我一直在关注一个创建多个tablesin 的教程database，但是我遇到了一些错误并且没有成功解决它们。
这对我来说是新的，所以如果我犯了一些愚蠢的错误，请放轻松，但我想在这里学习。这些是classes我遇到错误的地方：
主要活动：
public class MainActivity extends FragmentActivity implements
CustomPersonDialogFragment.CustomPersonDialogFragmentListener{
private Fragment contentFragment;
private PersonListFragment personListFragment;
private ContentFragment addPersonFragment;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
FragmentManager fragmentManager = getSupportFragmentManager();
CategoryDAO categoryDAO = new CategoryDAO(this);
//Initially loads departments
if (categoryDAO.getCategories().size() <= 0)
categoryDAO.loadCategories();
/*
* This is called when orientation is changed.
*/
if (savedInstanceState != null) {
if (savedInstanceState.containsKey("content")) {
String content = savedInstanceState.getString("content");
if (content.equals(ContentFragment.ARG_ITEM_ID)) {
if (fragmentManager
.findFragmentByTag(ContentFragment.ARG_ITEM_ID) != null) {
setFragmentTitle(R.string.add_person);
contentFragment = fragmentManager
.findFragmentByTag(ContentFragment.ARG_ITEM_ID);
}
}
}
if (fragmentManager.findFragmentByTag(PersonListFragment.ARG_ITEM_ID) != null) {
personListFragment = (PersonListFragment) fragmentManager
.findFragmentByTag(PersonListFragment.ARG_ITEM_ID);
contentFragment = personListFragment;
}
} else {
personListFragment = new PersonListFragment();
setFragmentTitle(R.string.app_name);
switchContent(personListFragment, PersonListFragment.ARG_ITEM_ID);
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.action_add:
setFragmentTitle(R.string.add_person);
addPersonFragment = new ContentFragment();
switchContent(addPersonFragment, ContentFragment.ARG_ITEM_ID);
return true;
}
return super.onOptionsItemSelected(item);
}
@Override
protected void onSaveInstanceState(Bundle outState) {
if (contentFragment instanceof ContentFragment) {
outState.putString("content", ContentFragment.ARG_ITEM_ID);
} else {
outState.putString("content", ContentFragment.ARG_ITEM_ID);
}
super.onSaveInstanceState(outState);
}
/*
* We consider PersonListFragment as the home fragment and it is not added to
* the back stack.
*/
public void switchContent(Fragment fragment, String tag) {
FragmentManager fragmentManager = getSupportFragmentManager();
while (fragmentManager.popBackStackImmediate())
;
if (fragment != null) {
FragmentTransaction transaction = fragmentManager
.beginTransaction();
transaction.replace(R.id.content_frame, fragment, tag);
// Only PersonAddFragment is added to the back stack.
if (!(fragment instanceof PersonListFragment)) {
transaction.addToBackStack(tag);
}
transaction.commit();
contentFragment = fragment;
}
}
protected void setFragmentTitle(int resourseId) {
setTitle(resourseId);
getActionBar().setTitle(resourseId);
}
/*
* We call super.onBackPressed(); when the stack entry count is > 0. if it
* is instanceof PersonListFragment or if the stack entry count is == 0, then
* we prompt the user whether to quit the app or not by displaying dialog.
* In other words, from PersonListFragment on back press it quits the app.
*/
@Override
public void onBackPressed() {
FragmentManager fm = getSupportFragmentManager();
if (fm.getBackStackEntryCount() > 0) {
super.onBackPressed();
} else if (contentFragment instanceof PersonListFragment
|| fm.getBackStackEntryCount() == 0) {
//finish();
//Shows an alert dialog on quit
onShowQuitDialog();
}
}
public void onShowQuitDialog() {
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setCancelable(false);
builder.setMessage("Do You Want To Quit?");
builder.setPositiveButton(android.R.string.yes,
new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int id) {
finish();
}
});
builder.setNegativeButton(android.R.string.no,
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int id) {
dialog.cancel();
}
});
builder.create().show();
}
/*
* Callback used to communicate with PersonListFragment to notify the list adapter.
* Communication between fragments goes via their Activity class.
*/
@Override
public void onFinishDialog() {
if (personListFragment != null) {
personListFragment.updateView();
}
}
}
类别DAO：
public class CategoryDAO extends PersonDBDAO {
private static final String WHERE_ID_EQUALS = DataBaseHelper.ID_COLUMN
+ " =?";
public CategoryDAO(Context context) {
super(context);
}
public long save(Category category) {
ContentValues values = new ContentValues();
values.put(DataBaseHelper.NAME_COLUMN, category.getName());
return database.insert(DataBaseHelper.CATEGORY_TABLE, null, values);
}
public long update(Category category) {
ContentValues values = new ContentValues();
values.put(DataBaseHelper.NAME_COLUMN, category.getName());
long result = database.update(DataBaseHelper.CATEGORY_TABLE, values,
WHERE_ID_EQUALS,
new String[] { String.valueOf(category.getId()) });
Log.d("Update Result:", "=" + result);
return result;
}
public int deleteCategory(Category category) {
return database.delete(DataBaseHelper.CATEGORY_TABLE,
WHERE_ID_EQUALS, new String[] { category.getId() + "" });
}
public List<Category> getCategories() {
List<Category> categories = new ArrayList<Category>();
Cursor cursor = database.query(DataBaseHelper.CATEGORY_TABLE,
new String[] { DataBaseHelper.ID_COLUMN,
DataBaseHelper.NAME_COLUMN }, null, null, null, null,
null);
while (cursor.moveToNext()) {
Category category = new Category();
category.setId(cursor.getInt(0));
category.setName(cursor.getString(1));
categories.add(category);
}
return categories;
}
public void loadCategories() {
Category movies = new Category("Movies");
Category music = new Category("Music");
Category games = new Category("Games");
Category books = new Category("Books");
Category food = new Category("Food");
List<Category> departments = new ArrayList<Category>();
departments.add(movies);
departments.add(music);
departments.add(games);
departments.add(books);
departments.add(food);
for (Category dept : departments) {
ContentValues values = new ContentValues();
values.put(DataBaseHelper.NAME_COLUMN, dept.getName());
database.insert(DataBaseHelper.CATEGORY_TABLE, null, values);
}
}
}
这是我得到的错误：
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: FATAL EXCEPTION: main
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: Process: com.dusandimitrijevic.dontforget, PID: 19544
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dusandimitrijevic.dontforget/com.dusandimitrijevic.dontforget.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor android.database.sqlite.SQLiteDatabase.query(java.lang.String, java.lang.String[], java.lang.String, java.lang.String[], java.lang.String, java.lang.String, java.lang.String)' on a null object reference
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2693)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.ActivityThread.access$900(ActivityThread.java:177)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.os.Looper.loop(Looper.java:145)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5942)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:372)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor android.database.sqlite.SQLiteDatabase.query(java.lang.String, java.lang.String[], java.lang.String, java.lang.String[], java.lang.String, java.lang.String, java.lang.String)' on a null object reference
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at helper.CategoryDAO.getCategories(CategoryDAO.java:51)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at com.dusandimitrijevic.dontforget.MainActivity.onCreate(MainActivity.java:38)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.Activity.performCreate(Activity.java:6289)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.ActivityThread.access$900(ActivityThread.java:177)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.os.Looper.loop(Looper.java:145)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5942)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:372)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)
10-28 18:42:30.868 19544-19544/com.dusandimitrijevic.dontforget E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)