 
    String dateForButton = dateFormat.format(mCalendar.getTime()); 
    mDateButton.setText(dateForButton);
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putLong(RemindersDbAdapter.KEY_ROWID, mRowId);
}



private void saveState() {
    String title = mTitleText.getText().toString();
    String body = mBodyText.getText().toString();

    SimpleDateFormat dateTimeFormat = new SimpleDateFormat(DATE_TIME_FORMAT); 
    String reminderDateTime = dateTimeFormat.format(mCalendar.getTime());

    if (mRowId == null) {

        long id = mDbHelper.createReminder(title, body, reminderDateTime);
        if (id > 0) {
            mRowId = id;
        }
    } else {
        mDbHelper.updateReminder(mRowId, title, body, reminderDateTime);
    }
   /* Intent i=new Intent(ReminderEditActivity.this,ReminderService.class);
    i.putExtra("title", title);
    i.putExtra("description", body);
    i.putExtra("date", reminderDateTime);
    startService(i);*/
    new ReminderManager(this).setReminder(mRowId, mCalendar); 


}

}

Logcat

03-09 15:29:56.554: I/System.out(758): Reminders 1
03-09 15:29:56.733: D/gralloc_goldfish(758): Emulator without GPU emulation detected.
03-09 15:29:56.774: I/dalvikvm(758): threadid=3: reacting to signal 3
03-09 15:29:56.804: I/dalvikvm(758): Wrote stack traces to '/data/anr/traces.txt'
03-09 15:30:06.613: I/System.out(758): RowID 1
03-09 15:30:06.613: D/AndroidRuntime(758): Shutting down VM
03-09 15:30:06.623: W/dalvikvm(758): threadid=1: thread exiting with uncaught exception (group=0x409c01f8)
03-09 15:30:06.643: E/AndroidRuntime(758): FATAL EXCEPTION: main
03-09 15:30:06.643: E/AndroidRuntime(758): java.lang.RuntimeException: Unable to resume activity {com.dummies.android.taskreminder/com.dummies.android.taskreminder.ReminderEditActivity}: java.lang.NullPointerException
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2444)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2472)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1986)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.app.ActivityThread.access$600(ActivityThread.java:123)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.os.Handler.dispatchMessage(Handler.java:99)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.os.Looper.loop(Looper.java:137)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.app.ActivityThread.main(ActivityThread.java:4424)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at java.lang.reflect.Method.invokeNative(Native Method)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at java.lang.reflect.Method.invoke(Method.java:511)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at dalvik.system.NativeStart.main(Native Method)
03-09 15:30:06.643: E/AndroidRuntime(758): Caused by: java.lang.NullPointerException
03-09 15:30:06.643: E/AndroidRuntime(758): 	at com.dummies.android.taskreminder.ReminderEditActivity.populateFields(ReminderEditActivity.java:181)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at com.dummies.android.taskreminder.ReminderEditActivity.onResume(ReminderEditActivity.java:92)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1154)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.app.Activity.performResume(Activity.java:4539)
03-09 15:30:06.643: E/AndroidRuntime(758): 	at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2434)
03-09 15:30:06.643: E/AndroidRuntime(758): 	... 12 more
03-09 15:30:06.893: I/dalvikvm(758): threadid=3: reacting to signal 3
03-09 15:30:06.993: I/dalvikvm(758): Wrote stack traces to '/data/anr/traces.txt'
03-09 15:30:07.013: D/dalvikvm(758): GC_CONCURRENT freed 146K, 3% free 12859K/13127K, paused 7ms+62ms
03-09 15:30:07.243: I/dalvikvm(758): threadid=3: reacting to signal 3
03-09 15:30:07.273: I/dalvikvm(758): Wrote stack traces to '/data/anr/traces.txt'

Answer 1

在RemindersDbAdapter.java中，使用这部分代码

public Cursor fetchReminder(long rowId) throws SQLException {
        mCursor = mDb.query(true, DATABASE_TABLE, new String[] { KEY_ROWID,
                KEY_TITLE, KEY_BODY, KEY_DATE_TIME }, KEY_ROWID + "="
                + rowId, null, null, null, null, null);
    if (mCursor != null  && mCursor.getCount()>0) 
    { mCursor.moveToFirst(); }

    return mCursor;

}

而不是

public Cursor fetchReminder(long rowId) throws SQLException {
    if (mCursor != null && (mCursor.getCount() > 0)) { //this line have to remove
        mCursor = mDb.query(true, DATABASE_TABLE, new String[] { KEY_ROWID,
                KEY_TITLE, KEY_BODY, KEY_DATE_TIME }, KEY_ROWID + "="
                + rowId, null, null, null, null, null);
    } // this line have to remove for early removal
    if (mCursor != null  && mCursor.getCount()>0) 
    { mCursor.moveToFirst(); }

    return mCursor;

}

因为主要是mCursor 为空。执行查询后，他会得到一些数据，也可能得不到一些数据。然后你必须执行下一行。