Question

我开始使用片段，因为我习惯使用活动，我只想把事情带到另一个层面。到目前为止我学到的是在片段之间传递数据，现在我想尝试做一些包含数据库的事情。

这只是一个示例应用程序，它将向我展示如何处理这类内容。

所以基本上我有一个用于添加联系人的片段和第二个片段，用于在列表视图中显示联系人的名字。我在数据库中添加联系人，该部分工作正常。但是当我尝试在包含listview的片段内的数据库中显示数据时，我的应用程序会崩溃。

以下是代码：

public class MainActivity extends AppCompatActivity implements Contacts_Add.sendData {

DatabaseHandler handler;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    handler = new DatabaseHandler(this, null, null, 1);

    //____________________________PRINT DATABASE_____________________

    FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
    fragmentTransaction.replace(R.id.fragment_container, new Contacts_All(), "allFrag");
    fragmentTransaction.commit();

    getSupportFragmentManager().executePendingTransactions();

    Contacts_All fragAll = (Contacts_All) getSupportFragmentManager().findFragmentByTag("allFrag");
    fragAll.printDatabase();

    //______________________________________________________________________

//etValues method is outside of onCreate I just didnt put the whole code.

  @Override 
public void etValues(String fname, String lname, String phone, String email) {

    ContactsObjects contact = new ContactsObjects(fname, lname, phone, email);

    handler.insertContact(contact);

    FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
    fragmentTransaction.replace(R.id.fragment_container, new Contacts_All(), "allFrag");
    fragmentTransaction.commit();

    getSupportFragmentManager().executePendingTransactions();

    Contacts_All fragAll = (Contacts_All) getSupportFragmentManager().findFragmentByTag("allFrag");
    fragAll.printDatabase();


}

所有联系人片段：

public class Contacts_All extends ListFragment {


DatabaseHandler handler;

public Contacts_All() {
    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    View view = inflater.inflate(R.layout.contacts_all, container, false);


    return view;
}

public void printDatabase() {

    List<ContactsObjects> contactsList = handler.getAllContacts();
    ArrayList<ContactsObjects> newContactsList = new ArrayList<ContactsObjects>();
    for (ContactsObjects cn : contactsList) {

        newContactsList.add(cn);

    }

    ListAdapter la = new ArrayAdapter<ContactsObjects>(getActivity(), android.R.layout.simple_list_item_1, newContactsList);

    setListAdapter(la);


}


}

数据库处理器：

 public List<ContactsObjects> getAllContacts() {
    List<ContactsObjects> contactList = new ArrayList<ContactsObjects>();

    String selectQuery = "SELECT  * FROM " + TABLE_CONTACTS;

    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);


    if (cursor.moveToFirst()) {
        do {
            ContactsObjects contact = new ContactsObjects();
            contact.set_id(Integer.parseInt(cursor.getString(cursor.getColumnIndex("_id"))));
            contact.setFirstName(cursor.getString(cursor.getColumnIndex("firstname")));
            contact.setLastName(cursor.getString(cursor.getColumnIndex("lastname")));
            contact.setPhoneNumber(cursor.getString(cursor.getColumnIndex("phonenumber")));
            contact.setEmail(cursor.getString(cursor.getColumnIndex("email")));

            contactList.add(contact);
        } while (cursor.moveToNext());
    }


    return contactList;
}

logcat错误：第一个错误出现在这一行：

 List<ContactsObjects> contactsList = handler.getAllContacts();

第二个错误出现在这一行：

  fragAll.printDatabase(); //in onCreate

Answer 1

在listfragment（Contacts_All类）的printDatabase（）方法中，您的处理程序永远不会被初始化。您缺少这一行：

handler = new DatabaseHandler(this, null, null, 1);

如何使用片段内的数据库数据填充listview？

1 个答案: