Error in Cursor in getting data from SQLiteDatabase

Question

I want populate my ListView with the data from SQLiteDataBase but I'm having error on my cursor. I am a beginner in android programming please help. thanks. I am using fragments. . this my fragment code:

    package com.android.jayzero2013.estudiante;

import android.database.Cursor;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;

public class deleteSchedulePage extends Fragment {
    private ListView listview;
    scheduleDataBase mydb;

    View viewPage;
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        viewPage= inflater.inflate(R.layout.delete_schedule_layout,container,false);
        listview=(ListView)viewPage.findViewById(R.id.deleteSchedList);
        return viewPage;
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        mydb=new scheduleDataBase(getActivity());

        getDb();
    }

    public void getDb(){
            Cursor get = mydb.getalldata();
            String[] fields = new String[]{mydb.Col1, mydb.Col2};
            int[] textViewId = new int[]{R.id.itemNumTExt, R.id.SubjectCodeTextView};
            SimpleCursorAdapter myCursorAdapter;
            myCursorAdapter = new SimpleCursorAdapter(getActivity().getBaseContext(), R.layout.sched_list, get, fields, textViewId, 0);
            listview = (ListView) viewPage.findViewById(R.id.deleteSchedList);
            listview.setAdapter(myCursorAdapter);

        }
}

this is my code for SQLiteDatabase:

package com.android.jayzero2013.estudiante;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

/**
 * Created by jayzero2013 on 23/11/2015.
 */
public class scheduleDataBase extends SQLiteOpenHelper {

    public  static final String Database_Name="Schedule.db";
    public  static final String Table_Name="My_Schedule";
    public  static final String Col1="Number";
    public  static final String Col2="Subject_Code";
    public  static final String Col3="Subject_Description";
    public  static final String Col4="Type";
    public  static final  String Col5="Room";
    public  static final String Col6="Day";
    public  static final String Col7="TimeStart";
    public  static final String Col8="TimeEnd";

    private SQLiteDatabase db;

    public static final String [] ALL_KEYS=new String[]{Col1, Col2};


    public scheduleDataBase(Context context) {

        super(context, Database_Name, null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        sqLiteDatabase.execSQL("create table " + Table_Name + " (" +
                Col1 + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                Col2 + " TEXT NOT NULL, " + Col3 + " TEXT, " +
                Col4 + " TEXT NOT NULL, " + Col5 + " TEXT, " +
                Col6 + " TEXT NOT NULL, " + Col7 + " TEXT NOT NULL, " +
                Col8 + " TEXT NOT NULL)");
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
        sqLiteDatabase.execSQL("DROP TABLE IF EXISTS " + Table_Name);
        onCreate(sqLiteDatabase);
    }

    public boolean insertData(String sbjCD, String sbjDsc, String room, String typ, String day, String tymstrt, String tymnd){
        SQLiteDatabase sqLiteDatabase=this.getReadableDatabase();
        ContentValues content=new ContentValues();
        content.put(Col2, sbjCD);
        content.put(Col3, sbjDsc);
        content.put(Col4, typ);
        content.put(Col5, room);
        content.put(Col6, day);
        content.put(Col7, tymstrt);
        content.put(Col8, tymnd);

        long result=sqLiteDatabase.insert(Table_Name, null, content);
        if(result==-1){
            return false;
        }
        else{
            return true;
        }
    }

    public Cursor getalldata(){
        String where= null;
        Cursor c=db.query(true, Table_Name, ALL_KEYS,where,null,null,null,null,null);
        if(c!=null){
            c.moveToFirst();
        }
        return c;
    }


}

this is the error message from LogCat:

11-27 23:57:52.758  10346-10346/com.android.jayzero2013.estudiante E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at com.android.jayzero2013.estudiante.scheduleDataBase.getalldata(scheduleDataBase.java:76)
            at com.android.jayzero2013.estudiante.deleteSchedulePage.getDb(deleteSchedulePage.java:42)
            at com.android.jayzero2013.estudiante.deleteSchedulePage.onActivityCreated(deleteSchedulePage.java:37)
            at android.support.v4.app.Fragment.performActivityCreated(Fragment.java:1973)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1103)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1259)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1624)
            at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:517)
            at android.os.Handler.handleCallback(Handler.java:725)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5041)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

Your db object is null,before you use it,you should call this:

db = getWriteabledatabase();