CODEWITHSUNDEEP
androidlistviewandroid-arrayadapter
Sid Chopra
Sid Chopra

Reputation: 19

Error in initialization of custom array adapter

enter image description here

I am trying to make a custom listview. The list is declared as below

List<DocRow> doctors = new ArrayList<>();

This list is then being populated.

My custom array adapter is in a separate class with its constructor declared as below.

public class DocAdapter extends ArrayAdapter<DocRow>{
    Context context;
    int resource;
    ArrayList<DocRow> doctors;
    private LayoutInflater inflater;

    public DocAdapter(@NonNull Context context, @LayoutRes int resource, ArrayList<DocRow> doctors) {
        super(context, resource, doctors);
        this.context = context;
        this.resource = resource;
        this.doctors = doctors;
        inflater = LayoutInflater.from(context);
    }

Now in my main activity, I am trying to create a new custom array adapter by passing off my list (which is a valid parameter), it isn't accepted. The code for creation and setting of adapter for linking the listview with the list is below.

DocAdapter adapter = new DocAdapter(getApplicationContext(), R.layout.doc_row, doctors);
docList.setAdapter(adapter);

Can anyone explain what is the issue? The link for error screenshot is above. I tried searching for this specific issue, but haven't been able to find a solution that works.

Upvotes: 1

Views: 201

Answers (2)

Bhavit S. Sengar
Bhavit S. Sengar

Reputation: 8934

A parent class's reference can store subclass's object, but the reverse is not true.

Here, in the constructor of your adapter, you have ArrayList<DocRow> as your parameter type, but your doctors list is of type List<DocRow>. You, you're passing a List<> object to an ArrayList<> reference.

To solve it, either change your doctors variable type to ArrayList<>, or your constructor parameter type to List<>

Upvotes: 0

Nayan Srivastava
Nayan Srivastava

Reputation: 3725

Change your constructor argument to List instead of ArrayList as you are passing list in it.

 List<DocRow> doctors;

 public DocAdapter(@NonNull Context context, @LayoutRes int resource, List<DocRow> doctors) {
        super(context, resource, doctors);
        this.context = context;
        this.resource = resource;
        this.doctors = doctors;
        inflater = LayoutInflater.from(context);
    }

As pointed by @Tim, here is a little detail about why this is needed.

When an instance is initialized, it may be initialized with one of its child classes but the object remains an instance of Super class only(Due to runtime polymorphism) and therefore the methods that consume this instance either expect super class or the instance should be casted to superclass before passing it on.

The easiest way to identify is to always look at the type on the left-hand side instead.

List a=new ArrayList();

In above example, the instance is actually an arraylist but it is of Type List.

Upvotes: 2

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