Creating multiple objects within a class

Question

Hi all,

I am trying to create a class. Let's call it Questionnaire. In Questionnaire, I want to instantiate a number of classes called Question. The constructor for Questionnaire accepts a string array of words, and for each word, there should be a Question class to accept it as an argument for its constructor. In other words, for every string that is passed as an argument, I want to create a Question.

I need to be able to put Questionnaire within a bigger class and still call the methods of the Question classes. The size of the array passed to Questionnaire varies as well. I don't know how I can put multiple classes within another class when I don't know how many classes there will be, and how to reference them from an outer class. If the array was a fixed length I'd just create Questions as question1, question2, question3... but since this is not the case I am not sure anymore. I do know there is a maximum number of Question classes I want to create though.

edit: I'm using J2ME CDLC 1.1 MIDP 2.0 for this (it's part of a mobile phone app) so my solutions are constrained by what it offers.

Answer 1

You don't want to have a separate variable per question - use a List<Question> to have one variable which refers to a collection of questions:

public class Questionnaire
{
    private final List<Question> questions;

    public Questionnaire(String[] words)
    {
        questions = new ArrayList<Question>();
        for (String word : words)
        {
            questions.add(new Question(word));
        }
    }

    // Use questions here
}

EDIT: If you're in an environment without generics, you could use the non-generic form, like this:

    private final List questions;

    public Questionnaire(String[] words)
    {
        questions = new ArrayList();
        for (String word : words)
        {
            questions.add(new Question(word));
        }
    }

or like this if you don't have List/ArrayList:

    private final Vector questions;

    public Questionnaire(String[] words)
    {
        questions = new Vector();
        for (String word : words)
        {
            questions.add(new Question(word));
        }
    }

In either case you'll need to cast on every access. Alternatively, you could use an array:

    private final Question[] questions;

    public Questionnaire(String[] words)
    {
        questions = new Question[words.length];
        for (int i = 0; i < words.length; i++)
        {
            questions[i] = new Question(word);
        }
    }

Answer 2

Why don't you use a List on the questionaire and for each question you add you add one to the List, for example.

public class Questionnaire
{
    public List<Question> myQuestions {get; private set;}

    public Questionnaire(string[] questions)
    {

       myQuestions = new List<Questions>();

       foreach(string q in questions)
       {
          myQuestions.Add(new Question {questionText = g});
       }
    }
}
public class Question
{
    public string questionText {get; set;}
}

Then you will be able to check the questions from outside... this is what you want?

Hope this helps

EDIT: ohh is Java, but you got the idea right?

Answer 3

import java.util.*;

public class Questionnaire implements Iterable<Question> {
    private final List<Question> questions;

    public Questionnaire(String ...words) {
       questions = new ArrayList<Question>(words.length);
       for(String word : words) {
           questions.add(new Question(word));
       }
    }

    // allows you to use a Questionnaire object in a for-each loop
    public Iterator<Question> iterator() { return questions.iterator(); }

    @Override
    public String toString() { return questions.toString(); }
}

You could add many more goodies to your class to make it more useful. An example of using the class above follows:

public class QuestionnaireTest {
    public static void main(String[] words) {
        Questionnaire questionnaire = new Questionnaire(words);
        for(Question q : questionnaire) {
            System.out.println("You asked: " + q);
        }
    }
}

You could also use it as follows:

public class QuestionnaireTest3 {
    public static void main(String[] words) {

        // because I declared the constructor to accept "String ...words", I can specify as many questions as I want using simple syntax
        Questionnaire questionnaire = new Questionnaire("How deep the ocean?", "How high the moon?");
        for(Question q : questionnaire) {
            System.out.println("I asked: " + q);
        }
    }
}

Even though I did so in my first example above, you should really accept an array of String objects as questions. Here's a better design:

public class Questionnaire implements Iterable<Question> {
    private List<Question> questions = new ArrayList<Question>();

    public void add(Question q) {
        if(q == null) throw new IllegalArgumentException("can't add null question!");

        questions.add(q);
    }

    public Question get(int index) {
       if(index < 0 || index >= questions.size()) throw new IndexOutOfBoundsException("invalid question index: " + index);

       return questions.get(index);
    }

    // allows you to use a Questionnaire object in a for-each loop
    public Iterator<Question> iterator() { 
        return Collections.unmodifiableList(questions).iterator();
    }

    @Override
    public String toString() { return questions.toString(); }
}

public abstract class Question {
    public String getText();
    public String getAnswer();
    public String getOptions();
    // ...
}

public class YesNoQuestion extends Question {
    private final String text;
    private final String answer;

    public YesNoQuestion(String text, boolean answer) {
       if(!(text.startsWith("Is"))) throw new IllegalArgumentException("Must start with is: " + text);

        this.text = text;

        this.answer = answer ? "Yes" : "No"; // if answer == true, the "Yes",...
    }

     @Override
     public String getText() { return text; }

     public String getAnswer() { return answer; }

     public String getOptions() { return "Yes or No ?"; }
}

And now you can use it as follows:

public class QuestionnaireTest4 {
    public static void main(String[] words) {

        Questionnaire test = new Questionnaire();

        test.add(new YesNoQuestion("Is dogs animals?", false));
        test.add(new YesNoQuestion("Is me has cheezburgers?", true));

        for(Question q : questionnaire) {

            System.out.println(q);
            System.out.println(q.getOptions());

            String input = null; // you need to code this part
            if(q.getAnswer().equals(input))
                System.out.println("CORRECT!");
            else
                System.out.println("YOU IS STUPID!!!!");
        }
    }
}

Answer 4

As J2ME API lacks the Collections API, your best bet is to grab a Vector.

Vector questions = new Vector();
for (int i = 0; i < words.length; i++) {
    questions.addElement(new Question(words[i]));
}

Answer 5

You probably want to go with a Set of questions rather than List. Try :

private final Set<Question> questions;

This will prevent duplicate questions. Everything else will be the same as JonSkeet's answer.