Exception While Retrieving Data from SQlite

Question

I am trying to retrieve data from SQLite database using AndroidStudio 3.0.1 but getting the following exception. I have been searching for the Error but couldn't found a good solution. Here is My Code.

inserting Data to Database

public boolean insertCoins(int coins){
   SQLiteDatabase db=this.getReadableDatabase ();
    ContentValues values=new ContentValues ();
     values.put(Col1,coins);
    long result=db.insert(Table_Name,null,values);
    if (result != -1) {
        return true;
    } else {
        return false;
    }

}

The code is giving Error Here

public Cursor getData(){
Cursor res = this.getReadableDatabase ().rawQuery("select sum("+Col1+") from "+Table_Name+"",null);
return res;
}

here is the method id Plying_Window Class that is calling insertCoins method of Database_Helper

  public void insertCoins( ){
    Database_Helper  db=new Database_Helper ( this );
    boolean isInserted=db.insertCoins( 5 );
    if(isInserted){
        Toast.makeText ( Playing_Window.this, getString( R.string.c_add),Toast.LENGTH_LONG ).show ();}
}

here is the Levels Class that is calling getData method of Database_Helper .

public class Levels extends AppCompatActivity {
static int Easy_lvl_counter=0,Medium_lvl_counter=0,Hard_lvl_counter=0;
public  TextView display_coins;
int tot_coins   =coin;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_levels );
    Actions ();get ();
    set ();
}
void set(){
    int c=get();
    tot_coins=c;
    display_coins.setText ( String.valueOf ( c ) );

}
Button btn_level1,btn_level2,btn_level3,btn_level4,btn_level5,btn_level6,btn_level7,btn_level8,btn_level9,btn_level10,btn_mainMenu;
public void Actions(){
    display_coins=findViewById ( R.id.coins );

    btn_level1= findViewById ( R.id.btn_level1 );
    btn_level2=findViewById ( R.id.btn_level2 );
    btn_level3=findViewById ( R.id.btn_level3 );
    btn_level4=findViewById ( R.id.btn_level4 );
    btn_level5=findViewById ( R.id.btn_level5 );
    btn_level6=findViewById ( R.id.btn_level6 );
    btn_level7=findViewById ( R.id.btn_level7 );
    btn_level8=findViewById ( R.id.btn_level8 );
    btn_level9=findViewById ( R.id.btn_level9 );
    btn_level10=findViewById ( R.id.btn_level10 );
    btn_mainMenu=findViewById ( R.id.btn_mainMenu );
    btn_mainMenu.setOnClickListener ( new View.OnClickListener () {
        @Override
        public void onClick(View v) {
            finish ();
            Medium_lvl_counter=0;Easy_lvl_counter=0;Hard_lvl_counter=0;

        }
    } );

    btn_level1.setOnClickListener ( new View.OnClickListener () {
        @Override
        public void onClick(View v) {
            switch (Choice){
                case 1:{Easy_lvl_counter = 1; Medium_lvl_counter=0;Hard_lvl_counter=0;
                    if(tot_coins>=0)startActivity ( new Intent ( getApplicationContext (),Playing_Window.class ) );break;}

                case 2:{Medium_lvl_counter=1;Hard_lvl_counter=0;Easy_lvl_counter=0;
                    if(tot_coins>=50){startActivity ( new Intent ( getApplicationContext (),Playing_Window.class ) );}
                    else{Toast .makeText( Levels.this, R.string.txt_coins_short, Toast.LENGTH_SHORT).show();}break; }

                case 3:{Hard_lvl_counter=1;Easy_lvl_counter = 0; Medium_lvl_counter=0;
                    if(tot_coins>=100)startActivity ( new Intent ( getApplicationContext (),Playing_Window.class ) );
                    else{Toast .makeText( Levels.this,R.string.txt_coins_short, Toast.LENGTH_SHORT).show();}break; }

            }

        }
    } );............}
public int get(){
    Database_Helper dbHelper=new Database_Helper (Levels.this);
    Cursor cursor=dbHelper.getData ();
    if(cursor.getCount ()==0){return 0;}
    else{
        while(cursor.moveToNext ()){
            String db_coins=cursor.getString ( 0);
            if(coin  ==-1)coin=0;
            else coin=Integer.valueOf ( db_coins );
        }

    }
    return coin;
}
static int coin=0;

}

here is the playingWindow class

public class Playing_Window extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate ( savedInstanceState );
    setContentView ( R.layout.activity_playing__window );
    Actions ();
}
Levels obj_lev;
boolean timer_Count=false;
private EditText box1, box2 , box3, box4, box5, box6, box7, box8, box9, box10, box11, box12, box13, box14, box15, box16, box17, box18, box19    , box20, box21, box22, box23, box24, box25;
private AlertDialog.Builder builder;AlertDialog alertdialog;
private CountDownTimer timer;
void Actions() {
    obj_lev=new Levels ();
    builder = new AlertDialog.Builder(Playing_Window.this);
.
.
.
.
void dialogShow(){

    builder.setMessage ( getString( R.string.con_Text) );
    builder.setPositiveButton( R.string.txt_ok, new 
DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialogInterface, int i) {
            dialogInterface.dismiss();
            Levels obj=new Levels ();obj.get ();
            finish ();
        }
    });
    alertdialog=builder.create();
    alertdialog.show();
}
void dialogShowIncomplete(){

    builder.setMessage ( "Fill all the Boxes Correctly ! " );
    builder.setPositiveButton("Ok", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialogInterface, int i) {
            dialogInterface.dismiss();
        }
    });
    alertdialog=builder.create();
    alertdialog.show();

}
.
.
.
public void insertCoins( ){
    Database_Helper  db=new Database_Helper ( this );
    boolean isInserted=db.insertCoins( 5 );
    if(isInserted){
        Toast.makeText ( Playing_Window.this, getString( 
R.string.c_add),Toast.LENGTH_LONG ).show ();}
}

}

Here is the Stack Trace for My Exception . Thanks for Your Time

Process: com.sudoku.jamshaid.sudoku, PID: 23032
                                                                        java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
                                                                            at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:267)
                                                                            at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:223)
                                                                            at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:187)
                                                                            at com.sudoku.jamshaid.sudoku.Database_Helper.getData(Database_Helper.java:45)
                                                                            at com.sudoku.jamshaid.sudoku.Levels.get(Levels.java:241)
                                                                            at com.sudoku.jamshaid.sudoku.Playing_Window$3.onClick(Playing_Window.java:2555)
                                                                            at android.support.v7.app.AlertController$ButtonHandler.handleMessage(AlertController.java:162)
                                                                            at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                            at android.os.Looper.loop(Looper.java:135)
                                                                            at android.app.ActivityThread.main(ActivityThread.java:5221)
                                                                            at java.lang.reflect.Method.invoke(Native Method)
                                                                            at java.lang.reflect.Method.invoke(Method.java:372)
                                                                            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
                                                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

Answer 1

Here is your code, every where you are using db i,e SQLiteDatabase.

At the time of insertCoins again you are creating SQliteDatabase reference.

 public boolean insertCoins(int coins){
   SQLiteDatabase db=this.getReadableDatabase ();

Use one globally SQLiteDatabase db;

Try this :

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL ( "create table " + Table_Name + " (ID integer   PRIMARY KEY AUTOINCREMENT," + Col1 + " INTEGER not null " + ")" );

}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL ( "Drop table if exists "+Table_Name );
    onCreate ( db );
}
public boolean insertCoins(int coins){
    db=this.getReadableDatabase ();
    ContentValues values=new ContentValues ();
     values.put(Col1,coins);
    long result=db.insert(Table_Name,null,values);
    if (result != -1) {
        return true;
    } else {
        return false;
    }

}

Code for Getting data from Sqlite :

 private void setData() {

        Constants.cList = new ArrayList<LoadDataResult>();
        // Select All Query
        String selectQuery = "SELECT  * FROM " + Constants.TABLE_NAME;

        db = databaseHelper.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to list
        if (cursor.moveToFirst()) {
            do {
                LoadDataResult product = new LoadDataResult();
                product.setName(cursor.getString(1));
                product.setSku(cursor.getString(2));
                product.setUpc(cursor.getString(3));
                product.setAssoc_upc(cursor.getString(4));
                product.setPrice(cursor.getString(5));
                product.setDisplaySize(cursor.getString(6));
                product.setDisplaySizeYes(Integer.parseInt(String.valueOf(cursor.getInt(7))));
                product.setStatus(cursor.getString(8));
                Log.e("DATASQL",""+cursor.getString(3));

                // Adding contact to list
                Constants.cList.add(product);
            } while (cursor.moveToNext());
          //  utils.hideDialog();
        }

}

Try to use this, Change your getData() and try

public Cursor getData(){
    String selectQuery = "SELECT  * FROM " + Table_Name;
    Cursor res = this.getReadableDatabase().rawQuery(selectQuery,null);
    return res;
}

Answer 2

obj_lev=new Levels ();
... 
Levels obj=new Levels ();obj.get ();

This is never how you start an Activity in Android. Your error is because an Activity is a Context object, and your database needs a Context. When you call new on the Activity, none of the Activity lifecycle is started (onCreate for example, is never executed), and its Context is never setup, so it's null, and passed into the database handler, which is where you actually ran into the issue when attempting to use it.

Even looking at the rest of the code, I'm not sure how you expected the buttons in the Level class to work, or the text views to be set when none of the methods in onCreate are called.

You need to actually start it.

Overall, your get() method will not work from the "playing window" because holding onto an instance of an Activity from another is just the wrong pattern in Android development.

What you need instead is a wrapper around a Context that holds a database

public class LevelDB {  // this is not an Activity 
    private Context mContext;
    private Database_Helper mDb;

    public LevelDB(Context c) {
        mContext = c;
        mDb = new Database_Helper(c);
    }

    public int get() {
        SQLiteDatabase db = mDb.getReadableDatabase();
        return -1; // TODO: query the database 
    } 
} 

And now you would do

LevelDB obj=new LevelDB (Playing_Window.this);
int value = obj.get ();

And you can do the exact same thing in the other Activities, by passing the correct Context.

For example

public  TextView display_coins;
LevelDB mDb;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_levels );
    display_coins = findViewById(...);
    mDb = new LevelDB (Levels.this);
    int value = mDb.get ();
    display_coins.setText(""+value);
}

The alternative approach is to use your current code, but with a static method instead that takes a Context

public static int get(Context c){
    Database_Helper dbHelper=new Database_Helper (c);
    Cursor cursor=dbHelper.getData ();

Other than that, I pointed at the Room library in the comments. Use it - it'll help you manage the database better (but it won't solve your error of using Activities incorrectly)

Answer 3

I think it's because you are using getReadableDatabase, you should use getWritableDatabase. If not that, then maybe Col1 doesn't exist/has nothing in it, or perhaps your database hasn't been created. It's a null pointer, shouldn't be too hard to figure out