Android 如何使用带有片段的数据库
我使用的Android 如何使用带有片段的数据库,android,sqlite,fragment,android-sqlite,Android,Sqlite,Fragment,Android Sqlite,我使用的活动管理器选项卡布局有三个片段。我在TabtOneFragment中调用类Db extendsSQLiteHelper,但它不工作,并在Db文件的第行显示此错误 SQLiteDatabase db = getWritableDatabase(); 我使用的是Db=newdb(getActivity())以获取上下文,但它也不起作用。任何帮助都会非常有用,谢谢 下面是Db类: package com.example.hismartversearch; import android.c
活动
管理器选项卡布局有三个片段。我在TabtOneFragment
中调用类Db extendsSQLiteHelper
,但它不工作,并在Db文件的第行显示此错误
SQLiteDatabase db = getWritableDatabase();
我使用的是Db=newdb(getActivity())代码>以获取上下文,但它也不起作用。任何帮助都会非常有用,谢谢
下面是Db类:
package com.example.hismartversearch;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class Db extends SQLiteOpenHelper {
public Db(Context context) {
super(context, "Db.sqlite", null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
}
public Cursor getdata(String sql) {
// error at this line below
SQLiteDatabase db = getWritableDatabase();
Cursor c = db.rawQuery(sql, null);
return c;
}
public void querydata(String sql) {
SQLiteDatabase db = getWritableDatabase();
db.execSQL(sql);
}
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
public void delete_table(String table_name) {
SQLiteDatabase db = getWritableDatabase();
db.execSQL("delete from " + table_name);
}
public void delete_row(String tenbang, String tencot, String giatri) {
SQLiteDatabase db = getWritableDatabase();
db.delete(tenbang, tencot + "=" + giatri, null);
}
}
以下是TabOneFragment:
public class TabOneFragment extends Fragment implements SearchView.OnQueryTextListener {
Db db = new Db(getActivity());
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.tab_one_fragment, container, false);
recyclerView = (RecyclerView) view.findViewById(R.id.recyclerview);
final RecyclerView.LayoutManager mLayoutManager = new GridLayoutManager(getActivity(), 2);
recyclerView.setLayoutManager(mLayoutManager);
recyclerView.addItemDecoration(new GridSpacingItemDecoration(2, dpToPx(5), true));
recyclerView.setItemAnimator(new DefaultItemAnimator());
recyclerView.setAdapter(adapter);
return view;
}
@Override
public boolean onQueryTextSubmit(String query) {
return false;
}
@Override
public boolean onQueryTextChange(String newText) {
return false;
}
// error at line in logcat when I create table.
public void LoadDtaWeb() {
db.querydata("Create table if not exists tbl_mon_app (_ID integer primary key, IDMon integer not null, IDCH integer not null, TenMon text not null, Gia text not null, ImgUrl text not null, ImgLocal text)");
}
}
//some text hidden to save space display
这里是Logcat:
12-15 13:15:30.763 29533-29533/com.example.hieul.hismartversearch E/AndroidRuntime:致命异常:main
进程:com.example.hieul.hismartversearch,PID:29533
java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String,int,android.database.sqlite.SQLiteDatabase$CursorFactory,android.database.DatabaseErrorHandler)”
位于android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:223)
位于android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:163)
位于com.example.hieul.hismartversearch.Db.querydata(Db.java:33)
位于com.example.hieul.hismartversearch.TabOneFragment.LoadDtaWeb(TabOneFragment.java:309)
位于com.example.hieul.hismartversearch.TabOneFragment.onCreateView(TabOneFragment.java:131)
位于android.support.v4.app.Fragment.performCreateView(Fragment.java:2261)
位于android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1419)
在android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1750)
位于android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1819)
位于android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:797)
位于android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2590)
在android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2377)
在android.support.v4.app.FragmentManagerImpl.removedandtoperationsandexecute(FragmentManager.java:2332)
位于android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:2209)
位于android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:649)
在android.support.v4.app.FragmentStatePagerAdapter.finishUpdate(FragmentStatePagerAdapter.java:167)上
位于android.support.v4.view.ViewPager.populate(ViewPager.java:1238)
位于android.support.v4.view.ViewPager.populate(ViewPager.java:1086)
在android.support.v4.view.ViewPager.onMeasure上(ViewPager.java:1616)
在android.view.view.measure(view.java:20151)
位于android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6328)
位于android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:719)
位于android.support.design.widget.HeaderScrollingViewBehavior.onMeasureChild(HeaderScrollingViewBehavior.java:91)
位于android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onMeasureChild(AppBarLayout.java:1361)
位于android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:789)
在android.view.view.measure(view.java:20151)
位于android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6328)
public class TabOneFragment extends Fragment implements SearchView.OnQueryTextListener {
public Db db ;
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.tab_one_fragment, container, false);
db = new Db(getActivity());
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(CREATE_TABLE_STATEMENT);
}
public static final String TABLE_NAME = "table_name";
String CREATE_TABLE_SAMPLE = "create table " + TABLE_NAME + " (" + COLUMN_XXX_ID + " UNIQUE,"
+ COLUMN_XXX_NAME + "," + COLUMN_XXXX +
"," + COLUMN_XXXXX + ");";
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(CREATE_TABLE_SAMPLE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
onCreate(db);
}