Convert an ArrayList<String> to ArrayList<SomeClass> java

Question

I've managed to convert an arraylist of some class (in my case it's a class which extends ArrayAdapter<Question>) to an ArrayList<String>. I've passed it through an android intent bundle and received through the getStringArrayList method. Now I need to convert it back to the ArrayList<Question> to set my adapter up

Converting from ArrayList<Question> to ArrayList<String>

       questionlists = task.execute(myitem).get();

        for (Question object : questionlists) 
        {
            strings.add(object != null ? object.toString() : null);
        }
        simpleArray2 = new String[strings.size()];
        strings.toArray(simpleArray2);

Converting back to ArrayList<Question>

 List<Question> variable;
 lists = getArguments().getStringArrayList("Question");
     variable = (List<Question>)(List<?>) lists;

  @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.main_world, container, false);
    ListView lv = (ListView) view.findViewById(R.id.list_sessions);
    final QuestionAdapter adapter = new QuestionAdapter(getActivity(), variable);
    lv.setAdapter(adapter);
    //ArrayAdapter<String> adapter = new ArrayAdapter<String>(  
             //inflater.getContext(), android.R.layout.simple_list_item_1,  
             //lists);  
   // lv.setAdapter(adapter);             
    return view;
}

I've added a try/catch my adapter and I get the error:

java.lang.String cannot be cast to java.com.example.Question.

What am I doing wrong? Is it because I haven't converted back to a proper ArrayList first?

variable = (List<Question>)(List<?>) lists; what the. That won't work. — EpicPandaForce
– EpicPandaForce, Commented Jul 4, 2014 at 19:25
That is where I try to convert an arraylist<String>, which I've passed through my putStringArrayList call, to an arrayList of Questions so that I may display it in my adapter — user1397978
– user1397978, Commented Jul 4, 2014 at 19:27
List<Question> is not the same as List<String> because Question is not the same as String. You should make the Question class Parcelable using parcelabler.com and send an array of Parcelable instead of String. — EpicPandaForce
– EpicPandaForce, Commented Jul 4, 2014 at 19:30

Steve P. · Accepted Answer · 2014-07-04 19:18:51Z

3

In your first example, you're not actually casting a Question to a String. You're calling the toString() method for a Question.

For example,

public class Question {

    //instance member
    String question;

    public String toString() {
       return question;
    }
}

In order to go from a String to a Question, you're going to have to write a constructor or method that takes a String as a parameter and returns a Question.

For example, you could add this constructor to your Question class:

public Question(string myQuestion) {
     question = myQuestion;
}

edited Jul 4, 2014 at 19:18

answered Jul 4, 2014 at 19:12

Steve P.

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2 Comments

user1397978 Over a year ago

How would you suggest I do it? In my first example, strings is an Arraylist of strings.

Steve P. Over a year ago

@user1397978 Hopefully my update helps explain things.

Ridcully · Accepted Answer · 2014-07-05 04:54:45Z

1

Alright. To convert from List<Question> to List<String> you already do this:

Converting from ArrayList to ArrayList

   List<Question> questionlists = task.execute(myitem).get();
   List<String> strings = new ArrayList<String>();

    for (Question object : questionlists) 
    {
        strings.add(object != null ? object.toString() : null);
    }

Now to convert from List<String> to List<Question> you'd have to do exactly the same, just the other way round:

   List<String> strings = ... result from above
   List<Question> questionlists = new ArrayList<Question>();

    for (String object : strings) 
    {
        questionlists.add(object != null ? object.toQuestion() : null);
    }

Oh, wait! String does not have a toQuestion() method! What now? Well, we just make such a method ourselves. As it's going to be a method that creates a Question object, why not make it a Constructor of Question, taking a String as parameter?

   List<String> strings = ... result from above
   List<Question> questionlists = new ArrayList<Question>();

   for (String object : strings) 
   {
       questionlists.add(object != null ? new Question(object) : null);
   }

Now all that's left to do, is to add that Constructor to your question class.

class Question {
    public Question(String string) {
        // implementing this the right way is up to you...
    }
}

edited Jul 5, 2014 at 4:54

answered Jul 4, 2014 at 19:48

Ridcully

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1 Comment

user1397978 Over a year ago

Thanks! That seemed to help a lot and make so much sense!

DeeV · Accepted Answer · 2014-07-04 19:17:20Z

toString() is a method that all Java Object objects have. By default, it will list the name of the class and the memory location of the reference. You're not actually converting the object Question to String. You are generating a String that in some what interprets what Question is. For example, you can overwrite toString() to generate whatever you want.

public class Question {

  String mQuestion;

  @Override
  public String toString() {
     return mQuestion;
  }
}

Now, what you can do is make some sort of custom String in toString() that fully represents your Question object. Then have a fromString method that parses your custom String and builds new Question objects from it. However, this method is error prone and limiting.

A better method would be to either inherit Question from Serializable or better yet Parcelable. Doing this allows you to put the actual Question object in to a Bundle and pull it out from the Intent without any conversion.

Teddy · Accepted Answer · 2014-07-04 19:46:08Z

You have to understand Type Casting correctly.

Type casting is not conversion. It is asking java to assume for the rest of that context that the real type of that object is what you say it is. You are telling java that even though it is not obvious in the current context, you know better about what the actual type of the object is.

Information about the exact type of an object is not available in every context. As you pass objects around, methods treat them as any of their super class types right up to Object.

Type casting is a way to tell java that "I already know it is a List<Question> because I know who put that object there".

So, typecasting cannot help with conversion. The object should already be of that type for typecasting to work.

If it is of a different type, you have to write code to convert it.

public class ListProcessing {
public static void main(String[] args) {

    List<Integer> ints = Arrays.asList(1,2,3);

    //Take a list, convert and add to new list
    List<String> intStrs = listTransform(ints, new Converter<Integer, String>() {
        @Override
        public String convert(Integer input) {
            return input==null?null:"S"+input.toString();
        }           
    });

    System.out.println(intStrs);
}

//Generic list processor
protected static <T,U> List<U> listTransform(List<T> in, Converter<T,U> converter){
    if(in==null) return null;
    List<U> out = new ArrayList<>();
    for(T item:in){
        out.add(converter.convert(item));
    }
    return out;
}

//An interface for a converter, which converts a single item
private static abstract class  Converter <T,U>{
    public abstract U convert(T input);
}
}

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