Dynamically cast to yet unknown class in ArrayList of objects

Question

I have an ArrayList of type Object that contains 6 objects of different classes:

questions = new ArrayList<>();
questions.add(new Question1());
questions.add(new Question2());
questions.add(new Question3());
questions.add(new Question4());
questions.add(new Question5());

Depending on questionNum variable I have to get the object's method, i.e. isAnswered() from the ArrayList.

Example of what I want:

for (int i = 0; i < 5; i++)
{
     questionNum = i;
     if ((Question<questionNum>)questions.get(questionNum).isAnswered())
     {
          Log.d("Answered", String.valueOf(questionNum));
     }
}

What I am using right now:

else if (questionNum == 4 &&
((Question5)questions.get(questionNum)).isAnswered())

Any way to make it more elegant or should I go with interfaces or inheritance (doing Android project, so the every question class already inherits Fragment)?

You should use interface and delegate the functionality to the appropriate class — MaxZoom
– MaxZoom, Commented Apr 28, 2015 at 21:57

copeg · Accepted Answer · 2015-04-29 00:02:13Z

1

You can have each Question class implement an interface:

public interface Question{
    public boolean isAnswered(); 
}

public class Question1 implements Question{
    public boolean isAnswered(){
        //implementation
    }
    //other code
}

Then add these as Questions to the List

List<Question> questions = new ArrayList<Question>();
questions.add(new Question1());

Then you can get the Question from the List and call the appropriate method:

if ( questions.get(0).isAnswered() ){

}

edited Apr 29, 2015 at 0:02

answered Apr 28, 2015 at 21:57

copeg

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3 Comments

Nick Pakhomov Over a year ago

You have to do casting to (Question) interface to make it correct isAnswered() call. I do get Attempt to invoke interface method 'java.lang.Object java.util.List.get(int)' on a null object reference when I do the if statement. Any hints on avoiding it?

copeg Over a year ago

Casting shouldn't be necessary if the List is parametized to the class Question. java.lang.Object java.util.List.get(int)' on a null object reference is the List null?

Nick Pakhomov Over a year ago

My bad, I had the list declared twice: as a field and in the code. Changed the type in the field and it works like a charm!

Nikos M. · Accepted Answer · 2015-04-28 22:05:34Z

i see three solutions here:

A. Add a typecaster along with each question on ArrayList like this:

questions = new ArrayList<>();
questions.add(new ArrayList<>(Arrays.asList(new Question1(), "Question1")));
questions.add(new ArrayList<>(Arrays.asList(new Question2(), "Question2")));

Then have a switch statement depending on the second type argument and type cast (literally) as needed

B. Have each Question* class have a property type and then use a switch statement (as previously) to typecast appropriately, i.e:

class Question1
{
    public String type = "Question1";
    // ...
}
// ...

C. Have all Question* classes inherit from common Question superclass or implement common interface as needed, i.e:

class Question
{
    public String type = "Question";
    public Boolean isAnswered()
    {
        return false;
    }
    // ...
}

class Question1 extends Question
{
    public String type = "Question1";
    public Boolean isAnswered()
    {
        // ..
    }
    // ...
}
// ..

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