Java: Put objects of different types into a ArrayList for a ModelCollection. Interfaces?

Question

I want to have my models wrapped up in a ModelCollection which is mostly used for a ListView. The Collection has always the same attributes like title, totalResults (for Pagination) and it shall contain the listItem-Models in the ArrayList "items". However, these models have different types like "ModelCategory" or "ModelChain" and often have different Properties and Methods.

How can I achieve this in java with strong Typing discipline? I heart interface a the right way to do this. WHERE do I have to implement them?

public class ModelCollection {

              public ArrayList< ModelCategory OR ModelChain OR ModelXyz> items = new ArrayList<ModelCategory OR ModelChain OR ModelXyz>();

              private String id;
              private String title;
              private Long updated;

              private String linkSelf;
              private int totalResults;
              private int startIndex;

    /*
     more stuff like parsing a feed
    */

    }

Answer 1

Make your ModelCategory, ModelChain, and ModelXyz implement an interface. Then have your collection be on that interface.

For instance:

public interface MyModel {
}

public class ModelCategory implements MyModel {
}

List<MyModel> list = new ArrayList<MyModel>();

To reference specific methods of each class, you will need to cast your list objects to the correct type.

List<MyModel> list = new ArrayList<MyModel>();
list.add(new ModelCategory());

ModelCategory model = (ModelCategory) list.elementAt(0);

Obviously, you can use whatever methods you need to iterate through your collections.

Answer 2

Solution of Tauren is correct, but remember to check instanceof like deadsven proposed, and the result is like:

    List<MyModel> list = new ArrayList<MyModel>();
    list.add(new ModelCategory());

    for(MyModel mymodelListElement: list) {
        //mymodelListElement.sameMyModelMethods()

        if(ModelCategory instanceof mymodelListElement) {
            ModelCategory modelCategoryElement = (ModelCategory)mymodelListElement;
        } else if(ModelChain  instanceof mymodelListElement) {
            ModelChain  modelChainElement = (ModelChain )mymodelListElement;
        } else if(ModelXyz instanceof mymodelListElement) {
            ModelXyz modelXyzElement = (ModelXyz)mymodelListElement;
        } else {
            //ignore
            //or
            //throw new RuntimeException("Wrong MyModel implementation")
        }

    }

Answer 3

Define a common interface for all your model classes which gives access to the properties they share.

public interface MooModel
  String getTitle();
  // etc

And define the List<MooModel>