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我想在加載器和內容提供者上練習我有一個學生數據庫，其中包含（name，degree）activity_main.xml中我有一個recyclerview和2個editText一個其他學位的名字我的目標是，當用戶點擊按鈕的名稱將出現在recyclerview ，這裏是我的logcat嘗試調用空對象上的虛擬方法
致命異常：主要 過程：com.example.abdelmagied.myapplication， PID：16160 java.lang.NullPointerException：試圖調用虛方法'void com.examp在com.example.abdelmagied.myapplication.MainActivity.onLoadFinished（MainActivity.java:97） com.example.abdelmagied上的null對象引用 上的「le.abdelmagied.myapplication.recyclerview.swapcursor（android.database.Cursor）」。 myapplication.MainActivity.onLoadFinished（MainActivity.java:17） 在android.support.v4.app.LoaderManagerImpl $ LoaderInfo.callOnLoadFinished（LoaderManager.java:476） 在android.support.v4.app.LoaderManagerImpl $ LoaderInfo.onLoadComplete（ LoaderManager.java:444） at android.support.v4.content.Loader.deliverResult（Loader.java:126） at android.support.v4.content.CursorLoader.deliverResult（CursorLoader.java:105） at android.support.v4.content.CursorLoader.deliverResult（CursorLoader.java:37） at android.support.v4.content.AsyncTaskLoader.dispatchOnLoadComplete（AsyncTaskLoader.java:255） at android.support.v4.content.AsyncTaskLoader $ LoadTask.onPostExecute（AsyncTaskLoader.java:80） at android.support.v4.content.ModernAsyncTask.finish（ModernAsyncTask.java:485） at android.support.v4.content.ModernAsyncTask $ InternalHandler.handleMessage（ModernAsyncTask.java ：502） at android.os.Handler.dispatchMessage（Handler.java:102） at android.os.Looper.loop（Looper.java:148） at android.app.ActivityThread.main（Activ ityThread.java:5417） at java.lang.reflect.Method.invoke（Native Method） at com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:726） at com.android.internal .os.ZygoteInit.main（ZygoteInit。Java的：616）
這是我的MainActivity類別
package com.example.abdelmagied.myapplication;
import android.content.ContentValues;
import android.database.Cursor;
import android.net.Uri;
import android.support.v4.app.LoaderManager;
import android.support.v4.content.CursorLoader;
import android.support.v4.content.Loader;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
public class MainActivity extends AppCompatActivity implements LoaderManager.LoaderCallbacks<Cursor>{
public static final String[] MAIN_STUDENT_PROJECTION = {
studentContract.StudentEntry._ID ,
studentContract.StudentEntry.COLUMN_NAME ,
studentContract.StudentEntry.COLUMN_DEGREE ,
};
public static final int INDEX_COLUMN_ID = 0;
public static final int INDEX_COLUMN_NAME = 1;
public static final int INDEX_COLUMN_DEGREE = 2;
private recyclerview recyclerAdapter;
private int mposition = RecyclerView.NO_POSITION;
public static final int ID_STUDNET_LOADER = 99;
// details of the recyclerview
RecyclerView recycler;
RecyclerView.Adapter myadapter;
RecyclerView.LayoutManager mymanager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// adjust the recyclerview
recycler = (RecyclerView) findViewById(R.id.recyclerid);
myadapter = new recyclerview(this);
mymanager = new LinearLayoutManager(this);
recycler.setAdapter(myadapter);
recycler.setLayoutManager(mymanager);
getSupportLoaderManager().initLoader(ID_STUDNET_LOADER , null , this);
}
@Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
switch(id)
{
case ID_STUDNET_LOADER :
Uri Query = studentContract.StudentEntry.CONTENT_URI;
return new CursorLoader(this ,
Query ,
MAIN_STUDENT_PROJECTION ,
null ,
null ,
null
);
default:
throw new RuntimeException("Loader is not implemented : " + id);
}
}
@Override
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
recyclerAdapter.swapcursor(data);
if(mposition == RecyclerView.NO_POSITION) mposition = 0;
recycler.smoothScrollToPosition(mposition);
}
@Override
public void onLoaderReset(Loader<Cursor> loader) {
recyclerAdapter.swapcursor(null);
}
public void addtodatabase(View view) {
EditText name = (EditText) findViewById(R.id.name);
EditText degree=(EditText) findViewById(R.id.degree);
// make a contentvalues and put name , degree , salary on it...
ContentValues values = new ContentValues();
values.put(studentContract.StudentEntry.COLUMN_NAME , name.getText().toString());
values.put(studentContract.StudentEntry.COLUMN_DEGREE , degree.getText().toString());
getContentResolver().insert(studentContract.StudentEntry.CONTENT_URI , values);
}
}
這裏的recyclerview適配器
package com.example.abdelmagied.myapplication;
import android.content.Context;
import android.database.Cursor;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
/**
* Created by AbdELMagied on 8/4/2017.
*/
public class recyclerview extends RecyclerView.Adapter<RecyclerView.ViewHolder> {
Context context;
private Cursor mycursor;
public recyclerview(Context context) {
this.context = context;
}
@Override
public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(context).inflate(R.layout.recyclerrow , parent , false);
return new viewholder(view);
}
@Override
public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
mycursor.move(position);
viewholder myholder = (viewholder) holder;
myholder.mytxt.setText(mycursor.getString(MainActivity.INDEX_COLUMN_NAME));
}
void swapcursor(Cursor mycurso)
{
this.mycursor = mycurso;
notifyDataSetChanged();
}
@Override
public int getItemCount() {
if(mycursor == null) return 0;
return mycursor.getCount();
}
public class viewholder extends RecyclerView.ViewHolder
{
public TextView mytxt;
public viewholder(View itemView) {
super(itemView);
mytxt = (TextView) itemView.findViewById(R.id.textView);
}
}
}
這裏是我的內容提供商
package com.example.abdelmagied.myapplication;
import android.content.ContentProvider;
import android.content.ContentUris;
import android.content.ContentValues;
import android.content.UriMatcher;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.net.Uri;
import android.support.annotation.Nullable;
import android.text.TextUtils;
/**
* Created by AbdELMagied on 8/4/2017.
*/
public class studentprovider extends ContentProvider {
databaseopenhelper mydatabase;
static final int STUDENT = 100;
static final int STUDENT_WITH_ID = 101;
private UriMatcher urimatcher = matcher();
private UriMatcher matcher() {
UriMatcher mymatcher = new UriMatcher(UriMatcher.NO_MATCH);
mymatcher.addURI(studentContract.authority, "student", STUDENT);
mymatcher.addURI(studentContract.authority, "student/#", STUDENT_WITH_ID);
return mymatcher;
}
@Override
public boolean onCreate() {
mydatabase = new databaseopenhelper(getContext());
return true;
}
@Nullable
@Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
Cursor returncursor;
switch (urimatcher.match(uri)) {
case STUDENT:
returncursor = mydatabase.getReadableDatabase().query(
studentContract.StudentEntry.TABLE_NAME,
projection,
selection,
selectionArgs,
null,
null,
sortOrder
);
break;
case STUDENT_WITH_ID:
String where = "_id = " + uri.getLastPathSegment() + (!TextUtils.isEmpty(selection)? " AND " + selection : "");
returncursor = mydatabase.getReadableDatabase().query(
studentContract.StudentEntry.TABLE_NAME,
projection,
where,
selectionArgs,
null,
null,
sortOrder
);
break;
default:
throw new UnsupportedOperationException("not found uri " + uri);
}
returncursor.setNotificationUri(getContext().getContentResolver() , uri);
return returncursor;
}
@Nullable
@Override
public String getType(Uri uri) {
return null;
}
@Nullable
@Override
public Uri insert(Uri uri, ContentValues values) {
long rowid = mydatabase.getWritableDatabase().insert(studentContract.StudentEntry.TABLE_NAME, null , values);
if(rowid > 0)
{
Uri _uri = ContentUris.withAppendedId(studentContract.StudentEntry.CONTENT_URI, rowid);
getContext().getContentResolver().notifyChange(_uri, null);
return _uri;
}
throw new SQLException("Faild to add a record " + uri);
}
@Override
public int delete(Uri uri, String selection, String[] selectionArgs) {
if(selection == "") selection= "1";
int numberofdeleted = 0;
switch (urimatcher.match(uri))
{
case STUDENT:
String where = "_id = " + uri.getLastPathSegment() + (!TextUtils.isEmpty(selection)? "AND " + selection : "");
numberofdeleted = mydatabase.getWritableDatabase().delete("students" , where , selectionArgs);
break;
default:
throw new UnsupportedOperationException("uri not found " + uri);
}
if(numberofdeleted != 0)
{
getContext().getContentResolver().notifyChange(uri , null);
}
return numberofdeleted;
}
@Override
public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
return 0;
}
}
這裏的數據庫類
package com.example.abdelmagied.myapplication;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
/**
* Created by AbdELMagied on 8/4/2017.
*/
public class databaseopenhelper extends SQLiteOpenHelper {
public databaseopenhelper(Context context) {
super(context, "student.db", null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
final String SQL_CREATE_STUDENT_TABLE =
"CREATE TABLE " + studentContract.StudentEntry.TABLE_NAME + " (" +
studentContract.StudentEntry._ID + " INTEGER PRIMARY KEY , " +
studentContract.StudentEntry.COLUMN_NAME + " TEXT , " +
studentContract.StudentEntry.COLUMN_DEGREE + " TEXT); " ;
db.execSQL(SQL_CREATE_STUDENT_TABLE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("drop database if exists student");
onCreate(db);
}
}
這裏的
package com.example.abdelmagied.myapplication;
import android.content.ContentProvider;
import android.content.ContentUris;
import android.content.ContentValues;
import android.content.UriMatcher;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.net.Uri;
import android.support.annotation.Nullable;
import android.text.TextUtils;
/**
* Created by AbdELMagied on 8/4/2017.
*/
public class studentprovider extends ContentProvider {
databaseopenhelper mydatabase;
static final int STUDENT = 100;
static final int STUDENT_WITH_ID = 101;
private UriMatcher urimatcher = matcher();
private UriMatcher matcher() {
UriMatcher mymatcher = new UriMatcher(UriMatcher.NO_MATCH);
mymatcher.addURI(studentContract.authority, "student", STUDENT);
mymatcher.addURI(studentContract.authority, "student/#", STUDENT_WITH_ID);
return mymatcher;
}
@Override
public boolean onCreate() {
mydatabase = new databaseopenhelper(getContext());
return true;
}
@Nullable
@Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
Cursor returncursor;
switch (urimatcher.match(uri)) {
case STUDENT:
returncursor = mydatabase.getReadableDatabase().query(
studentContract.StudentEntry.TABLE_NAME,
projection,
selection,
selectionArgs,
null,
null,
sortOrder
);
break;
case STUDENT_WITH_ID:
String where = "_id = " + uri.getLastPathSegment() + (!TextUtils.isEmpty(selection)? " AND " + selection : "");
returncursor = mydatabase.getReadableDatabase().query(
studentContract.StudentEntry.TABLE_NAME,
projection,
where,
selectionArgs,
null,
null,
sortOrder
);
break;
default:
throw new UnsupportedOperationException("not found uri " + uri);
}
returncursor.setNotificationUri(getContext().getContentResolver() , uri);
return returncursor;
}
@Nullable
@Override
public String getType(Uri uri) {
return null;
}
@Nullable
@Override
public Uri insert(Uri uri, ContentValues values) {
long rowid = mydatabase.getWritableDatabase().insert(studentContract.StudentEntry.TABLE_NAME, null , values);
if(rowid > 0)
{
Uri _uri = ContentUris.withAppendedId(studentContract.StudentEntry.CONTENT_URI, rowid);
getContext().getContentResolver().notifyChange(_uri, null);
return _uri;
}
throw new SQLException("Faild to add a record " + uri);
}
@Override
public int delete(Uri uri, String selection, String[] selectionArgs) {
if(selection == "") selection= "1";
int numberofdeleted = 0;
switch (urimatcher.match(uri))
{
case STUDENT:
String where = "_id = " + uri.getLastPathSegment() + (!TextUtils.isEmpty(selection)? "AND " + selection : "");
numberofdeleted = mydatabase.getWritableDatabase().delete("students" , where , selectionArgs);
break;
default:
throw new UnsupportedOperationException("uri not found " + uri);
}
if(numberofdeleted != 0)
{
getContext().getContentResolver().notifyChange(uri , null);
}
return numberofdeleted;
}
@Override
public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
return 0;
}
}
但沒有任何問題。儘管如此，我還是給你+1，以便在他們關於例外的問題中實際包含一個幸運的堆棧跟蹤。 – hoodaticus
你可能想在你的代碼中加入一些斷點來看看爲什麼NPE正在發生。看起來像它從97行onLoadFinished – DroiDev