使用 sqlite 数据库中的数据设置 ListView 时出现 NullPointerException答案

【问题标题】：NullPointerException while setting up ListView with data from sqlite database使用 sqlite 数据库中的数据设置 ListView 时出现 NullPointerException
【发布时间】：2014-05-27 13:06:29
【问题描述】：

我已经用 sqlHandler、DatabaseHandler 和一个带有 ContactListAdapter 和 contact.java 的 ListView 建立了一个数据库。

现在我在尝试启动应用程序时收到 Nullpointerexception。 Logcat 说没有 TABLE_CONTACTS，但是 DatabaseHandler 应该在 onCreate 设置它！

这里是错误：

05-27 14:46:51.316: E/SQLiteLog(17898): (1) no such table: TABLE_CONTACTS
05-27 14:46:51.336: E/AndroidRuntime(17898): FATAL EXCEPTION: main
05-27 14:46:51.336: E/AndroidRuntime(17898): Process: de.poetjan.powerconsumption, PID: 17898
05-27 14:46:51.336: E/AndroidRuntime(17898): java.lang.NullPointerException
05-27 14:46:51.336: E/AndroidRuntime(17898):    at de.poetjan.powerconsumption.Zahlerstand.showList(Zahlerstand.java:230)
05-27 14:46:51.336: E/AndroidRuntime(17898):    at de.poetjan.powerconsumption.Zahlerstand.onCreateView(Zahlerstand.java:67)
05-27 14:46:51.336: E/AndroidRuntime(17898):    at android.support.v4.app.Fragment.performCreateView(Fragment.java:1500)
05-27 14:46:51.336: E/AndroidRuntime(17898):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:927)
05-27 14:46:51.336: E/AndroidRuntime(17898):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)

这里是从我的 Fragment（head）到调用 showList()-Method 的代码。

public class Zahlerstand extends Fragment {

    int mYear3;
    int mMonth3;
    int mDay3; 
    EditText dateDisplay3;
    EditText kwhDisplay1;
    String neuformatjahr3;
    String neuformatmonat3;
    String neuformattag3;
    TextView test;
    String gesamt3o;
    Button pickdate;
    SqlHandler sqlHandler;
     ListView lvCustomList;
     EditText etName, etPhone;
     Button btnsubmit;

    @Override
     public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);

      sqlHandler = new SqlHandler(getActivity());


    }


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

        View rootView = inflater.inflate(R.layout.zahlerstand, container, false);
        kwhDisplay1 = (EditText) rootView.findViewById(R.id.kwhDisplay1);
        dateDisplay3 = (EditText) rootView.findViewById(R.id.dateDisplay3); 
        pickdate = (Button) rootView.findViewById(R.id.pickDate);
        lvCustomList = (ListView) rootView.findViewById(R.id.lv_custom_list);
          btnsubmit = (Button) rootView.findViewById(R.id.btn_submit);

          test = (TextView) rootView.findViewById(R.id.textViewverb);

          showList();
          registerForContextMenu(lvCustomList);
          btnsubmit.setOnClickListener(new OnClickListener() {

               @Override
               public void onClick(View v) {
             if (dateDisplay3.getText().toString().isEmpty() || kwhDisplay1.getText().toString().isEmpty()) {
                 Toast.makeText(getActivity(), R.string.beideeingeben, Toast.LENGTH_SHORT).show();
             }

             else {  
                String name = dateDisplay3.getText().toString();
                String phoneNo = kwhDisplay1.getText().toString();

                String query = "INSERT INTO TABLE_CONTACTS(id, name, phone_number) values ('"
                  + name + "','" + phoneNo + "')";
                sqlHandler.executeQuery(query);
                showList();

在发生错误的showList()方法的第230行：

private void showList() {

  ArrayList<Contact> contactList = new ArrayList<Contact>();
  contactList.clear();
  String query = "SELECT * FROM TABLE_CONTACTS ";
  Cursor c1 = sqlHandler.selectQuery(query);
  if (c1 != null && c1.getCount() != 0) {
   if (c1.moveToFirst()) {
    do {
     Contact contactListItems = new Contact();

     contactListItems.setID(c1
       .getColumnIndex("slno"));
     contactListItems.setName(c1.getString(c1
       .getColumnIndex("name")));
     contactListItems.setPhoneNumber(c1.getString(c1
       .getColumnIndex("phone")));
     contactList.add(contactListItems);

    } while (c1.moveToNext());
   }
  }
  c1.close();

  ContactListAdapter contactListAdapter = new ContactListAdapter(
    getActivity(), contactList);
  lvCustomList.setAdapter(contactListAdapter);

 }

为了确保问题不存在，sqlHandler.java 和 DatabaseHandler.java

SQLHandler：

public class SqlHandler {

     public static final String DATABASE_NAME = "MY_DATABASE";

     Context context;
     SQLiteDatabase sqlDatabase;
     DatabaseHandler dbHelper;

     public SqlHandler(Context context) {

      dbHelper = new DatabaseHandler(context);
      sqlDatabase = dbHelper.getWritableDatabase();
     }

     public void executeQuery(String query) {
      try {

       if (sqlDatabase.isOpen()) {
        sqlDatabase.close();
       }

       sqlDatabase = dbHelper.getWritableDatabase();
       sqlDatabase.execSQL(query);

      } catch (Exception e) {

       System.out.println("DATABASE ERROR " + e);
      }

     }

     public Cursor selectQuery(String query) {
      Cursor c1 = null;
      try {

       if (sqlDatabase.isOpen()) {
        sqlDatabase.close();

       }
       sqlDatabase = dbHelper.getWritableDatabase();
       c1 = sqlDatabase.rawQuery(query, null);

      } catch (Exception e) {

       System.out.println("DATABASE ERROR " + e);

      }
      return c1;

     }

    }

这里是DatabaseHandler.java（只有头部，所有的CRUD-Operations当然都在下面）：

public class DatabaseHandler extends SQLiteOpenHelper {

    private static final int DATABASE_VERSION = 1;
    private static final String DATABASE_NAME = "contactsManager";

    // Contacts table name
    private static final String TABLE_CONTACTS = "contacts";

    // Contacts Table Columns names
    private static final String KEY_ID = "id";
    private static final String KEY_NAME = "name";
    private static final String KEY_PH_NO = "phone_number";

    public DatabaseHandler(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    // Creating Tables
    @Override
    public void onCreate(SQLiteDatabase db) {
        String CREATE_CONTACTS_TABLE = "CREATE TABLE " + TABLE_CONTACTS + "("
                + KEY_ID + " INTEGER PRIMARY KEY," + KEY_NAME + " TEXT,"
                + KEY_PH_NO + " TEXT" + ")";
        db.execSQL(CREATE_CONTACTS_TABLE);
    }

    // Upgrading database
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // Drop older table if existed
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_CONTACTS);

        // Create tables again
        onCreate(db);
    }

最后是 Contact.java：

public class Contact {

    //private variables
    int _id;
    String _name;
    String _phone_number;

    // Empty constructor
    public Contact(){

    }
    // constructor
    public Contact(int id, String name, String _phone_number){
        this._id = id;
        this._name = name;
        this._phone_number = _phone_number;
    }

    // constructor
    public Contact(String name, String _phone_number){
        this._name = name;
        this._phone_number = _phone_number;
    }
    // getting ID
    public int getID(){
        return this._id;
    }

    // setting id
    public void setID(int id){
        this._id = id;
    }

    // getting name
    public String getName(){
        return this._name;
    }

    // setting name
    public void setName(String name){
        this._name = name;
    }

    // getting phone number
    public String getPhoneNumber(){
        return this._phone_number;
    }

    // setting phone number
    public void setPhoneNumber(String phone_number){
        this._phone_number = phone_number;
    }
}

最后但并非最不重要的是 ContactListAdapter.java：

public class ContactListAdapter extends BaseAdapter {

 Context context;
 ArrayList<Contact> contactList;

 public ContactListAdapter(Context context, ArrayList<Contact> list) {

  this.context = context;
  contactList = list;
 }

 @Override
 public int getCount() {

  return contactList.size();
 }

 @Override
 public Object getItem(int position) {

  return contactList.get(position);
 }

 @Override
 public long getItemId(int position) {

  return position;
 }

 @Override
 public View getView(int position, View convertView, ViewGroup arg2) {
     Contact contactListItems = contactList.get(position);

  if (convertView == null) {
   LayoutInflater inflater = (LayoutInflater) context
     .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
   convertView = inflater.inflate(R.layout.contact_list_row, null);

  }
  TextView tvSlNo = (TextView) convertView.findViewById(R.id.tv_slno);
  tvSlNo.setText(String.valueOf(contactListItems.getID()));
  TextView tvName = (TextView) convertView.findViewById(R.id.tv_name);
  tvName.setText(contactListItems.getName());
  TextView tvPhone = (TextView) convertView.findViewById(R.id.tv_phone);
  tvPhone.setText(contactListItems.getPhoneNumber());

  return convertView;
 }

}

【问题讨论】：

卸载您的应用并再次尝试运行。

标签： android sqlite android-listview nullpointerexception

【解决方案1】：

在showList() 的第 230 行中，输入 TABLE CONTACTS 作为表的名称，而在 DatabaseHandler 类中定义 TABLE_CONTACTS = "contacts" 并创建名称为 "contacts" 的表：

String CREATE_CONTACTS_TABLE = "CREATE TABLE " + TABLE_CONTACTS + "(" + KEY_ID + " INTEGER PRIMARY KEY," + KEY_NAME + " TEXT," + KEY_PH_NO + " TEXT" + ")";

因此，当您在showList() 中写入您的query 变量时，您需要确保它可以访问正确的表：

String query = "SELECT * FROM TABLE_CONTACTS ";

应该是：

String TABLE_CONTACTS = "contacts"
String query = "SELECT * FROM " + TABLE_CONTACTS;

你好像把变量名和表名弄混了。

【讨论】：