not able to retrieve data from an SQLite database on android. (returns an empty ArrayList)

Question

I got a problem in trying to extract data from SQLite database, I am trying to create an android app that serves as a Question Bank. when trying to Retrieve information from this Database I get an empty list of Questions the app runs quite well with no error, but the problem is why is my questionList empty. Can somebody help me figure out whats going on.

Code for QuestionDBHelper which extends SQLiteOpenHelper

package com.cyanidesystems.unitquizapp.data;

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

import com.cyanidesystems.unitquizapp.models.Question;

import java.util.ArrayList;
import java.util.List;

import static com.cyanidesystems.unitquizapp.data.QuizQuestionsContract.QuizquestionColumns.*;



public class QuestionDBHelper extends SQLiteOpenHelper {
    private static final int DATABASE_VERSION= 1;
    private static final String DATABASE_NAME= "questionBankDB.db";
    public static SQLiteDatabase  questionDB;

    public QuestionDBHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }


    @Override
    public void onCreate(SQLiteDatabase db) {
        questionDB=db;
        //Creating a string that has the sql statements to create a table with all its columns and properties
        final String SQL_CREATE_QUESTION_BANK_TABLE_COMMAND= "CREATE TABLE " + TABLE_NAME + "(" +
                COLUMN_SUBJECT + " TEXT NOT NULL, " + COLUMN_YEAR + " TEXT NOT NULL, " + COLUMN_PAPER_NUMBER + " TEXT NOT NULL, " +
                COLUMN_QUESTION_ID + " INTEGER PRIMARY KEY,  " + COLUMN_QUESTION_STRING + " TEXT NOT NULL, " +
                COLUMN_QUEST_OPTION_A + "TEXT NOT NULL, " + COLUMN_QUEST_OPTION_B + " TEXT NOT NULL, " + COLUMN_QUEST_OPTION_C  +
                " TEXT NOT NULL, " + COLUMN_QUEST_OPTION_D + " TEXT NOT NULL, " +  COLUMN_ANSWER  + " TEXT NOT NULL )";

        db.execSQL(SQL_CREATE_QUESTION_BANK_TABLE_COMMAND);
        initQuestions();


    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL(" DROP TABLE IF EXISTS "+TABLE_NAME);
        onCreate(db);

    }
    public void  initQuestions(){
//initializing the Database with some sample questions
        Question q1=new Question("Computer Science","2017","I","00","" +
                "What is the meaning of URL","Uniform resource link","Uniform resource locator","uniform risk link",
                "universal resource locator","Uniform resource locator");
        this.putThisValuesInColumn((q1));

        Question q2=new Question("Computer Science","2017","I","01","" +
                "Android was acquired by Google in","2000","2003","2005", "2007","2005");
        this.putThisValuesInColumn((q2));



    Question q3=new Question("Computer Science","2017","I","02","" +
                    "CPU stands for","central process unit","central powering unit","central processing unit","control power unit","central processing unit");        
       this.putThisValuesInColumn((q3));

        Question q4=new Question("computer science","2017","I","03","What is the meaning of BIOS","Basic input output system","Basic instruction output system","Basic in/out system","bits in/out System","Basic input output system");
        this.putThisValuesInColumn((q4));
    }

        public void putThisValuesInColumn(Question quest){
            //this method actually loads the data to be inserted for a particular column

        //questionDB=this.getWritableDatabase();
        ContentValues values=new ContentValues();
        values.put(COLUMN_SUBJECT,quest.getSubject());
        values.put(COLUMN_YEAR,quest.getYear());
        values.put(COLUMN_PAPER_NUMBER,quest.getPaper_number());
        values.put(COLUMN_QUESTION_ID,quest.getQuestID());
        values.put(COLUMN_QUESTION_STRING,quest.getQuest_String());
        values.put(COLUMN_QUEST_OPTION_A,quest.getOptionA());
        values.put(COLUMN_QUEST_OPTION_B,quest.getOptionB());
        values.put(COLUMN_QUEST_OPTION_C,quest.getOptionC());
        values.put(COLUMN_QUEST_OPTION_D,quest.getOptionD());
        values.put(COLUMN_ANSWER,quest.getAnswer());



        // loads all of the initialized data into the database table
        questionDB.insert(TABLE_NAME,null,values);




    }


    public  ArrayList<Question> getAllQuestions(){
//this method extracts all the questions in the database
//and returns it as a list of question objects

            ArrayList<Question> questionList=new ArrayList<>();
            //string of selecting quest from DB
            String sqlSelectString="SELECT * FROM " + TABLE_NAME;

            questionDB=this.getWritableDatabase();
            Cursor cursor=questionDB.rawQuery(sqlSelectString,null);

            if (cursor.moveToFirst()){
                do {
                    Question question=new Question();

                    question.setSubject(cursor.getString(0));
                    question.setYear(cursor.getString(1));
                    question.setPaper_number(cursor.getString(2));
                    question.setQuestID(cursor.getString(3));
                    question.setQuest_String(cursor.getString(4));
                    question.setOptionA(cursor.getString(5));
                    question.setOptionB(cursor.getString(6));
                    question.setOptionC(cursor.getString(7));
                    question.setOptionD(cursor.getString(8));
                    question.setOptionD(cursor.getString(9));

                    //adding the question into the question list
                    questionList.add(question);

                }while (cursor.moveToNext());

            }
            return questionList;
        }

how Question class looks like

public class Question {


    String subject;
    String year;
    String paper_number;
    String questID;
    String quest_String;
    String optionA;
    String optionB;
    String optionC;
    String optionD;
    String answer;




    //class constructor With no Parameters
    public Question( ) {
        this.subject ="";
        this.year = "";
        this.paper_number = "";
        this.questID = "";
        this.quest_String = "";
        this.optionA = "";
        this.optionB = "";
        this.optionC = "";
        this.optionD = "";
        this.answer = "";
    }

    //Class Constructor

    public Question(String subject, String year, String paper_number, String questID, String quest_String,
                    String optionA, String optionB, String optionC, String optionD, String answer) {
        this.subject = subject;
        this.year = year;
        this.paper_number = paper_number;
        this.questID = questID;
        this.quest_String = quest_String;
        this.optionA = optionA;
        this.optionB = optionB;
        this.optionC = optionC;
        this.optionD = optionD;
        this.answer = answer;
    }


    //Getters and Setters

    public String getSubject() {
        return subject;
    }

    public void setSubject(String subject) {
        this.subject = subject;
    }

    public String getYear() {
        return year;
    }

    public void setYear(String year) {
        this.year = year;
    }

    public String getPaper_number() {
        return paper_number;
    }

    public void setPaper_number(String paper_number) {
        this.paper_number = paper_number;
    }

    public String getQuestID() {
        return questID;
    }

    public void setQuestID(String questID) {
        this.questID = questID;
    }

    public String getQuest_String() {
        return quest_String;
    }

    public void setQuest_String(String quest_String) {
        this.quest_String = quest_String;
    }

    public String getOptionA() {
        return optionA;
    }

    public void setOptionA(String optionA) {
        this.optionA = optionA;
    }

    public String getOptionB() {
        return optionB;
    }

    public void setOptionB(String optionB) {
        this.optionB = optionB;
    }

    public String getOptionC() {
        return optionC;
    }

    public void setOptionC(String optionC) {
        this.optionC = optionC;
    }

    public String getOptionD() {
        return optionD;
    }

    public void setOptionD(String optionD) {
        this.optionD = optionD;
    }

    public String getAnswer() {
        return answer;
    }

    public void setAnswer(String answer) {
        this.answer = answer;
    }
}

MainActivity.java file

public class MainActivity extends AppCompatActivity {
//Declaration of Variables
    int currentPosition = 0;//this denotes the current position of the question in our List
    int numberOfCorrectAnswers = 0;
    QuestionDBHelper questionDB=new QuestionDBHelper(this);
    ArrayList<Question> quizQuestions = new ArrayList<> ();



    //@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);

        progressBar.setProgress(0);

        //quizQuestions = setUpQuestions(new ArrayList<Question>());
        //it creates a new instance of an arraylist and passes it to the function
        //the function then loads it with data and returns a new loaded list
        //which is then assigned to quizQuestions


        //setData();

        setData();
    }

    public void setData() {

        subjTitle.setText(getString(R.string.Subject,"Computer Science"));
        subjYr.setText(getString(R.string.Year,"2017"));
        quizQuestions =questionDB.getAllQuestions();
        Log.i("size of quizQuestions",quizQuestions.toString());
    }

Log with info filter showing that the quizQuestions Arraylist is empty

04-23 00:39:39.596 2536-2536/com.cyanidesystems.unitquizapp W/dalvikvm: VFY: unable to resolve virtual method 323: Landroid/content/Context;.getColorStateList (I)Landroid/content/res/ColorStateList;
04-23 00:39:39.772 2536-2536/com.cyanidesystems.unitquizapp I/size of quizQuestions: []
04-23 00:39:39.836 2536-2536/com.cyanidesystems.unitquizapp W/dalvikvm: VFY: unable to find class referenced in signature (Landroid/graphics/drawable/Icon;)

Answer 1

Hey guys i finally found what was going wrong. i think by default when a database is created in SQLITE, the cursor to the Database is open for writing when i removed the questionDB=this.getWritableDatabase(); statement in getAllQuestions() method of the QuestionDBHelper class it worked.

thanks to everybody who contributed to my question luv the community spirit

Answer 2

Assuming the TABLE_NAME is correct, try to change the code

if (cursor.moveToFirst()){
    do {
        // create newObject
       questionList.add(question);
   } while (cursor.moveToNext());
}

To just

while(cursor.moveToNext()) {
    // create newObject
    questionList.add(question);
}
cursor.close(); //remember to close the cursor
return questionList;