我是android开发和sqlite的新手。当我尝试将对象插入数据库时，它会在getWritabledatabase()中抛出空指针异常。我不知道我在哪里弄错了。这是我的代码：
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import java.util.ArrayList;
import java.util.List;
public class Task_DB {
SQLiteDatabase sqLiteDatabase;
private Database_SF_APP dbHelper;
Context context;
public Task_DB(Context context){
dbHelper=new Database_SF_APP(context);
}
public void insert(Model_Task_List student) {
//Open connection to write data
Database_SF_APP dbHelper=new Database_SF_APP(context);
SQLiteDatabase db = dbHelper.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(Model_Task_List.KEY_username, student.getUserName());
values.put(Model_Task_List.KEY_subject,student.getSubject());
values.put(Model_Task_List.KEY_task, student.getTaskStatus());
// Inserting Row
db.insert(Model_Task_List.KEY_table, null, values);
db.close();
// Closing database connection
}
public void close() {
dbHelper.close();
}
public void delete(int student_Id) {
SQLiteDatabase db = dbHelper.getWritableDatabase();
// It's a good practice to use parameter ?, instead of concatenate string
db.delete(Model_Task_List.KEY_table, Model_Task_List.KEY_id + "= ?", new String[] { String.valueOf(student_Id) });
db.close(); // Closing database connection
}
public void update(Model_Task_List student) {
SQLiteDatabase db = dbHelper.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(Model_Task_List.KEY_username, student.getUserName());
values.put(Model_Task_List.KEY_subject,student.getSubject());
values.put(Model_Task_List.KEY_task, student.getTaskStatus());
// It's a good practice to use parameter ?, instead of concatenate string
db.update(Model_Task_List.KEY_table, values,Model_Task_List.KEY_id + "= ?", new String[] { String.valueOf(student.getId()) });
db.close(); // Closing database connection
}
public List<Model_Task_List> getStudentList() {
//Open connection to read only
SQLiteDatabase db = dbHelper.getReadableDatabase();
String selectQuery = "SELECT " +
Model_Task_List.KEY_id + "," +
Model_Task_List.KEY_username + "," +
Model_Task_List.KEY_subject + "," +
Model_Task_List.KEY_task +
" FROM " + Model_Task_List.KEY_table;
//Student student = new Student();
List<Model_Task_List>listobj=new ArrayList<Model_Task_List>();
Cursor cursor = db.rawQuery(selectQuery, null);
// looping through all rows and adding to list
if (cursor.moveToFirst()) {
while (cursor.moveToNext()) {
Model_Task_List modelobj=new Model_Task_List();
modelobj.setId(Integer.parseInt(cursor.getString(0)));
modelobj.setUserName(cursor.getString(1));
modelobj.setSubject(cursor.getString(2));
modelobj.setTaskStatus(cursor.getString(3));
listobj.add(modelobj);
}
}
return listobj;
}
}
这是数据库助手类：
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class Database_SF_APP extends SQLiteOpenHelper {
//version number to upgrade database version
//each time if you Add, Edit table, you need to change the
//version number.
Context context;
private static final int DATABASE_VERSION = 4;
// Database Name
private static final String DATABASE_NAME = "crud.db";
public Database_SF_APP(Context context ) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
//All necessary tables you like to create will create here
String CREATE_TABLE_STUDENT = "CREATE TABLE " + Model_Task_List.KEY_table + "("
+ Model_Task_List.KEY_id + " INTEGER PRIMARY KEY AUTOINCREMENT ,"
+ Model_Task_List.KEY_username + " TEXT, "
+ Model_Task_List.KEY_subject + " TEXT, "
+ Model_Task_List.KEY_task + " TEXT )";
db.execSQL(CREATE_TABLE_STUDENT);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// Drop older table if existed, all data will be gone!!!
db.execSQL("DROP TABLE IF EXISTS " + Model_Task_List.KEY_table);
// Create tables again
onCreate(db);
}
}
这是我插入数据库的片段：
public class Task extends Fragment {
private static final String MY_PREFERENCE_KEY = "yogan";
private List<Model_Task_List> model_task_lists;
//Creating Views
String currentDateTimeString;
Context context;
SharedPreferences.Editor editor;
private RecyclerView recyclerView;
Task_List_Adapter taskadapter;
private RecyclerView.LayoutManager layoutManager;
SharedPreferences sharedPreferences;
private RecyclerView.Adapter adapter;
RequestQueue yog;
Task_DB taskobj = new Task_DB(getContext());
String yogan;
AppController app;
RequestQueue queue;
String Url;
Task_DB task_db=new Task_DB(getActivity());
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_task, container, false);
recyclerView = (RecyclerView) view.findViewById(R.id.my_recycler_view);
LinearLayoutManager layoutManager = new LinearLayoutManager(getContext());
layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
recyclerView.setItemAnimator(new DefaultItemAnimator());
if (model_task_lists == null) {
model_task_lists = new ArrayList<Model_Task_List>();
}
recyclerView.setLayoutManager(layoutManager);
/* sharedPreferences = PreferenceManager.getDefaultSharedPreferences(getContext());
if(sharedPreferences.contains("yogan")) {
editor = sharedPreferences.edit();
editor.putBoolean("yog", true);
String jsons = sharedPreferences.getString(MY_PREFERENCE_KEY, "");
Gson gsong = new Gson();
Type type = new TypeToken<ArrayList<Model_Task_List>>() {
}.getType();
model_task_lists = gsong.fromJson(jsons, type);
*/
taskadapter=new Task_List_Adapter(model_task_lists,getActivity());
recyclerView.setAdapter(taskadapter);
ConnectivityManager cn = (ConnectivityManager) getActivity().getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo nf = cn.getActiveNetworkInfo();
{
Toast.makeText(getActivity(), "Network Available", Toast.LENGTH_LONG).show();
/* SharedPreferences shared=getPreferenceManager().getDefaultSharedPreferences(getActivity().getBaseContext());
String yog=shared.getString("example_text","");
if(yog!=null){
Intent intent=new Intent(getActivity(),LoginActivity.class);
startActivity(intent);
}*/
SharedPreferences sharedPreferences = getActivity().getSharedPreferences(LoginActivity.login, 0);
yogan = sharedPreferences.getString("user_id", null);
queue= Volley.newRequestQueue(getContext());
Url = "http://xxx.xx.xx.x/xxxs/GetActivitiesByUserID.svc/getlist/GetTask/864" ;
//Creating a json array request
JsonObjectRequest jsonArrayRequest = new JsonObjectRequest(Request.Method.GET, Url, new JSONObject(),
new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
String server_response = response.toString();
try {
JSONObject json_object = new JSONObject(server_response);
JSONArray json_array = new JSONArray(json_object.getString("GetTaskResult"));
for (int i = 0; i < json_array.length(); i++) {
Model_Task_List modelobj = new Model_Task_List();
JSONObject json_arrayJSONObject = json_array.getJSONObject(i);
modelobj.setSubject(json_arrayJSONObject.getString("Subject"));
modelobj.setUserName(json_arrayJSONObject.getString("UserName"));
modelobj.setTaskStatus(json_arrayJSONObject.getString("TaskStatus")) ;
model_task_lists.add(modelobj);
task_db.insert(modelobj);
}
//Finally initializing our adapter
} catch (JSONException e) {
e.printStackTrace();
}
}
},
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Toast.makeText(getContext(), error.toString(), Toast.LENGTH_SHORT).show();
Log.e("yog", error.toString());
}
});
//Creating request queue
queue.add(jsonArrayRequest);
}
return view;
}
@Override
public void onResume() {
super.onResume();
}
@Override
public void onStop() {
//queue.stop();
/* SharedPreferences sharedpref = PreferenceManager.getDefaultSharedPreferences(getContext());
editor = sharedpref.edit();
Gson gson = new Gson();
String god = gson.toJson(model_task_lists);
editor.putBoolean("yogan", true);
editor.putString(MY_PREFERENCE_KEY, god);
editor.commit();
*/
super.onStop();
}
}
这是我的logcat：
ntime: FATAL EXCEPTION: main
java.lang.NullPointerException
at xx.xx.Task_DB.insert(Task_DB.java:27)
at xxx.xxx.Task$1.onResponse(Task.java:128)
at pxx.xxx.Task$1.onResponse(Task.java:111)
at com.android.volley.toolbox.JsonRequest.deliverResponse(JsonRequest.java:72)
at com.android.volley.ExecutorDelivery$ResponseDeliveryRunnable.run(ExecutorDelivery.java:99)
at android.os.Handler.handleCallback(Handler.java:733)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5113)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:609)
at dalvik.system.NativeStart.main(Native Method)
当我尝试插入时，它会抛出空指针异常。
答案 0 :(得分：0)
您的context成员变量未初始化，但您正在将其提供给SQLiteOpenHelper构造函数，如下所示：
Database_SF_APP dbHelper=new Database_SF_APP(context);
与许多其他API相反，上下文仅在您调用时使用，例如帮助者getWritableDatabase()。