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我已經遇到了一個與我的Android開發有關的真氣問題 - 每當我呼叫startActivityForResult(this, Notepadv3.class);，我的應用程序skipps直接在它上面，而不是啓動新的活動，也不會返回任何結果。就好像代碼不在那裏！Android正在跳過startActivityForResult，拋出異常
當我改變this到mContext與NullPointerException（I定義mContext如在類的開頭Context mContext;），應用程序崩潰。
我在不同的類中使用完全相同的代碼佈局，並且運行完美無缺。 我已驗證我正確地宣佈了清單中的活動。
我已經花了幾個小時搜索stackoverflow的答案，以及查找無數如何做這個特定的活動的例子，無濟於事。我正在學習如何編寫Android應用程序，因此使用Google的記事本教程來開發我的應用程序。感謝您提前給予的大力幫助！
代碼和堆棧跟蹤如下：
NoteEdit.java：（我已經脫脂的可讀性的緣故一些無關緊要的代碼），而不是推出新的意圖，從一個啓動的活動，從另一個活動
public class NoteEdit extends Activity implements OnClickListener {
Context mContext;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mDbHelper = new NotesDbAdapter(this);
mDbHelper.open();
setContentView(R.layout.note_edit);
setTitle(R.string.edit_item);
}
@Override
public void onActivityResult(int requestCode, int resultCode, Intent intent) {
if (requestCode == REQUEST_BARCODE) {
if (resultCode == RESULT_OK) {
barcode = (intent.getStringExtra("SCAN_RESULT"));
new updateBarcodeField().execute("");
} else if (resultCode == RESULT_CANCELED) {
}
}else if (requestCode == REQUEST_NEW) {
System.out.println("REQUEST_NEW onActivityResult().");
}
}
@Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
saveState();
outState.putSerializable(NotesDbAdapter.KEY_ROWID, mRowId);
}
@Override
protected void onPause() {
super.onPause();
saveState();
}
private void saveState(){
System.out.println("saveState()");
doorCall();
mDbHelper.open();
String title = mTitleText.getText().toString();
String barcode = mBarcodeText.getText().toString();
String price = mPriceText.getText().toString();
Intent requestNew = new Intent(mContext, Notepadv3.class);
if (returnCode == 1){
System.out.println("returnCode is 1. Calling validateFields()...");
String errors = validateFields(title);
if (errors.length() > 0) {
Toast.makeText(this, "Oops! Need a title!", duration).show();
System.out.println("Calling Notepadv3...");
startActivityForResult(requestNew, REQUEST_NEW);
}
}
if (returnCode == 2){
System.out.println("returnCode == 2 - Cancelling activity");
}else{
if (title.matches("")){
System.out.println("Variable is null. Returning...");
doorCall();
mDbHelper.close();
return;
}else{
System.out.println("Checking mRowId for null");
if (mRowId == null) {
Toast.makeText(this, "Success, product saved successfully", duration).show();
System.out.println("Switching activity to 'NotesDbAdapter'");
long id = mDbHelper.createNote(title, barcode, price);
mRowId = id;
}else{
System.out.println("mRowId is not null. Calling updateNote");
mDbHelper.updateNote(mRowId, title, barcode, price);
}
}
}
System.out.println("saveState() Finished");
if (doorCall()==true){
System.out.println("Database is Open");
}else{
System.out.println("Database is Closed");
}
}
}
Notepadv3.java:
public class Notepadv3 extends ListActivity implements OnClickListener {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.notes_list);
mDbHelper = new NotesDbAdapter(this);
mDbHelper.open();
mAddButton = (Button) findViewById(R.id.addButton);
mAddButton.setOnClickListener(this);
mScanButton = (Button) findViewById(R.id.scanButtonList);
mScanButton.setOnClickListener(this);
fillData();
registerForContextMenu(getListView());
}
@SuppressWarnings("deprecation")
private void fillData() {
Cursor notesCursor = mDbHelper.fetchAllNotes();
startManagingCursor(notesCursor);
// Create an array to specify the fields we want to display in the list (only TITLE)
String[] from = new String[]{NotesDbAdapter.KEY_TITLE, NotesDbAdapter.KEY_PRICE};
// and an array of the fields we want to bind those fields to (in this case just text1)
int[] to = new int[]{R.id.text1, R.id.text2};
// Now create a simple cursor adapter and set it to display
SimpleCursorAdapter notes =
new SimpleCursorAdapter(this, R.layout.notes_row, notesCursor, from, to);
setListAdapter(notes);
}
public void onClick(View v) {
switch (v.getId()) {
case R.id.addButton:
createNote();
break;
case R.id.scanButtonList:
Intent intent = new Intent("com.google.zxing.client.android.SCAN");
intent.putExtra("SCAN_MODE", "ONE_D_MODE");
startActivityForResult(intent, REQUEST_BARCODE);
}
}
@Override
public void onCreateContextMenu(ContextMenu menu, View v,
ContextMenuInfo menuInfo) {
super.onCreateContextMenu(menu, v, menuInfo);
menu.add(0, DELETE_ID, 0, R.string.menu_delete);
}
@Override
public boolean onContextItemSelected(MenuItem item) {
switch(item.getItemId()) {
case DELETE_ID:
AdapterContextMenuInfo info = (AdapterContextMenuInfo) item.getMenuInfo();
mDbHelper.deleteNote(info.id);
fillData();
return true;
}
return super.onContextItemSelected(item);
}
public void createNote() {
Intent i = new Intent(this, NoteEdit.class);
startActivityForResult(i, ACTIVITY_CREATE);
}
private void viewNote() {
System.out.println("viewNote(). Starting NoteEdit...");
Intent i = new Intent(this, NoteView.class);
i.putExtra(NotesDbAdapter.KEY_BARCODE, barcode);
startActivityForResult(i, ACTIVITY_EDIT);
}
@Override
public void onListItemClick(ListView l, View v, int position, long id) {
super.onListItemClick(l, v, position, id);
Intent i = new Intent(this, NoteEdit.class);
i.putExtra(NotesDbAdapter.KEY_ROWID, id);
startActivityForResult(i, ACTIVITY_EDIT);
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent intent) {
super.onActivityResult(requestCode, resultCode, intent);
System.out.println("Notepadv3. requestCode is " + requestCode + ", and resultCode is " + resultCode);
if (requestCode == REQUEST_BARCODE) {
if (resultCode == RESULT_OK) {
barcode = (intent.getStringExtra("SCAN_RESULT"));
System.out.println("Calling viewNote()...");
viewNote();
} else if (resultCode == RESULT_CANCELED) {
} else if (resultCode == RESULT_FIRST_USER) {
}
}else if (requestCode == REQUEST_NEW){
System.out.println("Notepadv3. requestCode is REQUEST_NEW. Calling createNote()...");
createNote();
}
super.onActivityResult(requestCode, resultCode, intent);
fillData();
}
}
Stack trace:
FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to pause activity {com.android.demo.notepad3/com.android.demo.notepad3.NoteEdit}: java.lang.NullPointerException
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:2706)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:2662)
at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:2640)
at android.app.ActivityThread.access$800(ActivityThread.java:123)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1158)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4424)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at android.content.ComponentName.<init>(ComponentName.java:75)
at android.content.Intent.<init>(Intent.java:3148)
at com.android.demo.notepad3.NoteEdit.saveState(NoteEdit.java:206)
at com.android.demo.notepad3.NoteEdit.onPause(NoteEdit.java:187)
at android.app.Activity.performPause(Activity.java:4590)
at android.app.Instrumentation.callActivityOnPause(Instrumentation.java:1195)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:2693)
爲什麼你需要mContext？你已經在活動中，所以爲什麼不使用Activity_name.this本身 – pixelscreen 2012-08-17 10:59:26
什麼是doorCall（）;有可能在saveState（）內有一些變量爲空。將變量打印到logcat並查看返回的內容null – pixelscreen 2012-08-17 11:01:30
另外，在'onPause（）中開始一個新的活動對我來說看起來很可疑。 （saveState（）在onPause（）中調用） – 2012-08-17 11:03:52