Question

我正在使用包含三个片段的Activity经理标签布局。我在SQLiteHelper中调用了类Db扩展TabtOneFragment，但它无效，并在Db文件中的Line处显示此错误。

 SQLiteDatabase db = getWritableDatabase();

我使用Db db = new Db(getActivity());获取上下文，但它也无法正常工作。任何帮助都会非常有帮助，拜托！

这是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);
       }
    }

这是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

这是Logcat：

12-15 13：15：30.763 29533-29533 / com.example.hieul.hismartversearch E / AndroidRuntime：FATAL EXCEPTION：main 过程：com.example.hieul.hismartversearch，PID：29533 java.lang.NullPointerException：尝试调用虚方法＆＃39; android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase（java.lang.String，int，android.database.sqlite.SQLiteDatabase $ CursorFactory，android。 database.DatabaseErrorHandler）＆＃39;在null对象引用上在android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked（SQLiteOpenHelper.java:223）在android.database.sqlite.SQLiteOpenHelper.getWritableDatabase（SQLiteOpenHelper.java:163）在com.example.hieul.hismartversearch.Db.querydata（Db.java:33）在com.example.hieul.hismartversearch.TabOneFragment.LoadDtaWeb（TabOneFragment.java:309）在com.example.hieul.hismartversearch.TabOneFragment.onCreateView（TabOneFragment.java:131）在android.support.v4.app.Fragment.performCreateView（Fragment.java:2261）在android.support.v4.app.FragmentManagerImpl.moveToState（FragmentManager.java:1419）在android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState（FragmentManager.java:1750）在android.support.v4.app.FragmentManagerImpl.moveToState（FragmentManager.java:1819）在android.support.v4.app.BackStackRecord.executeOps（BackStackRecord.java:797）在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.removeRedundantOperationsAndExecute（FragmentManager.java:2332）在android.support.v4.app.FragmentManagerImpl.execSingleAction（FragmentManager.java:2209）在android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss（BackStackRecord.java:649）在android.support.v4.app.FragmentStatePagerAdapter.finishUpdate（FragmentStatePagerAdapter.java:167）在android.support.v4.view.ViewPager.populate（ViewPager.java:1238）在android.support.v4.view.ViewPager.populate（ViewPager.java:1086）在android.support.v4.view.ViewPager.onMeasure（ViewPager.java:1616）在android.view.View.measure（View.java:20151）在android.view.ViewGroup.measureChildWithMargins（ViewGroup.java:6328）在android.support.design.widget.CoordinatorLayout.onMeasureChild（CoordinatorLayout.java:719）在android.support.design.widget.HeaderScrollingViewBehavior.onMeasureChild（HeaderScrollingViewBehavior.java:91）在android.support.design.widget.AppBarLayout $ ScrollingViewBehavior.onMeasureChild（AppBarLayout.java:1361）在android.support.design.widget.CoordinatorLayout.onMeasure（CoordinatorLayout.java:789）在android.view.View.measure（View.java:20151）在android.view.ViewGroup.measureChildWithMargins（ViewGroup.java:6328）在android.widget.FrameLayout.onMeasure（FrameLayout.java:194）在android.support.v7.widget.ContentFrameLayout.onMeasure（ContentFrameLayout.java:139）在android.view.View.measure（View.java:20151）在android.view.ViewGroup.measureChildWithMargins（ViewGroup.java:6328）在android.widget.LinearLayout.measureChildBeforeLayout（LinearLayout.java:1464）在android.widget.LinearLayout.measureVertical（LinearLayout.java:747）在android.widget.LinearLayout.onMeasure（LinearLayout.java:629）在android.view.View.measure（View.java:20151）在android.view.ViewGroup.measureChildWithMargins（ViewGroup.java:6328）在android.widget.FrameLayout.onMeasure（FrameLayout.java:194）在android.view.View.measure（View.java:20151）在android.view.ViewGroup.measureChildWithMargins（ViewGroup.java:6328）在android.widget.LinearLayout.measureChildBeforeLayout（LinearLayout.java:1464）在android.widget.LinearLayout.measureVertical（LinearLayout.java:747）在android.widget.LinearLayout.onMeasure（LinearLayout.java:629）在android.view.View.measure（View.java:20151）在android.view.ViewGroup.measureChildWithMargins（ViewGroup.java:6328）在android.widget.FrameLayout.onMeasure（FrameLayout.java:194）在com.android.internal.policy.PhoneWindow $ DecorView.onMeasure（PhoneWindow.java:3143）在android.view.View.measure（View.java:20151）在android.view.ViewRootImpl.performMeasure（ViewRootImpl.java:2594）在android.vi

Answer 1

纠正 FRAGMENT 部分。
DB onCreate(SQLiteDatabase db) 。

onCreate(SQLiteDatabase db) 这是我们编写create table语句的地方。这是在创建数据库时调用。

代码

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());

并且

@Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(CREATE_TABLE_STATEMENT);
    }

检查Android SQLite Database Tutorial。

Answer 2

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);
        }

如何使用片段

2 个答案: