我是android的新手，请不要介意这个愚蠢的问题，但如果您知道的话，请回答...！当我使用Intent启动新活动时，但是启动新活动时我的应用程序意外停止。错误是java.lang.NullPointerException。我在此上浪费了很多时间，但我无法理解问题出在哪里。
这是第一个活动：
package com.people.rock;
import com.android.taskreminder.R;
import android.app.ListActivity;
import android.content.Intent;
import android.database.Cursor;
import android.os.Bundle;
import android.view.ContextMenu;
import android.view.ContextMenu.ContextMenuInfo;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView.AdapterContextMenuInfo;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
public class ReminderListActivity extends ListActivity {
private static final int ACTIVITY_CREATE=0;
//private static final int ACTIVITY_EDIT=1;
private RemindersDbAdapter mDbHelper;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.reminder_list);
mDbHelper = new RemindersDbAdapter(this);
mDbHelper.open();
fillData();
registerForContextMenu(getListView());
}
@SuppressWarnings("deprecation")
private void fillData() {
Cursor remindersCursor = mDbHelper.fetchAllReminders();
startManagingCursor(remindersCursor);
// Create an array to specify the fields we want (only the TITLE)
String[] from = new String[]{RemindersDbAdapter.KEY_TITLE};
// and an array of the fields we want to bind in the view
int[] to = new int[]{R.id.text1};
// Now create a simple cursor adapter and set it to display
SimpleCursorAdapter reminders =
new SimpleCursorAdapter(this, R.layout.reminder_row, remindersCursor, from, to);
setListAdapter(reminders);
}
@Override
protected void onListItemClick(ListView l, View v, int position, long id) {
super.onListItemClick(l, v, position, id);
Intent i = new Intent(this, ReminderEditActivity.class);
i.putExtra("RowId", id);
startActivity(i);
}
@Override
public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo
menuInfo) {
super.onCreateContextMenu(menu, v, menuInfo);
MenuInflater mi = getMenuInflater();
mi.inflate(R.menu.list_menu_item_longpress, menu);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.reminder_list, menu);
MenuInflater mi = getMenuInflater();
mi.inflate(R.menu.list_menu, menu);
return true;
}
@Override
public boolean onMenuItemSelected(int featureId, MenuItem item) {
switch(item.getItemId()) {
case R.id.menu_insert:
createReminder();
return true;
}
return super.onMenuItemSelected(featureId, item);
}
public boolean onContextItemSelected(MenuItem item) {
switch(item.getItemId()) {
case R.id.menu_delete:
AdapterContextMenuInfo info = (AdapterContextMenuInfo) item.getMenuInfo();
mDbHelper.deleteReminder(info.id);
fillData();
return true;
}
return super.onContextItemSelected(item);
}
private void createReminder() {
Intent i = new Intent(this, ReminderEditActivity.class);
startActivityForResult(i, ACTIVITY_CREATE);
}
protected void onActivityResult(int requestCode, int resultCode, Intent intent)
{
super.onActivityResult(requestCode, resultCode, intent);
fillData();
}
}
这是我使用意图调用的新活动。
package com.people.rock;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import com.android.taskreminder.R;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.app.DatePickerDialog;
import android.app.Dialog;
import android.app.TimePickerDialog;
import android.database.Cursor;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.DatePicker;
import android.widget.EditText;
import android.widget.TimePicker;
import android.widget.Toast;
@SuppressLint("SimpleDateFormat") public class ReminderEditActivity extends Activity {
// all variables are declared before content view and are initiated after it.
private Button mTimeButton;
private Button mDateButton;
private static final String DATE_FORMAT = "yyyy-MM-dd";
private static final String TIME_FORMAT = "hour-minute";
public static final String DATE_TIME_FORMAT = "yyyy-MM-dd kk:mm:ss";
private static final int DATE_PICKER_DIALOG = 0;
private static final int TIME_PICKER_DIALOG = 1;
Calendar mCalendar;
private EditText mTitleText;
private Button mConfirmButton;
private EditText mBodyText;
private RemindersDbAdapter mDbHelper;
private Long mRowId;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.reminder_edit);
registerButtonListnerAndSetDefaultText();
//variable initiation
mDbHelper = new RemindersDbAdapter(this);
mDateButton=(Button) findViewById(R.id.reminder_date);
mTimeButton=(Button) findViewById(R.id.reminder_time);
mCalendar = Calendar.getInstance();
mConfirmButton = (Button) findViewById(R.id.confirm);
mTitleText = (EditText) findViewById(R.id.title);
mBodyText = (EditText) findViewById(R.id.body);
//to check whether any previous state exist which might be closed by killing of activity
mRowId = savedInstanceState != null
? savedInstanceState.getLong(RemindersDbAdapter.KEY_ROWID)
: null;
}
//to set id if we got it from intent declared in reminder_list activity
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();
}
//to fill the form of existing id which was killed by android
@SuppressWarnings("deprecation")
private void populateFields() {
if (mRowId != null) {
Cursor reminder = mDbHelper.fetchReminder(mRowId);
startManagingCursor(reminder);
mTitleText.setText(reminder.getString(
reminder.getColumnIndexOrThrow(RemindersDbAdapter.KEY_TITLE)));
mBodyText.setText(reminder.getString(
reminder.getColumnIndexOrThrow(RemindersDbAdapter.KEY_BODY)));
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);
}
}
updateDateButtonText();
updateTimeButtonText();
}
@Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
outState.putLong(RemindersDbAdapter.KEY_ROWID, mRowId);
}
private void registerButtonListnerAndSetDefaultText(){
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();
}
});
mDateButton.setOnClickListener(new View.OnClickListener() {
@SuppressWarnings("deprecation")
@Override
public void onClick(View v) {
showDialog(DATE_PICKER_DIALOG);
}
});
mTimeButton.setOnClickListener(new View.OnClickListener() {
@SuppressWarnings("deprecation")
@Override
public void onClick(View v) {
showDialog(TIME_PICKER_DIALOG);
}
});
updateDateButtonText();
updateTimeButtonText();
}
@SuppressWarnings("deprecation")
protected Dialog onCreateDialog(int id) {
switch(id) {
case DATE_PICKER_DIALOG:
return showDatePicker();
case TIME_PICKER_DIALOG:
return showTimePicker();
}
return super.onCreateDialog(id);
}
@SuppressLint("SimpleDateFormat") 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;
}
private void updateDateButtonText() {
SimpleDateFormat dateFormat = new SimpleDateFormat(DATE_FORMAT);
String dateForButton = dateFormat.format(mCalendar.getTime());
mDateButton.setText(dateForButton);
}
private TimePickerDialog showTimePicker() {
TimePickerDialog timePicker = new TimePickerDialog(this, new TimePickerDialog.OnTimeSetListener() {
@SuppressLint("SimpleDateFormat") @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;
}
@SuppressLint("SimpleDateFormat") private void updateTimeButtonText() {
SimpleDateFormat TimeFormat = new SimpleDateFormat(TIME_FORMAT);
String timeForButton = TimeFormat.format(mCalendar.getTime());
mDateButton.setText(timeForButton);
}
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);
}
new ReminderManager(this).setReminder(mRowId, mCalendar);
}
}
这是我得到的错误
07-16 08:04:30.620: D/dalvikvm(945): GC_FOR_ALLOC freed 44K, 4% free 3054K/3168K, paused 35ms, total 38ms
07-16 08:04:30.630: I/dalvikvm-heap(945): Grow heap (frag case) to 3.520MB for 500416-byte allocation
07-16 08:04:30.670: D/dalvikvm(945): GC_FOR_ALLOC freed 2K, 4% free 3540K/3660K, paused 33ms, total 33ms
07-16 08:04:31.570: I/Choreographer(945): Skipped 38 frames! The application may be doing too much work on its main thread.
07-16 08:04:31.800: D/gralloc_goldfish(945): Emulator without GPU emulation detected.
07-16 08:11:32.195: D/dalvikvm(945): GC_FOR_ALLOC freed 56K, 4% free 3717K/3840K, paused 108ms, total 109ms
07-16 08:11:32.195: I/dalvikvm-heap(945): Grow heap (frag case) to 4.142MB for 474384-byte allocation
07-16 08:11:32.425: D/dalvikvm(945): GC_FOR_ALLOC freed 13K, 4% free 4167K/4304K, paused 53ms, total 53ms
07-16 08:11:32.685: D/AndroidRuntime(945): Shutting down VM
07-16 08:11:32.685: W/dalvikvm(945): threadid=1: thread exiting with uncaught exception (group=0xb3a62ba8)
07-16 08:11:32.715: E/AndroidRuntime(945): FATAL EXCEPTION: main
07-16 08:11:32.715: E/AndroidRuntime(945): Process: com.android.taskreminder, PID: 945
07-16 08:11:32.715: E/AndroidRuntime(945): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android.taskreminder/com.people.rock.ReminderEditActivity}: java.lang.NullPointerException
07-16 08:11:32.715: E/AndroidRuntime(945): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
07-16 08:11:32.715: E/AndroidRuntime(945): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
07-16 08:11:32.715: E/AndroidRuntime(945): at android.app.ActivityThread.access$800(ActivityThread.java:135)
07-16 08:11:32.715: E/AndroidRuntime(945): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
07-16 08:11:32.715: E/AndroidRuntime(945): at android.os.Handler.dispatchMessage(Handler.java:102)
07-16 08:11:32.715: E/AndroidRuntime(945): at android.os.Looper.loop(Looper.java:136)
07-16 08:11:32.715: E/AndroidRuntime(945): at android.app.ActivityThread.main(ActivityThread.java:5017)
07-16 08:11:32.715: E/AndroidRuntime(945): at java.lang.reflect.Method.invokeNative(Native Method)
07-16 08:11:32.715: E/AndroidRuntime(945): at java.lang.reflect.Method.invoke(Method.java:515)
07-16 08:11:32.715: E/AndroidRuntime(945): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-16 08:11:32.715: E/AndroidRuntime(945): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-16 08:11:32.715: E/AndroidRuntime(945): at dalvik.system.NativeStart.main(Native Method)
07-16 08:11:32.715: E/AndroidRuntime(945): Caused by: java.lang.NullPointerException
07-16 08:11:32.715: E/AndroidRuntime(945): at com.people.rock.ReminderEditActivity.registerButtonListnerAndSetDefaultText(ReminderEditActivity.java:121)
07-16 08:11:32.715: E/AndroidRuntime(945): at com.people.rock.ReminderEditActivity.onCreate(ReminderEditActivity.java:47)
07-16 08:11:32.715: E/AndroidRuntime(945): at android.app.Activity.performCreate(Activity.java:5231)
07-16 08:11:32.715: E/AndroidRuntime(945): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
07-16 08:11:32.715: E/AndroidRuntime(945): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
07-16 08:11:32.715: E/AndroidRuntime(945): ... 11 more
07-16 08:11:36.505: I/Process(945): Sending signal. PID: 945 SIG: 9
提前致谢
初始化之前，您正在函数中使用按钮侦听器（mConfirmButton,mDateButton,mTimeButton）registerButtonListnerAndSetDefaultText()。
所以registerButtonListnerAndSetDefaultText()在将按钮初始化为后调用
mDbHelper = new RemindersDbAdapter(this);
mDateButton=(Button) findViewById(R.id.reminder_date);
mTimeButton=(Button) findViewById(R.id.reminder_time);
mCalendar = Calendar.getInstance();
mConfirmButton = (Button) findViewById(R.id.confirm);
mTitleText = (EditText) findViewById(R.id.title);
mBodyText = (EditText) findViewById(R.id.body);
registerButtonListnerAndSetDefaultText(); // use here
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