无法从 Android Studio 中 class 外部的公共 class 访问公共方法

Question

I'm building a SQlite database for a android quiz app. In Helper class I created a public method getAllQuestions(), but for some reason I can't access this method(or any other method in the helper class) in MainActivity. BTW I imported this class to Main but it doesn't help.

this is the code:

public class DB_Helper extends SQLiteOpenHelper {

public static final String DATABASE_NAME = "PRO_QUIZ";
public static final  int DATABASE_VERSION = 1;
private SQLiteDatabase db;

public ArrayList getAllQuestions(){

    ArrayList<QuestionsDataBase> questionsList = new ArrayList<>();
    db = getReadableDatabase();
    String Projection [] ={
            QuestionTable._ID,
            QuestionTable.Column_Question,
            QuestionTable.Column_Option1,
            QuestionTable.Column_Option2,
            QuestionTable.Column_Option3,
            QuestionTable.Column_Option4,
            QuestionTable.Column_Correct_Ans
    };
    Cursor c = db.query(QuestionTable.Table_Name,Projection,null,null,null,null,null);
    if(c.moveToFirst()){
        do{
            QuestionsDataBase questions = new QuestionsDataBase();
            questions.setQuestion(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Question)));
            questions.setQuestion(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Option1)));
            questions.setQuestion(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Option2)));
            questions.setQuestion(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Option3)));
            questions.setQuestion(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Option4)));
            questions.setQuestion(c.getString(c.getColumnIndexOrThrow(QuestionTable.Column_Correct_Ans)));
            questionsList.add(questions);

        }while(c.moveToNext());
    }
    c.close();
    return questionsList;
}

However, when I type in Main: DB_Helper. I cant see any methods.

Answer 1

The method getAllQuestions() is not static . That means that you need an instance of DB_Helper to access that method (you get an instance of an object with the keyword new ). So do this:

DB_Helper dbHelper = new DB_Helper();
dbHelper.getAllQuestions();

or alternatively you can declare the method as static this way:

public static ArrayList getAllQuestions(){
    ...
}

And then access it as you wanted in the first palce:

DB_Helper.getAllQuestions();

But beware that in this case (doing it static ) you will also need to declare all class variables used in that method as static too:

private static SQLiteDatabase db;

Recommended further reading: Difference between Static methods and Instance methods