我有一个列表视图，当我点击列表项时，用户将被重定向到编辑屏幕，该编辑屏幕将使用光标从数据库中获取数据并填充在编辑文本字段中。但是当我选择列表项时，我得到 NullPointerException ，并且错误显示在 cursor.getColumnIndexOrThrow（），即使数据库不为空。
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;
}
}
ReminderEditActivity
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;
}
私人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的
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'
答案 0 :(得分：0)
您没有mCursor值，因此在Cursor reminder = mDbHelper.fetchReminder(mRowId); reminder为空且使用reminder.getString时抛出NPE。
问题位于RemindersDbAdapter java文件中，您没有处理mCursor为空的情况，因此它将始终为null。
答案 1 :(得分：0)
在 RemindersDbAdapter.java 中，使用此部分代码
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;
}
主要是因为mCursor is null。执行查询后，他会得到一些数据。然后你必须执行下一行。